LIBRARY
OF THE
UNIVERSITY OF CALIFORNIA.
Class
GEOLOGY FOB ENGINEERS.
Demy 8vo. Handsome Cloth.
STRATIGRAPHICAL GEOLOGY AND PALEONTOLOGY,
ON THE BASIS OF PHILLIPS.
BY ROBERT ETHERIDGE, F.R.S.,
of the Natural Hist. Department, British Museum, late Palaeontologist to the Geological Survey of Great Britain, Past President of the Geological Society, etc.
With Map, Numerous Tables, and Thirty-six Plates.
" If Prof. SEKLEY'S volume was remarkable for its originality and the breadth of its views, Mr ETHERIDGE fully justifies the assertion made in his preface that his book differs in construction and detail from any known manual. . . . Must take HIGH RANK
AMONG WORKS OP REFERENCE."— A thenceum.
SIXTH EDITION, Thoroughly Revised. With Illustrations. Cloth. AIDS IN
PRACTICAL GEOLOGY:
WITH A SECTION ON PALEONTOLOGY.
BY PROFESSOR GRENVILLE COLE, M.R.I. A., F.G.S.
GENERAL CONTENTS.
Part I.— Sampling of Earth's Crust. I Part III.— Examination of Rocks. Part II.— Examination of Minerals. | Part IV.- Examination of Fossils.
"That the work deserves its title, that it is full of ' AIDS.' and in the highest degree ' PRACTICAL,' will be the verdict of all who use it." — Nature.
With 12 Full-page Illustrations from Photographs. Cloth. SECOND EDITION, Revised.
OPEN-AIR STUDIES IN GEOLOGY:
AN INTRODUCTION TO GEOLOGY OUT-OF-DOORS.
BY GRENVILLE A. J. COLE, F.G.S., M.R I.A., Professor of Geology in the Royal College of Science for Ireland,
and Examiner in the University of London.
"The FASCINATING 'OPEN-AiR STUDIES' of Prof. COLE give the subject a GLOW OF ANIMATION . . . cannot fail to arouse keen interest in geology."— Geological Magazine. "A CHARMING BOOK, beautifully illustrated."— A thenceum.
In Crown 8vo. Handsome Cloth. Fully Illustrated. A HANDBOOK ON
THEODOLITE SURVEYING AND LEVELLING.
FOR THE USE OF STUDENTS IN LAND AND MINE SURVEYING. BY PROFESSOR JAMES PARK, F.G.S.
"A book which should prove as useful to the professional surveyor as to the student." — Nature.
SECOND EDITION, Revised. Crown 8vo. Handsome Cloth. Illustrated.
MINING GEOLOGY.
A TEXT-BOOK FOR MINING STUDENTS AND MINERS. BY PROFESSOR JAMES PARK, F.G.S., M.Inst.M.M.,
Professor of Mining and Director of the Otago University School of Mines ; late Director Thames School of Mines, and Geological Surveyor and Mining Geologist to the
Government of New Zealand.
"A work which should find a place in the library of every mining engineer."— Mining World.
LONDON : CHARLES GRIFFIN & Co., LIMITED, EXETER ST., STRAND.
GEOLOGY FOR ENGINEERS
BY
LIEUT. -CoL. R. F. SORSBIE, R.E.
mattb nearly 100 ^figures.
LONDON:
CHARLES GRIFFIN & COMPANY, LIMITED, PHILADELPHIA: J. B. L1PPINCOTT COMPANY.
7
PREFACE.
IN these days of specialising in " watertight compartments," the bearing of geology in relation to almost every branch of engineer- ing is very frequently neglected or ignored. A knowledge of geology is, however, of the first importance to the practical engineer, but it is difficult for him to study the application of this science to his requirements without having recourse to a large number of different textbooks and other works. References to geology which are often of the greatest practical importance are often almost hidden away or treated in an obscure fashion, where- as the engineer requires the needful information to be put before him in a clear and concise manner. To meet this want I have endeavoured to compile the requisite information in one volume, in the hope that it may serve as a handy book of reference.
I am greatly indebted to the various authors and publishers of the books mentioned in the accompanying list for so kindly allow- ing me to take such extracts as I required, and desire to record my grateful thanks to Professors Lapworth, Cole, and Bauerman, and Mr Hayden of the Geological Survey of India, for their kind help and encouragement.
These extracts are referred to by a number at the end of each quotation corresponding with the number in the accompanying list.
R. F. SORSBIE.
January 1911.
226185
AUTHORITIES CONSULTED.
1 Author.
2 Geology in Systematic Notes and Tables, by Wintour F. Gwinnell, F.G S. ; 2nd edition. Allmann & Sons, 67 New Oxford Street, 1889.
3 An Intermediate Textbook of Geology, by Charles Lapworth, F.R.S. William Blackwood & Sons, 1899.
4 Geology: Chemical, Physical, and Stratigraphical, vol. i., Chemical and Physical, by Joseph Prestwich. Clarendon Press, 1 886.
5 Geology : A Manual for Students in Advanced Classes and for General Readers, by Charles Bird, B.A. Lond., F.G.S. Longmans, Green & Co., 1894.
6 Manual of Geology, Theoretical and Practical, by John Phillips, LL.D., F.R.S., Part I. Physical Geology and Palceon- tology, by H. G. Seeley, F.R.S. Charles Griffin & Co., 1885.
7 Physical Geology, by A. H. Green, M.A., F.G.S. Longmans, Green & Co., 1898.
8 filoxam's Chemistry (a few definitions only).
9 Physical Geology, by Ralph Tate, Weale's series. Crosby, Lockwood & Son, 1907.
10 The Standard English Dictionary (a few definitions).
11 Economic Geology, by David Page, LL.D., F.G.S. Wm. Blackwood & Sons, 1874.
12 Textbook of Systematic Mineralogy, by Hilary Bauerman, F.G.S. Longmans, Green & Co., 1903.
13 Elementary Course of Geology, Mineralogy, and Physical Geography, by D. T. Ansted. John Van Voorst, 1856.
14 Textbook of Descriptive Mineralogy, by Hilary Bauerman, F.G.S. Longmans, Green & Co., 1902.
15 Aids in Practical Geology, by Grenville A. S. Cole, M.R.I. A., F.G.S. Charles Griffin & Co., 4th edition, 1902.
16 The Study of Rocks, by Frank Rutley, F.G.S. Longmans, Green & Co., 6th edition, 1894.
Vlll GEOLOGY FOR ENGINEERS.
17 Vol. ii. of Prestwich's Geology (see No. 4).
18 Historical Geology, by Ralph Tate, Weale's series. Crosby, Lockwood & Sons.
19 How to Observe: Geology, by H. T. de la Beche, F.R.S., etc. Charles Knight, 1835.
!0 A Guide to Analysis in Geological and Agricultural Chemistry, by an Officer of the Bengal Engineers.
21 A First Book of Mineralogy, by J. H. Collins, F.G.S. William Collins & Sons.
22 The Principles of Waterworks Engineering, by J. H. T. Tudsbery, D.Sc., and A. W. Brightmore, D.Sc.; 3rd edition. E. & F. N. Spon, 1905.
23 The Water Supply of Cities and Towns, by W. Humber. Crosby, Lockwood & Sons, 1876.
24 Sanitary Engineering, by Vernon Harcourt. Longmans, Green & Co., 1907'.
25 Treatise on Waterworks, by S. Hughes. Crosby, Lockwood & Sons, 1875.
26 Quarrying and Blasting Rocks, by Sir J. Burgoyne, Weale's series. Crosby, Lockwood & Sons, 1895.
27 Treatise on Building and Ornamental Stones of Great Britain and Foreign Countries, by Edward Hall. Macmillan & Co., 1872.
28 Road-making and Maintenance, by T. Aitken. Charles Griffin & Co., 1900.
29 Appendix by R. Mallet in Dobson's Brick and Tile Making, Weale's series. Crosby, Lockwood & Sons.
30 Calcareous Cements, by G. R. Redgrave and Charles Spackman. Charles Griffin & Co., 1905.
31 Limes, Cements, Mortars, etc., by G. R. Burnell, Weale's series. Crosby, Lockwood & Sons.
32 Pioneer Engineering, by E. Dobson, Weale's series. Crosby, Lockwood & Sons.
33 Road-making and Maintenance, by T. Aitken. Charles Griffin & Co., 1900.
34 The Construction of Roads, Paths, and Sea Defences, by Frank Latham, C.E. The Sanitary Publishing Company, Ltd., 1903.
35 Professor Mahon's "Elementary Essay on Road-making," quoted in Rudiments of the Art of Constructing Roads, by H. Law, C.E., Weale's series. Crosby, Lockwood & Sons.
36 An article on "Broken Stone Roads," by Reginald Ryves in Engineering, 1905, pp. 76 and 205.
37 The Rudiments of Civil Engineering, by H. Law, C.E., Weale's series. Crosby, Lockwood & Sons, 1882.
AUTHORITIES CONSULTED. IX
S8 Hydraulic Tables, by Nathaniel Beardmore. Waterlow & Sons, 1852.
39 The General Principles of Mineralogy, by J. H. Collins, F.G.S. Wm. Collins & Sons.
40 Tidal Rivers, by W. H. Wheeler, M.I.C.E. Longmans, Green & Co., 1893.
41 Coast Erosion and Foreshore Protection, by J. S. Owens, M.D., A.M.I.C.E., F.R.G.S., and G. 0. Case. St Bride's Press, 1908.
42 An article on "Coast Erosion and Reclamation," in The Engineer of 27th April 1906, and subsequent numbers.
In addition to the above works from which extracts have been taken the following authorities have been consulted : —
Geikie's Textbook of Geology.
„ Class-book of Geology.
„ Field Geology. Lyell's Elements of Geology. Murchison's Siluria. Dana's Manual of Geology. Penning's Field Geology. Marker's Petrology for Students. Hatch's Textbook of Petrology. Chamberlin and Salisbury : " Geology : Processes and their
Results," Encyclopaedia Britannica. Chambers^ Encyclopaedia.
Stevenson's Principles of Canal and River Engineering. . Etc.
CONTENTS.
INTRODUCTION :— Practical Uses— Water-supply, Building, Road- making, Earthwork — Branches of Geology — Arrangement adopted 1_2
PART I.
DYNAMICAL AND STRUCTURAL GEOLOGY, INTRODUCTORY REMARKS 3
CHAPTER I. CHANGES ON THE EARTH'S SURFACE.
AGENCIES — DENUDATION — EFFECTS— SUB-HEADS .... 4
Section I.— The Work of the Atmosphere.
(i) Air. — Destructive Action — Changes of Temperature— JEolian Action— Transportive Action and Constructive Effects— Loess, Sand-drift, Sand-dunes 4.7
(ii) Rain — Chemical A ction.— Destructive Action — Weathering, Oxidation, Deoxidation, Carbonation, Hydration— Construc- tive Effects— Soil and Subsoil 7_8
(iii) Rain— Mechanical Action.— Destructive Action— Transport of Particles— Earth Pillars— Disintegration— Constructive Effects — Talus, Screes, Rain-wash ...... 8-9
Section II.— Underground Water.
Source — Amount 9-10
(i) Chemical Action.— Processes— Destructive Effects— Subterranean Channels, Caverns, and Swallow-holes — Constructive Effects —Stalactites, Stalagmites, Petrifying Springs . . . 10-11 (ii) Mechanical Action. — Destructive Effects — Landslips — Con- structive Effects . . 11-12
xi
Xll GEOLOGY FOR ENGINEERS.
Section III.— Running Water.
Source —Mechanical Action— Chemical Action ....
(i) Erosion. — Methods of Excavation — Rate of Erosion depends on (1) Nature of channel, (2) Rock formation, (3) Climate— De- velopment of Valleys
(ii) Transportation. — Transporting Power — Materials — Chemical Composition .........
(iii) Deposition. — Alluvium — Occurrence of Deposits: (a) Alluvial fans or cones ; (b) Alluvial plains ; (c) River terraces ; (d) Marine deltas ; (e) Lake deltas ; (/) Bars ....
Section IV. — Glacial Agencies.
(i) Frost and Snow. — Destructive Action — Protective Action .
(ii) Glaciers and Ice-sheets. — Formation — Movement of Glaciers — Work of Glaciers — Erosion— Transportation and Deposition, Moraines, Perched blocks, Roches moutonne'es
Section V. — Marine Action.
Gradation
(i) Oceanic Movements. — Wave - action — Breakers — Under-tow— Erosion — Transportation — Deposition — Ocean Currents
(ii) Oceanic Deposits. — Terrigenous Deposits — Pelagic Deposits — Globigerina Ooze — Red Clay
Section VI. — Organic Action.
(i) Vegetable. — Destructive Action— Constructive Action (ii) Animal. — Destructive Action — Constructive Action
CHAPTER II. CHANGES WITHIN THE EARTH.
Internal Forces. — Heat— Hot Springs — Pressure— Water
Volcanoes. — Volcanic Products — Lava, Rock - fragments, Bombs, Lapilli, Ash, Tuff— Volcanic Vents — Decline of Volcanic Activity— Mud Volcanoes— Mud Springs ....
Crust Movements. — Variation in the Sea-level — Elevation and Sub- sidence of Land — Causes of Secular Movements
Earthquakes. —Cause— Effects
Changes in Rocks. —Cause : Heat, Water, Pressure— Effects : Trans- formation, Plication, Metamorphism, Foliation, Cleavage — Consolidation .........
CONTENTS. Xlll
CHAPTER III.
STRUCTURAL CHARACTERS OF ROCKS. PAQB
INTRODUCTORY REMARKS 33
Section I. — Igneous Rocks.
Definition— Extrusive and Intrusive 33-34
Contemporaneous or Extrusive Rocks. — Lava — Fragments ... 34 Subsequent or Intrusive Rocks. — Necks — Veins and Dykes — Sills —
Laccolites — Bosses ........ 35
Joints. — Nature — Cause — Hexagonal Structure of Ice, Haematite, and
quartz— Columnar Structure of Basalt 36-37
Section II. — Aqueous Rocks.
Changes after Deposition 37
(i) Stratification.— Forms of Bedding— Laminae, Strata, False-bedding — Interposed Strata — Character of Strata — Alternation of
Beds 37-39
(ii) Inclination of Rocks. — Dip and Strike — Outcrop — Outliers and
Inliers — TJnconformability— Overlap 40-42
(iii) Curvature or Flexure. — Plication or Folds — Anticlinal, Syn- clinal, Monoclinal, etc 42-43
(iv) Joints. — Nature, Sandstone, Clay, Limestone — Master Joints . 43-44 (v) Dislocation. — Faults — Throw— Hade — Reversed Faults — Step
and Trough Faults— Shift— Fault-line— Dyke— Vein . . 44-45
Section III. — Altered and Metamorphic Rocks
Nature of Alteration — Causes ........ 45-46
Hydro-metamorphism. — Action — Results 46
Thermo- or Contact Metamorphism. — Action — Effects ... 47 Dynamo- or Regional Metamorphism. — Cleavage — Joints — Foliation
—Relation between Igneous, Aqueous, and Metamorphic Rocks 47-50
PART II.
ROCKS AND MINERALS. INTRODUCTORY REMARKS 51
CHAPTER IV. THE STUDY OF MINERALS.
SOIL, ROCK, AND MINERALS 52
XIV GEOLOGY FOR ENGINEERS.
Section I. — Mineral Chemistry. PAQK
Definitions. — Element — Compound — Compound Radicle — Acid — Base— Salt— Oxide— Terminations — Earth — Metal — Metall oid — Quantivalence — Monad, Dyad, Triad, Tetrad, Organic
Radicles — Anhydride . . 52-54
Constituents of Earth. — Elements — Compounds — Water . . . 54-56 Chemical Characters. — Solubility in Acids — Odour — Taste — Be- haviour (B.B.) 57
Section II. — Mineral Forms.
Mode of Occurrence. — Amorphous — Crystalline — Massive — Amor- phous States — Colloidal— Vitreous . ..... 57-58
Crystal Forms. — The Crystal — Crystallography — Axes — Crystal Systems— Modified Forms — Irregular Grouping of Crystals — Pseudomorphism 58-62
Section III. — Physical Characters.
Cleavage.— Laws of Cleavage— Quality of Cleavage . . . . 62-63 Structure. — Columnar — Lamellar — Granular — Imitative Shapes —
Globular — Reniform — Botryoidal — Mammillary — Filiform —
Acicular— Stalactitic— Drusy 63-65
Fracture. — Form of Surface— Conchoidal, Even, Uneven — Nature of
Surface— Smooth, Splintery, Hackly, Earthy ... 65 Tenacity. — Frangibility, Tough, Brittle, Soft, Friable — Sectility —
Ductility— Malleability— Rigidity, Flexible, Elastic . . 65-66
Hardness.— Scale 66
Touch. — Soapy — Meagre — Harsh . 66
Specific Gravity. — Definitions, Density, Specific gravity ... 66
Translucency — Colour — Streak — Lustre. — Kinds — Intensity . . 66-68
CHAPTER V. ROCK-FORMING MINERALS.
Classification. — Native Elements — Sulphides — Fluorides — Chlorides — Anhydrous Oxides— Hydrous Oxides — Anhydrous Silicates — Hydrous Silicates — Carbonates — Sulphates — Phosphates
— Titanate— Hydrocarbons 69
Abbreviations.— Testing Minerals 69-70
List of Minerals. — Andalusite — Anhydrite— Apatite — Aragonite — Asphalt — Augite - Hornblende group — Barytes — Calcite — Celestine — Copper Pyrites — Dolomite — Epidote— Felspars — Fluor-spar — Galena — Garnet — Glauconite — Graphite — Gyp- sum — Iron Compounds — Kaolin — Leucite — Magnesite — Manganese Compounds— Micas and Talcs — Nepheline — Oliv- ine— Rock Salt — Silica Series— Sphene — Sulphur — Tourmaline —Zeolites— Zinc-blende 70-93
CONTENTS. XV
CHAPTER VI. THE STUDY OF ROCKS.
PAGE
DEFINITION— CLASSIFICATION 94
Section I. — Mode of Origin.
Igneous Rocks. — Plutonic — Volcanic — Hypabyssal .... 94-95
Aqueous Rocks. — Arenaceous — Argillaceous — Calcareous ... 95 Altered and Metamorphic Hocks. — Igneous Rocks — Arenaceous —
Argillaceous — Calcareous . 95
Section II. — Chemical and Mineralogical Composition.
General Terms 96
Igneous Rocks. — Groups — Acid — Intermediate — Basic — Ultra-basic
— Chemical Constituents — Mineral Constituents . . . 96-97
Aqueous Rocks. — Arenaceous — Argillaceous — Calcareous ... 97
Altered and Metamorphic Rocks. — Principal Changes ... 98
Section III. — Structure
General Terms . 98
Igneous Rocks. — Granitic — Porphyritic — Felsitic Matter — Columnar —Spheroidal — Drusy— Banded Structure. Group 1. Dis- tinctly Holocrystalline Rocks : Pegmatitic or Graphic — Fluidal Gneissic — Ophitic — Orbicular. Group 2. Lithoidal Rocks1: Hemicrystalline — Microcrystalline — Scoriaceous — Horny. Group 3. Glassy Rocks : Perlitic — Spherulitic — Lithophyse — Fluidal— Pumiceous and Scoriaceous— Amygdaloidal. Group 4. Volcanic Fragmental Rocks : Agglomerate — Brecciatad . 98-101
Aqueous Rocks. — Group 5. Coarsely Fragmental Rocks : Brecciated — Conglomerate. Group 6. Ordinary Stratified Rocks : Laminated — Oolitic — Pisolitic — Concretionary — Pebbly — Psammitic 101-102
Altered and Metamorphic Rocks. — Group 7. Rocks retaining traces of Bedding : Crystallisation — Cleavage — Fluidal structure. Group 8. Foliated or Schistose Rocks : Foliation — Eye- structure — Mylonitic — Granulitic. Group 9. Amorphous Rocks 102-103
Section IV. — Physical Characters.
Hardness— Fracture— Colour and Lustre— Streak— Feel and Smell . 104-105
XVI GEOLOGY FOR ENGINEERS.
CHAPTER VII. EOCKS.
PAGE
INTRODUCTORY REMARKS 106
Section I,— Igneous Rocks.
Plutonic Rocks.— Granites— Syenites— Porphyry— Granite Porphyry
— Quartz- Porphyry— Felspar- Porphyry— Diorite— Gabbro . 106-109 Volcanic Rocks. — Rhyolite — Trachyte — Andesites — Basalt Rocks . 109-111 Volcanic Fragmental Rocks. — Volcanic Sands — Volcanic Agglomerates
or Coarse Tuffs— Tuffs and Ashes 111-112
Section II. — Aqueous Eocks.
Fragmental or Clastic Rocks. — (i) Arenaceous: Sand — Sandstone Quartzite — Grit— Conglomerate— Greywacke— Arkose— Blue- stone, (ii) Argillaceous: Clay— Shale— Mudstone— Marl . 112-116
Rocks formed by Chemical or Organic Agencies. — (i) Calcareous : Limestone— Dolomite— Rock-Salt— Gypsum, (ii) Siliceous : Flint and Chert, (iii) Phosphatic : Phosphatite— Bone-beds — Coprolitic — Guano, (iv) Carbonaceous : Humus — Peat —Lignite — Coal, (v) Ferruginous: Ironstones . . . 116-122
Section III. — Altered and Metamorphic Eocks.
Classification 122
Altered Rocks. — Quartzite — Lydian-stone — Spotted Shale — Purcel-
lanite— Slate— Crystalline Limestone — Serpentine. . . 123-124
Distinctly Foliated Rocks. — Classification — Gneiss— Mica Schist — Chlorite Schist— Talc Schist— Hornblende Schist— Calc Schist — Mylonite — Granulite — Flaser gneiss — Augen gneiss . , 124-128
Section IV. — Eock Decomposition.
Igneous Rocks— Felspars— Origin of Clays — Decomposition of other Silicates— Origin of Quartzose Sands and Sandstones— Extent of Disintegration 128-132
Sedimentary Strata. — Alteration of Colour — Freestones — Green
rocks— Argillaceous— Deoxidisation— Bleached gravels . . 132-134
CONTENTS. XV11
PAKT III.
HISTORICAL GEOLOGY. PAGE
INTRODUCTION 135
CHAPTER VIII.
PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. Section I. — Classification of Stratified Rocks.
Formations— Periods and Systems 136-138
-Table I.— Sedimentary Strata in Great Britain . . . . 138-143 Table II.— Classified List of the Chief Groups of Strata in North
America .......... 144-147
Table III.— List of the Formations in India 148-149
Table IV. — List of the Sedimentary and Metamorphic Strata of
Australia 150
Table V.— List of the Sedimentary Strata of New Zealand . . 151
Table VI.— List of the Sedimentary Strata of South Africa . . 152
Section II. — Palaeontology.
Definitions— Classification of Animals ...... 152-153
Invertebrata. — Protozoa — Spongida — Coslenterata — Echinodermata— Annulosa or Vermes — Arthropoda or Articulata — Molluscoida
— Mollusca 153-160
Vertebrata.— Fishes— Amphibia— Reptilia— Birds— Mammals. . 160-162
Classification of Plants 162
Phanerogams. — Angiosperms — Gymnosperms 162
Cryptogams. — Pteridophyta— Bryophyta— Thallophyta . . . 162
CHAPTER IX. THE GEOLOGICAL SYSTEMS.
CLASSIFICATION OF STRATA 163
Section I. — Anthropozoic or Quaternary Period.
Introduction 163
Recent or Post-glacial Formations. — Human Relics — Non-glacial
deposits 163-165
Pleistocene or Glacial Formations. — Glacial Deposits — Great Britain — Continental Europe — North America — Asia — Africa — Australasia 165-168
Section II. — Cainozoic or Tertiary Period.
Introduction — Fossils — Great Britain — Continental Europe —
North America — Asia— Australasia ... . 168-171
XV111 GEOLOGY FOR ENGINEERS.
Section III. — Mesozoic or Secondary Period. PAGE Introduction ........... 171
Cretaceous System — Fossils — Great Britain — Continental Europe —
North America— South America — Asia— Africa— Australasia . 172-174
Jurassic System. — Fossils — Great Britain — Continental Europe —
North America — South America— Asia— Africa — Australasia . 174-175
Triassic System. — Types — Fossils — Great Britain — Continental
Europe— World-wide Distribution 175-177
Section IV. — Palaeozoic Period
Introduction 177
Dyas or Permian System. — Fossils— Great Britain — Continental Europe — North America — South America — Asia — Africa — Australasia 177-179
Carboniferous System. — Fossils — Great Britain — Continental Europe
— North America— Asia — Africa — Australia .... 179-181
Devonian System — Fossils — Great Britain — Continental Europe —
North America 181-182
Silurian System (Upper). — Fossils — Great Britain — Continental
Europe — North America — Asia — Australia .... 182-184
Ordovician System (Lower Silurian). — Fossils— Great Britain— Con- tinental Europe — North America — Asia — Australasia . . 184-185
Cambrian System. — Fossils — Great Britain — Continental Europe —
Asia— North America— Australasia 185-186
Section V. — Eozoic Period.
Archaean and Pre-Cambrian Rocks. — Introduction — Fossils — Great Britain — Continental Europe — Asia — North America — South America — Africa— Australasia . . . 186-188
PART IV.
GEOLOGICAL OBSERVATION. INTRODUCTORY REMARKS 189
CHAPTER X. OUTDOOR WORK.
Equipment. — Hammer— Chisel — Bag and Belt — Walking-stick- Compass — Tape-measure — Abney's Level — Pocket-leiis — Note- book . 190-191
CONTENTS. XIX
Section I.— Geological Surveying. PAGE
Preliminary Remarks ......... 191-192
Maps. — Contours— Tracing Boundary Lines 192-194
Geological Sections . . . . . . . « . , .
Section II.— Structural Characters of Rocks.
Introductory Remarks ......... 195
Strata and their Inclination.— Principle of Stratification — Dip and Strike— Measurement of Thickness of Strata — Curvature- Overlap— Unconformity 195-199
Dislocation.— Presence of a Fault— Tracing Faults . . . .199-200
Section III. — Determination of Rocks.
Selection of Specimens.— Position — Rock-specimens. . . . 200
Easily distinguishable Characters.— Structure— Hardness — Streak —
Feeling— Smell—Effervescence— Colour and Lustre— Fracture 200-202 Table VII.— Easily Distinguishable Characters of Rocks . . . 203-206
CHAPTER XI. INDOOR WORK.
Section I. — Further Examination of Rocks.
Physical Characters.— Hardness — Specific Gravity .... 207-209 Chemical Examination. — Detection of Carbonates — Preparation of Material — Summary of determinative Chemical Analysis of a Rock— Fusibility 209-212
Section II.— Isolation of Constituents.
Mechanical Analysis. — Crushing — Washing — Magnetic Separation — Dense Liquids — Use of Acids — Determination of Propor- tions 213-218
Section III. — Determination of Minerals.
Mode of Occurrence — Extraction . 218
External Form. — Preliminary Examination — Measuring Crystal
Angles 218-219
Physical Characters. — Determining Cleavage — Hardness — Determina- tion of Specific Gravity— Fracture 219-224
Chemical Characters. — Taste and Odour — Solubility — Action of
Solvents . 224-226
XX GEOLOGY FOR ENGINEERS.
Section IV. — Blowpipe Examination. PAGE
Apparatus and Reagents. — Apparatus — Reagents — Use of Blowpipe . 226-229 Blowpipe Operations. — Assay — Observation of Flame -col oration — Observation of Fusibility — First Operation (Closed Tube)— Second Operation (Open Tube) — Third Operation (Reactions on Charcoal) — Fourth Operation (Cobalt) — Fifth Operation (with Soda)— Sixth Operation (Borax Bead)— Seventh Opera- tion (Microcosmic Salt)— Eighth Operation— Test for Sulphur 229-236 Table VIII. — Colours of Beads . 233-234
PART V.
PRACTICAL GEOLOGY.
INTRODUCTORY REMARKS . 237
CHAPTER XII.
WATER-SUPPLY.
Section I. — Rainfall and Evaporation.
Rainfall. — Rain — Quantity of Rain — Estimation of Mean Annual
Fall— Maximum and Minimum Fall 238-241
Evaporation and Absorption. — Effect on Water-Supply — Loss —
Evaporation from Surfaces of Water — Dry Weather Flow . 241-243
Section II.— Underground and Surface Waters.
Underground Water. — Water-slope— Saturation and Imbibition- Capacity of Rocks for Water — Water-bearing Strata — Yield of Water— Porosity of Rocks— Bournes— Quality of Water . 243-251
Surface Waters. — Surface of Saturation — Conditions of Flow —
Forests 251-253
Section III. — Springs and Wells.
Springs. — Ordinary Springs — Intermittent Springs — Line of Satura- tion— Fault Springs — Artesian Springs — Springs as a Source of Supply 253-260
Wells.— Shallow Wells— Deep Wells— Causes of Success or Failure
—Wells as a Source of Supply— Quality of Water . . . 260-262
Section IV. — Rivers.
Flow of Water — Quality of Water dependent on Strata — River
Schemes— Flow of Streams and Rivers 263-266
Table IX. —Summer Discharge of Rivers 266
CONTENTS. XXI
Section V. — Lakes and Impounding Reservoirs. PAGE
Comparative Advantages 267
Drainage Areas. — Source of Supply — Size of Catchment Area —
Available Rainfall— Tendula Project 267-270
Lakes. —Advantages 270
Impounding Reservoirs. — Sites — Geological Features . . . 271
CHAPTER XIII.
BUILDING-STONES.
INTRODUCTION 272-273
Section I. — Granites and Granitoid Rocks.
Granites and Syenites. — Constituents of Granites — Qualities — Geological Age of Granite— Syenite— British Granites and Syenites — European Granites — European Syenites — Table X. Analyses 273-279
Granitoid Hocks. — Gneiss — Porphyry — Serpentine — Crystalline
Schists 279-281
Trap Hocks. —Greenstone— Basalt— Lavas— Table XI. Analyses . 281-287
Section II. — Sandstones, Limestones, and Argillaceous Rocks.
Weathering Properties of Sandstones and Limestones . . . 287-289
Sandstones. — Lithological Character — Cambrian and Silurian — Old Red Sandstone — Carboniferous — Permian — Triassic — Jurassic —Cretaceous— Tertiary— Table XII. Analyses of Sandstones . 289-293
Limestones. — Lithological Character — Marbles— Archaean — Silurian — Devonian —Carboniferous — Permian — Jurassic — Cretaceous —Tertiary 293-299
Argillaceous Hocks (Slates, Shales, and Clays). — Lithological Characters — Cambrian— Silurian — Devonian — Carboniferous —Selection of Quarry— Table XIII. Analyses . . . 299-304
CHAPTER XIV. BRICKS AND CLAYS.
Clays.— Kaolin and Felspathic Mud — Loam, Shales, Marls, etc. —
British Clays— Colouring— Qualities— Brick and Tile Clays . 305-309
Fire-clays, Fire-bricks, etc. — Fire-clays — Dinas Bricks — Firestones
—Floating Bricks— Terra-cottas 309-311
Science of Brick -making. — Choice of Clay — Clays — Foreign Bodies — Normal Constituents — Laws of Induration — Contraction — Colours— Table XIV. Analyses . . . . . . 311-317
XX11 GEOLOGY FOR ENGINEERS.
CHAPTER XV. LIMES, CEMENTS, AND PLASTERS.
PAGE
DEFINITION OF CEMENTS AND LIMES— INTERMEDIATE LIMES . . 318-319 Limes. — Combination of Lime with Water — Quicklime — Slaked Lime
—Lime slowly recombines with Carbonic Acid— Classification
of Limes .......... 319-321
Hydraulic Limes. — The Influence of Clayey Matters — Artificial
Admixture of Clayey Matters — Pozzuolana, Trass, etc. —
Influence of Heat on the Silicates 321-322
Limestones. —Subdivisions — Chemical Nature of Stones furnishing
different Sorts of Lime . . . 322-323
Calcination. — Kilns and Fuel — Admixture with Ashes — Results of
Calcination 323-324
Testing Limes and Limestones. — Berthier's Mode of Analysis— The
Condition of the Silica— Treatment with Muriatic Acid . . 325-326 Cements. — Energy — Influence of Calcination — Roman Cement —
Magnesium Cements of America — Portland, Selenitic, and
Sesvage Sludge Cements 326-328
Plasters.— Plaster of Paris— Keene's and Parian Cements . . 328-329 Geological Distribution.— General Laws —Probable Position of Different
Materials— Lias Lime— British Limestones . . . . 329-332
CHAPTER XVI. ROADS AND CANALS. Section I. — Road-making.
Selection of Route. — Value of Geological Knowledge— Determination
of Route— Laying out New Roads 333-334
Road Construction. — Road-cuttings — Side-slopes — Methods of Drain- age—Subsoil Drainage 334-338
Mountain Roads. — Crossing Watersheds— Mountain Passes— Line of
Descent . . . . . . ... . 338-341
Section II. — Road Materials.
Influence of Weather.— Classes of Roads— Water . . . .341-343 Materials for " Wearing" Roads.— Local Circumstances— Suitable
Road Metal 343-344
Materials for Weather-resisting Roads. — Limestone — Gravel . . 345-346 Binding Material. — Choice — On Main Roads — On By-roads . . 346-347
Paving Materials. — Asphalt— Tar-macadam 347-348
Selection of Materials. — Requisites in a Road Stone — Physical Tests
—Durability of Road Stones— Coefficients of Quality . . 348-351
CONTENTS. xxiii
Section III. — Canal-making. PAQE
Level Surface — Natural Feeder —Strata passed through — Leakage- General Remedy 351-353
CHAPTER XVII. RIVERS.
Motion of Water in Rivers. — Motion of Water — Retarding Force — Velocity — Contour — Rotary Motion of Particles — Dynamic Action 354-358
The Transporting Power of Water.— Transport of Material — Erosion — Quantity of Material — Motion of Particles of Matter in Suspension — Effect of Alteration in Dimensions of Channel — Proportion of Deposit carried — Material transported . . 358-362
The Physical Condition of Tidal Rivers. — Origin and Description of Rivers — Agents of Maintenance — Regime of Rivers — Junction of Rivers with the Sea — Source of Detritus in Rivers — Effect of obstructing the Free Flow of the Tide .... 362-365
Bars at the Mouth of Rivers. — Description — Bars composed of Hard Material not affected by the Scour of the Current — Bars due to the Deposit of Alluvial Matter — Bars at the Mouths of Sandy Estuaries — Formation of Sandbars — Channels where Bars are absent — Theories as to the Cause of Bars . . . 365-368
River Improvement Schemes. — Geological Formation of River Bed . 368
Land Reclamation. — Embanking and Warping .... 368-369
CHAPTER XVIII.
COAST EROSION.
INTRODUCTORY REMARKS 370
Section I. — Coast-lines and their Origin.
Outline — Influence of Altitude — Minor Features — Headlands — Inlets
—The Shore— Sea-cliffs 370-373
Section II. — Forces acting on Coast and Sea-bed.
Waves. — Free Waves — Waves of Translation — Forced Waves — Close to the Breaker Line — Breakers — Percolation — Overtaking of One Wave by Another — Direction of Waves — Oblique Waves . 373-378 Tidal Action.— Slow Rise and Fall— Tidal Currents . . . 379-380 Joint Action of Waves and Currents. — Movement of Material . . 380 Wind-formed Currents. — Effect of Wind — Undercurrents — Along- shore Currents . . 380-381
XXIV GEOLOGY FOR ENGINEERS.
Section III. — Coast Erosion and Eeclamation. PAQE
Physical Causes of Denudation. — Subsidence and Upheaval of the Earth's Crust — Physical Causes of Sea Encroachment — River Detritus — Effect of Deposits on the Deep-sea Bed — Relation of Littoral Drift to Eroded Material— Deep-sea Erosion . . 381-384
Protective Works. — Impossibility of Entire Prevention of Erosion — Effect of Protective Works on Adjoining Coast-line — National Aid in Coast Protection — Effect of Pier Works and other Artificial Projections 384-387
Littoral Drift. — Effects of Coast Contour and River Estuaries —
Effects of Tide and Wind 387-388
Sea Watts and Groynes.— Se* Walls— Groynes .... 388-390
CHAPTER XIX. USES OF MINERALS.
Distribution of Valuable Minerals and Rocks. — Coal — Iron — Gold —
Silver— Platinum— Mercury— Tin— Copper .... 391-393
Other Useful Minerals. — Barytes — Anhydrite — Gypsum — Asbestos —
Mica 393-394
Mineral Pigments. — Ochre — Bole — Reddle — Umber — Whiting —
Ultramarine— Metallic Pigments— Table .... 394-396
INDEX . 397-423
LIST OF ILLUSTRATIONS.
PIG. PAGE
1. Millstone grit, Yorkshire 6 6
2. Rocks passing up into soil 19 8
3. Section of ossiferous cavern with stalactites and stalagmites 19 . 11
4. Fan at Tigar in Nubra at Ladakh 6 16
5. The Mer de Glace 6 18
6. Diagram of crag and tail 4 .19
7. Action of the sea on the rocks of the coast 19 21
8. Volcanic dykes 19 35
9. Columnar structure of basalt 19 ....... 37
10. Jointed structure of granite 19 37
11. False-bedding9 38
12. Lenticular, interposed, and divided beds 6 39
13. Exchange or alternation of beds 6 .39
14. Section of outlier 6 - . 40
15. Map of outlier 6 . . 40
16. Mapofinlier6 ... 40
17. Section of inlier 6 40
18. Unconformity of stratification 6 41
19. Diagram of overlap 6 ......... 41
20. Anticlinal dip 6 42
21. Synclinal dip 6 42
22. Breadth and throw of a fault 9 44
23. Dislocation of strata 6 44
24. Dislocation of vein 6 45
25. Reversed fault 6 45
26. Showing that cleavage does not pass through a bed of sandstone 6 . 48
27. Parallel cleavage in contorted strata of North Devon 6 . . . 48
28. Ideal section 6 50
29. Cubic system 13 . . . . • 59
30. Tetragonal system 39 60
31. Rhombic system 13 60
32. Oblique system *>• 13 61
33. Doubly oblique system 13 ........ 61
XXVI GEOLOGY FOR ENGINEERS.
F*G- PAGE
34. Hexagonal system 13 ......... 61
35. Imitative shapes »» 64
36. Nummulites 6 153
37. Monograptus (MurcMson) . . 154
38. Diplograptus 17 154
39. Didymograptus 17 154
40. Rastrites (Lyell) . . . .154
41. Lithostrotion 3 154
42. Calceola 3 154
43. Madrepora3 155
44. Favosites ( Murchison) . . . . . . . . .155
45. Heliolites (Dana) 155
46. Syringopora (Dana) 155
47. Pentacrinus 6 155
48. Encrinus liliiformis 3 . . . . . . . . .155
49. Cypris15 • ., ' . . . .156
50. Estheria3 156
51. Eurypterus 17 ...''. . .156
52. Olenellus3 ..'..'. . .157
53. Paradoxides (Murchison) . .157
54. Fenestella15 157
55. Spirifer15 " 157
56. Rhynchonella 15 . . . ^. . . . " , . . . . 158
57. Productus 15 . . . . ' ' .• . . ' . . . .158
58. Terebratula6 .......... 158
59. Gryphsea3 . 158
60. Cyrena 3 . .... . . . ^ • . . .158
61. Hippurites 3 158
62. Gasteropods : (a) Bellerophon ; (b) Limnaea ; (c) Planorbis (Lyell) ;
(d) Paludina 3 159
63. Nautilus 6 '...,. 159
64. Goniatites (Lyell) . . . . v . . . . .159
65. Ceratites3 . . . . . . . . , . .159
66. Ammonites 6 159
67. Turrilites3 . . ' . • 159
68. Scaphites3 ...".. . 159
69. Orthoceras3 . . v . ., . ... . . 160
70. Belemnites 15 .... . . .... . .160
71. Hamites (Geikie) 160
72. Measurement of dip ls , \ 197
73. Calculating thickness of strata 9 ....... 198
74. Thoulet's washing apparatus 15 . . . . . . .214
75. Spring at outcrop of permeable stratum w 254
76. Hollow collecting water ^ 254
77. Spring arising from water falling on outcrop 23 254
78. Syphon action 23 255
LIST OP ILLUSTRATIONS. XXV11
FIG.
79. Water at outcrop of permeable between two impermeable beds * . 256
80. Inclined line of saturation * ....... 256
81. Inclined line of saturation25 ....... 257
82. Origin of two kinds of springs * . . ..... 257
83. Spring in valley caused by fault25 ...... 258
84. Spring on hill caused by fault ^ ....... 258
85. Spring thrown out by a dyke w ....... 259
86. Water held down in porous bed by superimposed impervious
stratum23 .......... 259
87. Natural fissure giving rise to artesian spring w 259
88. Surface of saturation near a river ^ ...... 263
89. Road-cuttings in mountain pass 32 ...... 340
90. Road-cutting in mountain pass ^ . . . . . . . 341
91. Oscillation of particles of water41 ....... 374
92. Action of oblique waves 41 ........ 378
93. Erosion by parallel waves 41 . ...... 379
94. Joint action of waves and currents 41 ...... 380
GEOLOGY FOR ENGINEERS.
INTRODUCTION.
GEOLOGY is the science which investigates the history of the earth. It treats of the nature and formation of the rocks which form the solid framework of the globe ; of the agents which produce changes in these rocks ; and of the history of the past life, whose remains (fossils) are buried in them.2
Practical Uses. — The advantages to engineers of a knowledge of this science will be palpable to all who study their profession, and especially to those employed abroad, who often must win from Nature the materials with which they may eventually defy her destructive efforts. The following are some of the practical uses of a knowledge of geology : — l
Water-supply, etc. — It explains the natural drainage of a district, both surface and subterranean ; and it shows where artesian wells are possible, as also where fissures and faults exist.
Building. — It affords indispensable information as to (1) the composition of various rocks fit for particular uses, e.g. for building -stone, for bricks, for mortar and cement, for tiles and slates : the way in which the rock has been affected by the weather, where exposed in cliffs and quarries, affords a valuable guide as to its durability ; (2) the areas covered by rocks yielding these materials, and their relative position among other strata, and how best worked.
Road-making. — It is of great importance in guiding the engineer (1) as to the choice of a line of road, so as to ensure its proper drainage, and prevent slipping: this will depend on the nature and succession of the strata and their dip ; (2) as to road- metal : what rocks are obtainable, what rocks are preferable, and what rocks are unsuitable.
Earthwork. — To the engineer making tunnels, cuttings, and
1
, <; G?QLOGY FOR ENGINEERS.
^, .foundations for bridges, cutting canals and docks, it is most necessary that he should know (1) the character of the rocks met with, especially whether hard or soft, permeable or not, to water ; (2) the succession of the strata in the district and their thickness; (3) the dip of the strata, and the direction of the drainage.2
The practical value of geology to the engineer is therefore to enable him to ascertain facts with regard to the present state of the earth's crust and to deduce from those facts what is likely to occur in the future, whereas the ordinary geological student is more often^concerned with what occurred jnjbhe^jsast.1
Branches of CreolCgy^^Th'e^cnlef branches of geology with which the engineer is concerned are : —
1. Dynamical Geology, relating to the causes of change in the earth's crust.
2. Geotectonic or Structural Geology, relating to the structure of rock-masses.
3. Petrological Geology, relating to the origin, occurrence, and structure of the constituents of the earth's crust.
4. Historical Geology, relating to the chronological order of strata and the succession of forms of life.
The arrangement adopted in this book will, it is thought, be the most useful to the engineer. Part I. includes the causes which tend to produce change (Dynamical Geology) and the structural features of rock-masses induced thereby (Structural Geology). In Part II. the characters of the Rocks and Minerals, which form the constituents of the earth's crust, are discussed and descriptions are given of the most important kinds. The Geological Systems and the traces of life contained in them are then described in Part III., and the remainder of the book is devoted to the subjects of Geological Observation (Part IV.) and Practical Geology (Part V.).1
PART I.
DYNAMICAL AND STRUCTUEAL GEOLOGY.
Dynamical Geology is the study of the agencies that have produced geological changes, their laws and modes of action.
The ultimate source of all geological energy both inside the earth and on its surface is, so far as we know at present, the sun. .
It is convenient to consider separately (i) changes on the earth's surface, sometimes called epigene or surface action, due principally to the movement of air and water actuated by the heat of the sun, and (ii) changes within the earth, sometimes known as hypogene or plutonic action, due to original internal heat.
Structural Geology. — The study of the structural characters of rocks, i.e. those of the large parts or whole of a rock-mass, is variously termed structural geology, architectural geology, and tectonic or geotectonic geology.1
[PT. I. CH. I.
CHAPTER I.
CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION.
THE agencies which effect change on the surface of the earth are air, water, and life.
For convenience, their action is considered separately ; but it is necessary to remember that the work of these agencies is so intimately connected that it is often impossible to say that the effects produced are due to any one of them.
The principal change effected by these agencies is termed denudation,1 or the process by which the surface of the ground is broken up, and its ruins carried away, so as to lay bare new surfaces.
The effects on the earth's surface of these various agencies, or agents of denudation, are in part destructive, in part transportive, and in part actually constructive.2
The work of the different agencies can best be considered under the following sub-heads, each being dealt with separately as regards their destructive, transportive, and constructive action : —
1. The work of the atmosphere, or seolian action.
2. The work of underground water.
3. The work of brooks and rivers.
4. The work of frost and ice, or glacial action.
5. The work of the sea, or marine action.
6. The work of plants and animals, or organic action.
Of these, 1, 3, 4, and 6 are said to be sub-aerial, the action taking place on the actual surface of the earth.
Section I. — The Work of the Atmosphere, (i) AIR.
Destructive Action. — Still, dry air, in localities where the changes of temperature are not great, has probably very little effect on rocks and minerals.1
SECT. I.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 5
The gases of the atmosphere (oxygen, nitrogen, and carbonic acid), after they have been taken up by rain-water, exert a wasting or degrading effect upon all rock surfaces 3 (see Rain).
Lightning. — In sandy strata there are occasionally found glassy tubes of variable lengths called fulgurites. These are found where beds of sand have been struck by lightning. They consist of hollow vitrified tubes, descending vertically into the ground, which in some instances have been traced to a depth of 30 feet, and varying in thickness from a quill to J or J inch in diameter. They are very brittle, rough, and angular, and consist of the grains of sand fused together. A considerable number have been found in the dunes near Drigg in Cumberland, and at Pillau near Koenigsberg.4
Changes of temperature in the air cause the rocks to split to pieces. Many extreme and striking instances of this are recorded by travellers and explorers, both in hot and cold countries. Heat causes rocks, as well as other things, to expand, and cold causes them to contract ; and as it is the outside which experiences the greatest changes, it is very apt to crack and split off from the inner portion.
The bare, splintered crags which form the summits of many of the almost inaccessible Alpine peaks have been formed in this way. During the day they become warmed, and the intense cold which follows the sunset causes rapid external contraction and fracture. Their bases are often found to be buried in the fragments chipped off. The foregoing action takes place when the rocks are quite dry ; but when they contain moisture, as they nearly always do, the disintegrating action caused by the expansion of the freezing water is still more marked.5
In the Sahara and other desert regions where the daily range of the thermometer is excessive, the alternate expansion and contraction of the surface rocks is so great as to break them into rugged sheets and finally to shiver them into the finest fragments.3 Wind. — The agency of the wind as a denuding power is easily underestimated, though the amount of dust deposited from the atmosphere under ordinary circumstances demonstrates that much matter is carried by the air from a higher to a lower level. The modern invention of the sand-blast, by means of which glass, granite, and other substances are easily etched, illustrates experimentally the way in which wind, blowing in prevalent directions, abrades rocks. And when we remark that the contours of the sandhills of Holland are exactly the contours of mountain chains, it is quite possible that the outlines of mountains are in the main to be attributed to the agency of the wind.6 ^Eolian action is admirably seen in the pinnacles and crags on
6
GEOLOGY FOR ENGINEERS.
[FT. I. CH. I.
the top of Kinder Scout, a tableland of lower carboniferous rocks, on which pillars of sandstone are left, which often stand up in the shape of gigantic clubs or mushrooms.6 Fig. 1 is an instance of this action. Similar forms are very common in granite. Rocks weathered in this way are often mistaken for " Druidical remains."7
This destructive action of the wind results in the gradual lowering of the land level and the production of sandy wastes. The rock-erosion by seolian action often results in the under- mining of cliffs and the downfall of rock-masses.1 Wind also aids the sea and other large bodies of water in the work of denudation by causing waves and unusually high tides 7 (see Section V., pp. 20-21).
FIG. 1.— Millstone grit, Yorkshire.
Transportive Action and Constructive Effects. — Wind also acts as a transporting agent ; sand and dust, and any loose matters produced by the weathering of rocks, are swept by it into running water or the sea. But perhaps the most important work it does in this way is by transporting the light ashes thrown up by volcanoes ; these are carried by it to vast distances ; if they fall on the land, they are ready to be swept further on by rain and rivers ; or they may fall directly into the sea : in either case they furnish materials for subaqueous strata.7
In dry countries, such as large parts of Central Asia, a fine yellow dust often shrouds the sun and obscures the landscape. This dust settles everywhere, and after many years a deposit of considerable thickness accumulates. In this manner some of the ancient cities of the world, such as Babylon and Nineveh, have
SECT. I.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 7
been gradually covered over with this fine dust, which is rendered compact by the growth of weeds among the ruined houses and walls.1
Loess is a yellowish clay spread over the central parts of the Old World from Germany to China, the formation of which has been ascribed to the agency of the wind. In China it occasionally attains a thickness of from 1500 to 2000 feet.3
Sand-drift is sand driven and accumulated by the wind. Their grains are usually more rounded than the grains of sand accumu- lated under water, being subjected to more trituration than the latter. Moving sands are, at the present time, altering the contour of the land in many places. They cover extensive districts in the interior of Asia, Africa, and Australia.
Sand dunes are low hills formed entirely of sand on low sandy coasts and in sandy deserts, which sometimes attain the height of 200 to 300 feet. On the coast of the Bay of Biscay they are advancing at the rate of about 60 feet per annum, covering up everything as they go. Dunes are also found on the coasts of Nairn, Cornwall, Wexford, etc. The only method of stopping their advance is by planting sand-loving vegetation (see Section VI., p. 24).
(ii) RAIN — CHEMICAL ACTION.
Rain acts both chemically and mechanically. Its chemical action is largely dependent on the nature of the substances drawn by it from the air as it descends. The air is a mechanical mixture of nitrogen and oxygen, the former of which is very inert and passive, while the latter is very active. There is also present in the air variable quantities of carbonic acid as well as aqueous vapour and compounds of nitrogen and sulphur.1
Destructive Action. — Weathering is a term used to denote the action of air and rain on minerals and rocks. As this action is of considerable importance to the engineer, it is described more fully in Chapter VII., Section IV. ; but it will be as well to refer here very briefly to the processes which tend to produce decom- position.
Oxidation. — In the presence of moisture the oxygen of the air acts on various substances in the rocks, and brings about many changes. Most rocks contain iron, which oxidises very freely — the weathered rock usually acquiring a brown or yellow colora- tion. Oxidation generally involves the disintegration of the rock.
Deoxidisation. — Rain may also have the effect of deoxidising, or reducing from the state of an oxide, iron and other oxides. In its passage through the air and in contact with the soil it
8
GEOLOGY FOR ENGINEERS.
[PT. I. CH. t.
absorbs organic matter which has an affinity for oxygen (see Section VI., Organic Action, p. 24).
Carbonation. — Rain as it falls brings with it some of the carbon dioxide (C02) of the air, and as it sinks through the soil it takes up still more from decaying vegetable matter. This carbon dioxide, combined with water, forms a weak solution of carbonic acid (H2C03) which attacks limestone (CaC03) and dis- solves the resulting calcium bicarbonate (CaO . 2C02). In this manner cavities are formed in limestone (see Section II., pp. 10-11), and deposits of clay with flints are formed from chalk when the latter is dissolved.
Silicates of lime, soda, potash, iron, and manganese are also attacked by rain-water containing carbonic acid, with the result that carbonates of these bases are formed and silica is liberated. The felspars are decomposed in this manner l (see Chapter VII., Section IV.).
In some cases, where limestones contain a large admixture of siliceous matters, a sort of skeleton of the latter remains behind when the bicarbonate of lime is dissolved out, forming what is known as rotten-stone.7
Hydration. — Some anhydrous minerals, when exposed to air containing moisture, become hydrated (absorb water) and may then be more liable to additional change. Anhydrite thus becomes gypsum, its bulk increasing by about 33 per cent. Hydration thus often causes disruption of the rock.1
Constructive Effects. — Formation of soil and subsoil. — These are due to a variety of processes of which, however, the chemical
action of rain is, perhaps, the most important. The rock surface is broken up by the weathering processes referred to above as well as by the action of frost and vegetation. If the ground is level or con- cave, soil is formed in situ (see fig. 2), but, if convex, the disintegrated material is carried down by the rain (see Rain : Mechanical Action) into the hollows, or washed away by streams to be deposited in pools, lakes, or oceans, and eventually form new rocks.1
(iii) RAIN : MECHANICAL ACTION.
Destructive Action. — Transport of particles. — Rain exerts an important mechanical effect as a carrying agent. The loose
FIG. 2.— Rocks passing up into soil.
SECT. I.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 9
decomposed matter is washed off the higher ground, and as it moves it has a considerable erosive effect on the surface passed over. The amount and rapidity of this action do not depend on the annual amount of rain, but on the severity of the downfall. A few heavy rainstorms will carry off an enormous amount of sand and mud to lower levels. Again, the greater the slope of the ground the more rapid is the action of the rain.
Earth pillars. — In districts where conglomerate prevails it often happens that a large block preserves the soil immediately below it from disintegration, while the surrounding ground is washed away, leaving a pillar or column. The same effect is produced in certain valleys of the Alps where the clay is protected in places by large stones, the intervening portions being denuded.
Disintegration. — Besides acting as a carrier of loose materials rain softens many rocks, such as clay, and so makes them yield more easily to the weathering processes. Again, by washing off the soil on higher ground it exposes fresh surfaces to disintegra- tion, and the process of soil manufacture is thus continually renewed.1
Constructive Effects. — Talus. — Besides the formation of soil and subsoil, the mechanical action of rain accumulates material on the slopes below steep cliffs, forming what is called a talus.
Screes are long trails of loose blocks collected on the slopes beneath precipitous mountain sides.
Rain-wash is the name given to accumulations of soil, often mixed with angular fragments of rock, which are washed down into the hollows and often furnish brick-earths.1
Section II. — Underground Water.
Source. — A large portion of the rain which falls on the land sinks into the ground and is lost to sight. The remainder is either dissipated into the air by evaporation or flows off into streamlets, brooks, and rivers, and eventually most of it finds its way into the sea (see Section III., p. 12).
Water gets beneath the surface by obvious processes. Most soils and rocks are more or less porous, and the harder rocks are usually so broken by joints and fissures that water easily pene- trates to a considerable depth. The greatest depth reached may be assumed to be about 6 miles, as the zone of fracture of the rocky crust probably does not extend beyond that depth.
Springs are due to the intervention of impervious strata which hold up the water and enable it to reappear at the surface — see Chapter XII., p. 253, in which both springs and wells are dealt with.1
10 GEOLOGY FOR ENGINEERS. [PT. I. CH. I.
Amount of underground water. This depends on the following : —
(1) Amount of rainfall.
(2) Rate of rainfall. — The heavier the fall the less water sinks into the ground, as the surface soon becomes waterlogged.
(3) Formation of the surface. — The natter the ground, the more water will sink in ; the steeper the slope, the quicker the water runs off.
(4) Texture of the soil.
(5) Texture and structure of the underlying rock. — Stratified rock is usually more favourable for the entrance of water than massive rock.1
(i) CHEMICAL ACTION.
Processes. — The various processes of oxidation, deoxidation, carbonation, and hydration which have been described as set in motion by the action of rain (see Rain : Chemical Action, above), are likewise set in motion by underground water and produce changes, analogous to weathering, which are often intensified by internal heat and pressure.1
Destructive Effects. — The subtraction of soluble mineral matter from rock renders it porous. This subtraction is accomplished by underground water charged with carbonic acid as well as with the products of organic decay. The amount depends on the nature of the rock, the readiness with which it is reached by water, and the properties of the water.
The substitution of certain mineral substances for others extracted from the rock is frequently effected. Thus the car- bonate of lime in shells may be replaced by some other substance such as silica, or buried logs may be petrified or converted into stone by the substitution of mineral for vegetable matter.
Subterranean channels and caverns. — In districts containing rocks which are easily soluble, subterranean channels and caverns are often found. The solution and removal of rock-salt frequently results in local sinkings of the surface of the ground, causing depressions in which pools and lakes are formed. In calcareous districts vertical cavities called swallow-holes or sinks are often formed, and the surface water is thus carried below in such quantities that large tunnels and caverns are dissolved out of the rock.1
Ossiferous caves are so named because in them the remains of various animals, such as bears, hysenas, elephants, etc., are detected, often enveloped by mud or other deposits, and in such cases concealed from ordinary observation. Caverns are far more abundant in limestone rocks than in others ; and hence the
SECT. II.] CHANGES ON THE EARTHS SURFACE, OR EPIGENE ACTION. 11
frequent occurrence of stalactical and stalagmitical matter in
ossiferous caves, which often masks the organic riches contained
beneath it. The conditions of ossiferous caverns vary ; but fig. 3
may serve to illustrate
one kind by no means
relatively uncommon.
Let 1 1 be a section of
a limestone hill in
which there is a cavern,
bb, communicating with
a valley, v, by the
entrance, a. Let d d be
a floor of stalagmite (see
Constructive Effects,
below) covering cavities, ^IG. 3- — Section of limestone cavern.
cc, in which there is
an accumulation up to the stalagmite, dd, of the remains of
animals, intermingled with mud, silt, sands, or gravel, as the case
may be.19
Such caves are of great assistance in the study of historical geology (see Part III.).
Constructive Effects. — Stalactites, or the pendent, icicle-like forms of calcium carbonate and stalagmites, their complement forms which rise erect from the floors of caves and such like, are the most notable instances of deposition (see fig. 3).
Petrifying springs, as they are popularly called, are calcareous springs which incrust vegetable matter with carbonate of lime, giving the plants, etc., the appearance of being converted into stone.
Travertine is a limestone deposited from calcareous waters, chiefly springs. It is usually soft and cellular, and hence is also called calcareous tufa or calc sinter.1
(ii) MECHANICAL ACTION.
Destructive Effects. — When underground water collects into definite streams the channels are enlarged by mechanical erosion as well as by solution.
Landslips are common in volcanic districts. The chief agent, however, is water, which most commonly acts by insinuating itself into minute cracks which are widened and deepened by frost. When the fissure becomes sufficiently deep, on the melting of the ice, a landslip occurs.
Sometimes when the strata are very much inclined and rest on an impermeable bed like clay, the water which percolates down
12 GEOLOGY FOR ENGINEERS. [PT. I. CH. I.
through the more porous rocks above softens the clay, which becomes slippery, and the superincumbent mass slides over it to a lower level.1
Constructive Effects. — The mechanical sediment carried off by underground water may be deposited either below the surface or after the streams emerge from underground.1
Section III. — Running Water.
Source. — A large proportion, of the rain which falls on the earth is carried off at once by a vast natural drainage system which forms a network over the land. Passing rapidly from the higher ground by streamlets, brooks, and streams into rivers which eventually find their way into the sea, the running water carries with it a large amount of material in the shape of mechanical sediment or in solution, the major portion of which is deposited in the lower levels, though some finds its way into the sea.
Brooks and rivers would cease to flow in dry weather but for the fact that they are fed by springs which originate as described in Section II., p. 9, and in greater detail in Chapter XII., p. 263 ; also by mist, dew, and melted snow.1
Mechanical Action. — The work done by running water is chiefly mechanical, and may be subdivided into (1) erosion, (2) trans- portation, and (3) deposition ; but, -as in the case of most geological action, these cannot be separated in nature, although it is con- venient to discuss them separately, for they are interdependent. Erosion is increased and accelerated by the amount of sediment transported, and deposition depends on the rate of transportation as well as on the amount of sediment carried.1
Chemical Action. — The mineral matter carried in solution by running water is derived from rain passing over rocks or from springs. It increases the mechanical action, but is not otherwise of very great importance.1
(i) EROSION.
Methods of Excavation. — The gravel rolled along the bed of a stream serves as a tool to excavate the channel owing to the friction set up between the moving pebbles and the stones of the bed. These pebbles are themselves rounded in the first instance by this friction, and are gradually worn smaller and smaller and ultimately become fine particles or are dissolved.
The matter carried in suspension also has an excavating and erosive effect, the particles brought into contact with the sides and bed of the stream having a considerable wearing action owing
SECT. III.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 13
to the innumerable blows which they strike on the resisting surfaces.1
Rate of Erosion. — Those conditions which are favourable to the most rapid erosion of the channel of a stream are not always the same as those which tend to produce the most rapid abrasion of the surrounding country. The rate of erosion depends on (i) the nature of the channel, (ii) the rock formation, (iii) the climate.
(i) Nature of channel. — The greater the slope the more rapid is the rate of erosion, both in the channel of a stream and in the basin which it drains. A steep channel is, however, not favour- able to weathering, as owing to rapid removal of the water the work of solution is retarded ; but the rapid wearing action induced by the greater slope brings fresh surfaces to undergo the action of weathering.
In places where an eddy occurs and there is a gravelly bottom the circular motion of the gravel excavates pot-holes or depressions in the river bottom.
(ii) Rock formation. — The rate of erosion is dependent on both the structural and petrological characters of the rock (see Chapters III. and VI.). Stratified and jointed rocks, or those possessing cleavage properties like slate, are more easily eroded than massive rocks, and fine-grained, compact rocks resist erosion much better than those which cohere loosely.
Again, if rocks split up into angular fragments, the latter have far more eroding effect than the rounded fragments afforded by conglomerates, etc.
The chemical composition is also a matter of much importance from this point of view. If the rock itself is soluble, it will be easily eroded; but, if the cementing material of the rock is soluble while the harder portions remain undissolved, the rock will be an efficient eroding agent.
If the river bottom is covered with debris, only the upper portion of which is disturbed by the current, the underlying rocks will be protected, but violent floods will sweep the debris away and lay the rock bare and subject it to erosion.
(iii) Climate. — The effects of atmospheric agencies have already been discussed in Section I., pp. 4-9. The most important factor in promoting erosion is rain, and, where conditions are favourable to weathering, the rate of erosion will be more rapid than where but little weathering takes place.1
Development of Valleys. — If the rainfall is sufficient small depressions in the ground soon become watercourses and a gully is started. The latter tends to collect still more drainage, and the water entering at the head lengthens it by cutting back, while the water which flows through it tends to deepen it. The form
14 GEOLOGY FOR ENGINEERS. [PT. I. CH. I.
of valley excavated by rivers is determined in part by the nature of the rocks and in part by the climate. In rainless or arid regions steep- walled canons or ravines, e.g. Indian nullahs, are cut to a great depth across high plateaus ; in rainy regions subaerial denudation leads to the formation of wide valleys of much gentler slopes.
Valleys are also a guide to the nature of the agents which have developed the topography of the land. If a surface is characterised by open valleys which lead into other and lower ones and eventually to the sea or into an inland basin, it is clear that running water has been the principal agent. If, however, the depressions are enclosed or hills and ridges occur in such a way as to be independent of lines of drainage, it is obvious that other agents have been the chief factor in the development of the surface of the land.1
(ii) TRANSPORTATION.
The transportation effected by a stream depends on (i) transporting power of the current, (ii) accessibility of materials, (iii) chemical composition of the water.1
Transporting Power. — This depends on the velocity, and varies as the sixth power of the velocity ; e.g. if the velocity of a stream is doubled, the transporting power is increased 64-fold. The velocity of a stream depends chiefly on its gradient, its volume, and the amount of sediment it moves. "As both gradient and volume increase, so does the velocity; but as the sediment in- creases the velocity diminishes, for the effort of moving sediment absorbs a certain amount of energy which reduces the velocity.1
The velocity of a current is greatest in the centre of a river and least at the borders. The velocity of the particles in contact with the bed is about as much less than the mean velocity as the greatest is greater than the mean. In ordinary cases the least, mean, and greatest velocities may be taken as bearing to each other nearly the proportion of three, four, and five.
The following are the effects in the removal and transport of various materials by currents of given velocities acting on the bed of a river : —
Soft clay requires a velocity of . . 0*25 foot per second
Fine sand „ „ . . 0'50 „ „
Gravel as large as French beans re- quires a velocity of . . . I'OO „ ,,
Gravel of pebbles 1 inch in diameter
requires a velocity of . . . 2*25 ,, „
Larger blocks of rock require a velocity of 6*00 „ „ and
•upwards,4
SECT. III.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 15
Or it may be said that bottom velocities of —
30 feet per minute will not disturb clay with sand and stones.
40 ,, ,, will sweep along coarse sand.
60 „ „ „ „ fine gravel.
120 „ ,, „ „ rounded pebbles.
180 „ ,, „ „ angular stones.38
Materials. — The average specific gravity of the materials varies from two to three times that of water, and consequently, when stones, etc., are carried along by the water, they lose from one-half to one-third of their weight in air and thus large blocks are easily carried along.
Coarse materials such as small stones, gravel, and coarse grains of sand are rolled along the bottoms of streams, but finer particles of matter are held in suspension, although their specific gravity is considerably greater than that of water. If such particles were only acted on by gravity and the onward rush of water, they would infallibly sink to the bottom, but they are maintained in suspension (1) by subordinate upward or rotatory currents which are set in motion by obstacles such as boulders met with by the stream, (2) by different velocities in different parts of the stream which exert different pressure on the sides of the particles in suspension.1
Chemical Composition. — Water chemically impure contains a considerable amount of mineral matter in solution which reduces its transporting capacity below that of pure water.1
(iii) DEPOSITION.
Deposition cannot take place without transportation having previously occurred, and is due to the transporting power being rendered deficient. The latter, we have already seen, is chiefly influenced by velocity, hence deposition takes place when the velocity of a stream is checked. For, a certain load of sediment is carried by a stream with a certain velocity, but if the latter is checked or reduced by any cause, the stream becomes overloaded and a portion of its burden is deposited. The sediment thus deposited is called alluvium.1
Occurrence of Deposits. — Deposits usually occur under the following conditions : —
(a) Where the gradient is suddenly decreased, alluvial fans or cones are formed, e.g. at the base of mountain slopes where the gradient changes suddenly, and at various points in the course of every stream where slight changes in gradient occur suddenly and cause a check to the stream (see fig. 4).1
16
GEOLOGY FOR ENGINEERS.
[PT. I. CH. I.
(b) Where the gradient is gradually reduced, deposits will form gradually, covering the flood plains of streams and forming alluvial plains. The continual deposition sometimes has the effect of raising the river bed above the surrounding country.
(c) In these alluvial plains or flats, owing to the gentle current, there is a tendency to meander, and both deposition and erosion take place at the same time, alluvium being deposited on the concave side of each bend, while the bank is undercut on the convex side, the sinuosities being thereby gradually increased.
Sudden floods, however, will often form short cuts, eliminating the bends, and they will also carry away some of the alluvium
FIG. 4. — Fail at Tigar in Nubra, at Ladakh.
previously deposited, making the bed deeper, and leaving part of the old bed high and dry. River terraces are formed in this way.
(d) Where rivers and streams reach the sea and the tides are low, deltas occur which spread out to sea, often to some distance. Strong tides, however, prevent the formation of deltas, and coast- wise currents have the same effect.
Deltas are similar to alluvial fans, but, being formed in deeper water, their front slope is steeper than that of fans or cones. The thickness of deposit in some deltas is enormous. At Calcutta the alluvial matter is about 500 feet thick, and at the mouth of the Mississippi it is still thicker.
(e) Similar action occurs in lakes, which get gradually filled up with alluvium, the delta gradually extending over the whole lake.
SECT. IV.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 17
Rivers also give rise to lakes, either by obstructing their tributaries by deposition at the junction of the latter and thereby damming them up, or, when the tributaries contribute more sediment than the main stream can carry, the latter drops part of its load and forms a bar which dams up the main stream and forms a lake.
(/) Bars are formed at the mouths of tidal rivers by the deposition of alluvium, due to the oscillation between the river and sea water.1
Section IV. — Glacial Agencies.
(i) FROST AND SNOW.
Destructive Action. — Frost assists weathering (see Section I., p. 7) and accelerates landslips (see Section II., p. 11). It acts with great intensity at high levels and in high latitudes, but even in temperate regions its action is very marked and productive of great disin- tegration of rocks. Indeed, in the production of the weathered crusts of rocks frost is hardly less active than rain. It is in the Arctic and mountainous regions, however, that its action is most conspicuous. The rocks under its influence are ruptured and shattered to such a degree that frequently the parent masses become buried under shivered heaps of their own debris.1
Frost will also split open stone full of "quarry-sap" if they are brought to the surface in winter, and advantage is taken of this circumstance by some stone- workers.6
Snow, in the shape of avalanches, sweeps away rocks and trees on steep hillsides and often causes floods by temporarily blocking up valleys.1
Protective Action. — Snow protects the surface of the ground from the action of frost.1
(ii) GLACIERS AND ICE-SHEETS.
Formation. — At different points on the earth's surface there is a certain line, called the snow-line, above which more snow falls than melts. The height of this snow-line varies from about 18,000 feet in the 'equatorial regions to the sea-level in the Arctic and Antarctic regions.
Above the snow-line there is a continual process of accumulation of snow, which presses downwards and converts the lower portion of the accumulated mass into ice, forming ice-sheets. The continual pressure from above gradually forces the ice to escape downwards by any available outlets. On the steeper slopes great masses of snow break away in the form of avalanches, and on the gentler
2
18
GEOLOGY FOR ENGINEERS.
[FT. I. CH. I.
slopes glaciers which are, in effect, rivers of ice, are formed. These are usually found on or just below the snow-line in temperate climates, but in the higher altitudes the ice-sheets cover the land and break off at the edge of the sea and the portions thus detached form icebergs.1
The most favourable conditions for the formation of a glacier
FIG. 5.— The Mer de Glace.
are that the valley should ascend up to, or nearly up to, the snow-line, and should have, as indeed most mountain valleys have, a great semicircular recess at its head (cirque) and above it, a great snowfield. The snow and ice are then forced down the slopes of the cirque and pushed down the valley. The mass of ice and snow which fills the cirque and covers the ground
SECT. IV.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 19
round about it is called ihefirn or neve. It forms the gathering- ground or birthplace of the glacier.5
Movement of Glaciers. — There are two theories to explain the manner in which glaciers move. According to the first or plastic theory, the ice flows like a thick liquid ; according to the second or regelation theory, the glacier progresses by cracking, slipping, and again freezing. The motion resembles that of a river, quicker in the middle than at the sides and bottom.
The Her de Glace (fig. 5) moves as much as 34 inches a day in the summer.
Crevasses are large cracks which are caused by the strains set up by the movement of the glacier. They extend across the glacier in curves which are convex towards its source, and are often very deep.1
Work of Glaciers. — The work done by glaciers is similar to that accomplished by running water, and includes erosion, transportation, and deposition.1
Erosion. — The bottom of a glacier is usually charged with rock debris, part of which was embedded in the snow as it fell originally, and part collected by the glacier as it moves. This rock debris serves as a rasp to scour out and erode the bed of the glacier.1
Transportation and Deposition. — At the end of the glacier, where the ice melts more quickly than it is carried down, a mass of debris collects which is called the terminal moraine. The debris collected along the margin of the glacier is called lateral moraine, and when two glaciers meet, their adjacent lateral moraines form a medial moraine (see fig. 5).
Rocks subjected to glaciation are distinguished by scratches all in one direction, where they have scraped along the bottom. Erratic blocks are large stones carried down to lower levels by the ice, and are called perched blocks when they are left in precarious situations.
FIG. 6. — Diagram of crag and tail.
Rounded masses of glaciated rock are sometimes called roches moutonnees from their resemblance to reclining sheep,1 and while always presenting a continuous slope in the direction from which
20 GEOLOGY FOR ENGINEERS. [PT. I. CH. I.
the ice travels, they often retain their scraggy edges at the further end, under the lee of which a certain amount of debris finds shelter and forms a short tail. This form of structure is known as " crag and tail," and serves to indicate the direction of the ice movement on old " glaciated surfaces " 4 (see fig. 6).
Section V. — Marine Action.
Those portions of the earth's crust which are covered by seas are affected by the same three processes as the actual land surfaces, viz. : —
(1) Crust movements or diastrophism.
(2) Volcanic action or vulcanism.
(3) Gradation.
Of these the first two processes are discussed in Chapter II.1 Gradation. — On land degradation predominates and aggrada- tion is less important, but in the sea aggradation is far more important than degradation. The degrading or denuding action of the sea is termed marine denudation to distinguish it from subaerial denudation, and though the sudden destruction caused by the sea often appears very great, it is in reality of far less geological importance than the gradual action of subaerial denud- ing agents.
The gradational processes at work in the sea are greatest near its shores. These processes are effected —
(a) By mechanical means : by the movements of the water, the result being aggradational, except in shallow waters ;
(b) By chemical means: aggradation resulting from precipita- tion and degradation from solution ;
(c) By organic agencies which are chiefly aggradational : in the shape of corals, shells, and carbonaceous matter.
These processes and their results can best be considered under the heads of Oceanic Movements and Oceanic Deposits, both of which are dependent on all three of the above agencies.1
(i) OCEANIC MOVEMENTS.
The geological work effected by the sea is due to movements of the water which are actuated by (1) tides, (2) wind, (3) differ- ences of level due to influences exterior to the earth's surface, (4) volcanic disturbances or other earth movements.
These all tend to produce either (a) waves, or (b) ocean- currents.1
Wave-action. — Waves are caused by (a) tides, (b) wind, (c) volcanic disturbances ; but their action is similar in each case,
SECT. V.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 21
the difference being only as regards their intensity. Tidal waves are, of course, usually increased by wind. When passing through narrow straits the tide becomes a current and may be an effective agent of erosion.
Breakers. — When waves flow in on a shelving beach they gradually change in character : the velocity of the undulation diminishes, the troughs become flatter and the crests higher. At length the crest begins to curl over, and finally it topples over as a breaker upon the shore.
Under-toiv. — The water carried forward by waves recedes along the bottom and forms the " under-tow." When the wave is oblique to the shore, a longshore current is produced, but the under-tow remains at right angles to the coast.1
Erosion. — The action of the sea on a coast (see Chapter XVII.) is chiefly of an auxiliary nature in that its principal work is to communicate and dispose of material brought down from the cliffs, or on the shore, by atmospheric agency, but it has a direct action between high- and low-water levels. The downward effective range of wave action is very limited, and submarine structures are little disturbed at 15 to 25 feet below the surface.
Erosion is effected both by the waves themselves and by the detritus carried by them. The waves, armed with the loose material which falls from above, cut like a saw and will often undercut the cliffs, especially where a hard rock above high-water level overlies ^ a softer rock which is subjected to this saw- like process (see fig. 7). ^ 7<_Action of 3ea on ^ of coast> ^
As the undercutting hard rock . 6> soft rock . c> fallen rock .
continues, large rocks d, sea.
and boulders fall from
above which are soon reduced to smaller dimensions, and in their turn reinforce the waves in their eroding action.
The abrading power of the waves depends not only on the relative hardness of the rocks of which the coast is composed, but also on the position of the beds and on the planes of cleavage and of joints (see Chapter III.).
The power of the waves is often very great. On the Atlantic and North Sea coasts of Britain, breakers in winter will often exert a pressure of three tons per square foot, and blocks exceeding 100 tons in weight have been moved.
22 GEOLOGY FOR ENGINEERS. [PT. I. CH. 1.
The effect of breakers on a cliff is greatly increased by the alternate expansion and contraction of air in the cracks and fissures of the rocks. A partial vacuum is caused in this way and large masses of rock are often displaced — some even above the direct action of the breakers.
It is probable that the disruption of sea-walls in heavy gales is due to the same cause.1
It is easy to see that if the earth-crust remains stationary in any region the land of a country may in time all become cut down foot by foot, by shore erosion, to a common plain-like level, drowned by the waters of the sea. A plain-like expanse theoreti- cally formed in this way has been termed a plain of marine denudation? or base-level of erosion, but the denudation is often due rather to subaerial forces, and the action of the sea is often constructive rather than destructive ; see under Deposition, below.1
Transportation. — The eroded material is carried away by the action of the waves, under-tow, and shore-currents, which keep the sediment in transit and gradually sift it so that the coarsest materials accumulate where there is most agitation, and the finer parts remain in suspension or are deposited in calmer water. Shore currents actuated by prevailing winds or tides cause the shingle to travel along the coast.1
Deposition. — The incoming waves bring material to the shore and the under-tow carries out detritus, hence where the waves break ridges or barriers are formed which may increase until they enclose lagoons, and eventually the latter become filled with sediment. Deposition usually takes place opposite the mouth of a bay, owing to the shore current being checked in the deeper water of the bay.
The eroding action of the waves on a coast-line wears away the land until it is reduced below the level of breaker action, when it becomes covered with sand and other debris, and thus a sub- marine plain is formed protecting the coast-line from further injury.1
Ocean Currents. — Their erosive effect is not of much import- ance, since most ocean currents do not touch bottom. In places, however, where they are forced through narrow and shallow passages they have considerable abrading effect ; e.g. the Gulf Stream issues from the Gulf with a velocity of 4 or 5 miles an hour, and its shallow channel is abraded by the current. The nature of the bottom beneath the current will show the amount of erosive action at work.
The amount of transportation effected by ocean currents is comparatively slight, and the amount of deposition is also small,
SECT. V.] CHANGES ON THE EARTH'S SURFACE, Oft EPlGENE ACTION. 23
as it depends on transportation except in the lee of places where the bottom is eroded by the current.1
(ii) OCEANIC DEPOSITS,
These consist of (a) terrigenous deposits which are chiefly composed of debris from the land, and (b) pelagic deposits which are laid down in deep water and contain little or no land debris.
Terrigenous Deposits. — These are divided into shallow-water deposits up to the 100-fathom line and deep-sea deposits from the 100-fathom line to where terrigenous deposits merge into pelagic deposits. The selection of the 100-fathom line is an arbitrary one, but it is at about this depth that the sea bottom ceases to be affected by waves and currents. Shallow- water deposits are again divided into littoral deposits between high- and low-water mark, and non-littoral deposits between low-water mark and 100 fathoms.
Littoral deposits consist of boulders, gravels, sands, and other coarse materials derived from the land. Their nature is, generally speaking, determined by the nature of the adjoining land and organisms found locally.
Non-littoral deposits are composed of much the same materials as the littoral deposits, but are finer.
Terrigenous deep-sea deposits. — These consist of blue-green or reddish-coloured muds containing small particles of quartz, mica, or glauconite. Volcanic muds are found round the shores of volcanic islands, and coral sand and mud are found round coral islands.1
Pelagic Deposits. — The deep waters of the ocean formerly supposed to be barren have been proved to be rich in life. The deep-sea exploration has yielded many genera previously supposed to be extinct, and many types allied to extinct genera of the secondary strata (see Chapter IX., p. 172).
Deposits like the Chalk are now forming at the bottom of all the deep oceans, chiefly by the accumulation of foraminifera named Globigerina and Orbulina, with a few pteropods which live in the surface waters and sink to the bottom after death to become mixed with sponges, sea-urchins, shells, and crustaceans, which live at great depths.
Mr Murray reports that the deep-sea clays and deposits at a greater depth than 2000 fathoms appear to be always due to the decomposition of ashes and volcanic materials. The red clays owe their colour to oxide of iron; the chocolate-coloured clays are tinged with oxide of manganese, a mineral that abounds in sea- bed regions covered with augitic materials.6
24 GEOLOGY FOR ENGINEERS. [PT. I. CH. I.
Section VI. — Organic Action.
The living organisms of the vegetable and animal kingdoms produce certain effects on inorganic matter which, though com- paratively unimportant, must not be ignored.
(i) VEGETABLE.
Destructive Action. — Trees split rocks mechanically by forcing down their roots into tiny cracks and crevices. The roots of plants and trees open up the subsoil to the action of air and water, and the decay of plants furnishes strong acids which aid the action of water on rocks and minerals. Woods and forests attract rain and so increase the action due to rain and running water.1
Constructive Action. — Plants, by their growth and decay, are yearly adding to the soil at the same time that they protect its surface from the wasting action of rain, frost, and the like. Accumulations of plant -growth form peat-mosses, jungle, cypress and other swamps, and the surface of sand dunes (see Section I., p. 7) is often protected by plants. Coal is but a mass of mineralised vegetation ; and under favourable conditions, and in course of time, submerged peat-mosses, jungle-growths, forest-growths, and drifted rafts would form similarly mineralised deposits.3 All these aid in building up the crust of the earth. Moist wood is slowly converted by decay into a brown substance which has been called humus, and forms the chief part of the organic matter in soils ; 8 the regur or black-cotton soil of India is formed from decayed vegetation ; and bog-iron ore is formed by the action of decayed vegetation on iron.
Besides the carbonaceous or water deposits formed by the growth of plants, siliceous or flinty vegetable accumulations take place in lakes, marshes, and fresh-water estuaries through the growth and decay of microscopic forms (the diatoms) whose tiny frustules constitute beds of earthy matter (microphytal earths) such as the mountain-meal of the Swedes, the edible clay of the Indians, and the polishing slate of Tripoli. Even in the ocean itself the diatoms are busied in forming new and widely extended deposits (see Section V., p. 23).
Other rocks, etc., formed directly from organic matter are graphite, amber, and paraffin. Vegetation often checks erosion by forming a sort of carpet which protects the surface of the land.
Again, when the surface is bare of vegetation crystalline rocks are broken up into their constituent minerals in the process of weathering, but when covered with vegetation they are disin- tegrated into clays, etc.1 (see Chapter VII., Section IV., p. 128).
SECT. VI.] CHANGES ON THE EARTH'S SURFACE, OR EPIGENE ACTION. 25
(ii) ANIMAL.
Destructive Action. — Burrowing animals undermine the ground and expose the subsoil to the action of denuding agents. Dams made by beavers often alter the watercourses.
Marine-boring shells pierce limestone and promote its decay.1 Constructive Action. — Foraminiferal ooze (see Section V., p. 23) is formed from dead foraminifera, and limestone is chiefly formed from animal remains, whilst coral reefs are built by living organisms.1
[FT. I.
CHAPTER II. CHANGES WITHIN THE EARTH.
THE levelling tendency of the external agencies is continually opposed and counteracted by an antagonistic set of internal agencies. These are the volcano, the sudden earthquake, and the slow, long-continued crust movement. All of these are set in motion by certain forces acting within the earth.3
INTERNAL FORCES.
Heat. — An examination of the temperature of the earth's crust at various depths establishes the fact that the temperature below the cool surface increases on descending, and that at great depths there is still existing a vast reservoir of heat. From numerous observations made in mines and artesian wells in France, England, Prussia, Russia, and elsewhere, it is assumed as an approximation, though subject to many variations from the different conducting powers of different rocks, that below a depth of 100 feet — the stratum of variable temperature — the temperature increases 1° F. in 60 feet of depth. If the rate of increase were considered constant there would at 60,000 feet be a temperature of 1000° or that of low red heat. Descending still lower, the temperature, at a very moderate depth compared with the magnitude of the earth, would be found sufficient to retain mineral matter in a state of fusion ; and it is therefore unnecessary to place at a great depth the source of the melted rocks which are still poured out in so many parts of the earth.9
Hot springs which are found all over the earth also bear witness to the internal heat of the earth.1
Pressure. — In cooling, the earth contracts and the outer crust in settling down gets broken, crushed, and contorted. The lateral squeezing of the crust, as it contracts like the rind of a withered apple, generates additional heat.1
Water. — It is well known that in a closed vessel water may be made white hot without being converted into vapour ; and if we
26
CH. II.] CHANGES WITHIN THE EARTH. 27
suppose the water from the sea to penetrate down fissures in the neighbourhood of volcanoes, then, heated beneath the surface by contact with rocks at a high temperature, it would escape by the path where the pressure was least, flashing into steam with explosive energy as the pressure disappeared.6
Water, superheated in this manner, will also have a far more powerful solvent action than when at an ordinary temperature l
VOLCANOES.
A volcano is a hole or fissure in the earth's crust from which various materials, gaseous, liquid, and solid, are at times expelled and scattered round the opening or crater.5
The chief propulsive and explosive agents concerned in volcanic eruptions are generally acknowledged to be superheated waters (steam, etc.) or their component gases.3 These carry with them dust as well as coarser materials, but of themselves leave scarcely any lasting mark.
The permanent records of volcanic action are : —
(1) Volcanic products. — The ejected materials are not only spread out round the volcanic crater, but are often carried to considerable distances.
(2) Volcanic vents. — The vents and fissures through which the materials have been forced to the surface.1
Volcanic Products.— The steam and gases which are the first products of an eruption are followed by fragmentary materials and, after the shower of these has subsided, molten lava wells up from the interior of the volcano.1
Lava consists of molten or half-molten rocky material containing a large quantity of water, which escapes from it in the state of steam, filling the upper portion of . the lava stream with bubbles, and rendering it light and cindery. As it cools it becomes compact in the central and lower portions, and sometimes presents a peculiar columnar appearance, partly, perhaps, due to the development of cracks on cooling, and partly to a kind of rough attempt at crystallisation.5 When solidified it is still lava, and though the name is generally restricted to those volcanic rocks which are more or less cellular,9 it is at times used to denote all the molten rocks of volcanoes. The structure of these rocks is described in Chapter VII., Section L, pp. 109-12.
Coarsely cellular lava or fragments of lava are known as scorice.lQ
Fragmentary materials are rock-fragments, bombs, lapilli and dust. The fragments are torn off the throat of the volcano. The bombs and lapilli are masses and fragments of the more or less
28 GEOLOGY FOR ENGINEERS. [PT. I.
liquid lava, blown off by the ascending current of steam; the larger lumps, revolving in the air, cool on the outside into rounded bombs ; the finer and rapidly cooled fragments fall as angular lapilli.* The still finer particles are known as volcanic ask.1 The finest particles of the exploded lava float in the air in the form of volcanic dust, which spreads out in widely extended clouds around the volcano. This dust is of excessive fineness and may travel for enormous distances.3
Gradually the volcanic ash becomes more or less solidified, when it is called Tufa or Tuff. It is spread out in a more or less stratified manner, at one time on one side of the volcano, and at another time on another side, according to the direction of the wind, and is generally the most abundant product of volcanic action. Volcanic ash is not at all uncommonly met wilh inter- stratified with some of our most ancient aqueous rocks.5
Volcanic Vents. — All the time that the eruption is in progress, the volcano undergoes changes of form, partly from the accumula- tion of ejected materials on its flanks, partly from the building up of new lateral cones upon it. But more important changes are developed at the top of the mountain ; for, as the superheated water rises towards the surface, and flashes into steam in the throat, its explosive force blows out the loose materials of which the cone was composed; and thus the mountain becomes truncated, and its conical upward termination is often replaced by a funnel-shaped pit, which does not always become entirely obliterated by subsequent eruption.
Fissure eruptions. — - After the central cone has become sufficiently massive and consolidated to oppose a resistance which the explosive forces below cannot easily overcome, they oc- casionally find an outlet by producing rents on the mountain- side.6
Decline of Volcanic Activity. — After the solid materials cease to be ejected, and before the eruptive throat of a volcano is hermetically sealed, the existence of various gases may be detected, and the deposition of salts observed. Some of the gases appear to be given off all through an eruption, others chiefly at its close. Among the most frequent acids are sulphuric and hydrochloric. The gases comprise nitrogen, hydrogen, and carbon dioxide.
As the mountain cools and contracts, small cracks appear about its summit and its flanks. These are termed fumaroles, and give vent to steam and various vapours, which deposit brilliantly coloured crystals of salts, that are mostly soluble and are dissolved by rain.
The decline in eruptive power, however, is gradual, and at a
CH. II.] CHANGES WITHIN THE EARTH. 29
lower level on the flanks of mountains new phenomena often appear, and testify to the changed condition of the interior regions.
This is especially seen in the formation of solfataras, which are essentially hot springs wherein the dissolved acids decompose the rock through which the water flows, so that a good deal of mud is brought to the surface ; and as the sulphuretted hydrogen in the water is decomposed, sulphur is deposited in the clay in nodular masses. Such sources of sulphur-supply occur near Naples, near Girgenti in Sicily, in Iceland, and at Kalamaki, near the Isthmus of Corinth.6
Mud Volcanoes and Mud Springs. — Another phase of declining volcanic activity is exhibited in the formation of mud cones, which are common not only in the volcanic regions of Mexico and Peru, but in Iceland and many localities in the south of Europe. They occur also on the Mekran coast, which stretches from Scinde to the mouth of the Persian Gulf, where their situation is remark- able from the circumstance that there are no traces of volcanic action on the coast.0
CRUST MOVEMENTS.
Variation in the Sea-level. — From the statical property of water it is clear that if there be any permanent change of level between the land and the ocean, the solid land must be the part that is moved. An unstable change of sea-level is, however, due to the tidal wave, barometric pressure, and to the force of winds. These are, however, of slight importance.
The sedimentary rocks which constitute the main mass of the land either have been elevated to their present position, or the sea has been lowered. In which latter case the sea, which must have been equally lowered over its whole area, must have been reduced in depth equal in height to some of the highest mountains. But the quantity of water on the earth remains the same ; hence if the sea-level changes it must arise from the formation of hollows in the crust of the earth, the filling up of its deeper parts, or by the contraction of its capacity by the rising of the solid rock.9
Elevation and Subsidence of Land. — Evidences of oscillation of level are met with in the occurrence of sea-beaches now far removed from the action of the sea, sunken rocks, and of submerged forests, and such movements are indicated by accurate measurements referred to some standard of level which has not been disturbed.
Alterations of level, by elevation or depression, which are found
30 GEOLOGY FOR ENGINEERS. [PT. I.
in different parts of the world, are the effects of subterranean movements, and are of two kinds : —
(1) Secular, or movements progressing slowly.
(2) Paroxysmal, taking place suddenly, and which are intimately connected with earthquakes 9 (see Earthquakes}.
Causes of Secular Movements. — The causes of these slow movements may be sometimes local and due to removal in solution of rocks beneath ; e.g. of rock-salt, limestone, gypsum : or of certain constituents of such rocks as granite, basalt, etc., or to chemical change in minerals by addition of water (hydra- tion) or substitution of carbonic acid for silica; e.g. the change of felspars to kaolin, of magnetite or haematite to limonite, of silicates to carbonates, such changes necessitating increase of bulk. The movements are, however, usually widespread, and then almost certainly due to loss of the earth's internal heat by radiation into space. This cooling causes shrinkage of the interior, and this necessitates crumpling of the outer parts or crust, which has become too large for the shrunken core within.2
EARTHQUAKES.
Cause. — Earthquakes are earth waves due to a sudden shock, either —
(a) The cracking of rocks under strain, with production of faults (see Chapter III., p. 44), the throw of which may be very slight ;
(b) The collapse of the roofs of underground caverns ;
(c) The sudden generation of steam or other volcanic vapours owing to water getting access to heated rock ;
(d) The sudden condensation of steam under pressure, owing to access of water through fissures.2
Earthquakes are more frequent near the sea than far from it, and they are common among many of the great mountain ranges of the world.5
Effects. — The geological effect of earthquakes is not so great as might be supposed, in spite of the widespread destruction to life and property which they frequently occasion. They sometimes cause a permanent elevation or depression of the land, as well as landslips and rents of the ground. Indirectly they may produce derangements of lakes, rivers, and springs.1
CHANGES IN ROCKS.
Cause. — The forces — heat, pressure, and water — which set in motion the larger earth movements have also a considerable effect on the actual rocks.
CH. II.] CHANGES WITHIN THE EARTH. 31
Heat. — Not only does the original heat of the globe, as well as the heat due to the transformation of mechanical energy in the crushing and crumpling of rocks, act upon the rocks themselves, but the heat due to chemical changes within the earth's crust must also be taken into account. Rocks expand on fusion and contract on solidification.
Water. — All rocks contain water within their pores, which is known as interstitial water, and the minute cavities in crystals are usually filled with water. This water usually contains other matter in solution, and thus has a powerful chemical effect which is greatly enhanced by heat.
Pressure acts (1) vertically, producing consolidation (see below) ; (2) laterally, producing or tending to produce metamorphism (see below) ; and (3) as a heat producer (see above).1
Effects. — The newest water-formed rocks are similar in appear- ance to deposits which are now being deposited ; but the older strata have often undergone changes which have obliterated some of their original features which were due to deposition, and have imparted characters which sometimes make it difficult or impossible to discover from observation that they were ever deposited in water at all.
Transformation.— These changes are partly the consequence of the slow infiltration of water, which dissolves certain mineral constituents from one place or one rock and deposits them again elsewhere, sometimes as crystalline minerals, but almost always in different mineral combinations; and when a rock is thus altered by the action of water, it may be said to be transformed.
Plication. — Other changes of a more varied and important character result from the action of pressure, when rocks are forced by folding to occupy less space. See Chapter III., Section II., p. 42, as regards plication.1
Metamorphism. — When from the action of pressure the original distinction between minor layers of rock disappears and is replaced by new planes of division, and when the original mineral character of the rock disappears to give rise to a crystalline texture, and to minerals which are never found in the strata, the rocks are said to be metamorphosed. Afterwards it may be seen that these changes go so far, that lavas and granites appear to be formed out of the sands and mud by the action of the heat to which pressure gives rise.6 See Chapter III., Section III., p. 46, as regards metamorphism.
Foliation and cleavage are structures induced by metamorphism (see Chapter III., Section III., pp. 47-49).1
Consolidation. — The hardening process begins soon after a
32 GEOLOGY FOR ENGINEERS. [PT. I.
deposit is formed. The pressure of overlying material squeezes the particles closer together, forces out a portion of the water, and causes a certain amount of consolidation ; and in the cases of some beds of clay and sand, even of considerable antiquity, this is all that has taken place. Generally, however, various substances, such as carbonate of lime, oxide of iron, or silica, are chemically deposited by percolating water among the particles, and cement them together into a solid mass. In some instances when the deposit is very deeply buried, it is influenced by the subterranean heat, and subjected to a process of baking in addition. Thus, under the action of pressure, infiltration, and heat, soft aqueous deposits are converted into hard rocks.5
CH. III. SECT. I.]
CHAPTER III. STRUCTURAL CHARACTERS OF ROCKS.
THE principal structural characters are massive, i.e. the rocks are compact, homogeneous, and have no joints or divisions ; bedded, or stratified ; and foliated, i.e. have division planes imposed by pressure. It will, however, be more convenient to consider the structural characters of rocks according to their mode of origin : viz. Igneous, or generated by heat ; Aqueous, or water-formed ; and Altered and Metamorphic, or those which have undergone change. Most igneous rocks are massive, but some aqueous rocks have this characteristic ; in some aqueous rocks the bedding planes are indistinguishable, while some igneous rocks are bedded ; and both altered and metamorphic rocks are not all foliated.1
Section I. — Igneous Rocks.
Igneous Rocks are generally and evidently crystallised masses, often analogous to igneous or volcanic products, or compounds containing essentially minerals which are not known to be pro- ducible from water, but in several instances are obtainable by artificial heat, or generated in the deep furnaces of which volcanic mountains are the vents ; and the greater number of the crystal- line rocks are unstratified or have no true bedded structure.
Igneous rocks contain no evidences of aqueous origin or mechanical aggregation, and they rarely possess organic remains except when volcanic ashes or mud have entombed the life of the time. They generally abound along mountain chains and groups and form their axis or nucleus.6
Among the igneous rock-masses we can distinguish two main groups — first, those which have been actually emitted at the surface of the earth-crust in the manner of the lavas, ashes, and tuffs of recent volcanoes ; and second, those which did not reach the surface at the time of their formation, but were injected into subterranean cavities and fissures in the earth-crust, and after-
33 3
34 GEOLOGY FOR ENGINEERS. [PT. I. CH. III.
wards cooled and consolidated in that position. The igneous rocks belonging to the first of these groups are classed as Extrusive or Ejected, because they were forced out to the surface ; as Inter- stratified, because their ashes and tuffs are found interbedded with ordinary aqueous deposits ; and as Contemporaneous, because they are necessarily of the same geological age as the strata with which they are associated. The igneous rocks belonging to the second group are classed as Intrusive or Injected, because they were forced into the subterranean cavities and fissures in which they after- wards consolidated ; and as Subsequent, because their date of origin, intrusion, and consolidation must have been subsequent to that of the already consolidated rocks into whose fissures they were intruded.3
CONTEMPORANEOUS OR EXTRUSIVE ROCKS.
These are either massive or crystalline lavas or fragmentary ashes and lapilli, etc. The lavas radiate from the mouth of the crater in sheets, thickest usually near their point of origin, and dying away gradually as they pass outwards from the base of the volcanic pile. The ashes not only occur in thick sheets lapping round the flanks of the mountain itself, but their finer materials are scattered far and wide ; and, where they fall into the waters of lakes and seas, they mix more or less with sedimentary matter, and form what are called tuffs. The throat or neck of the crater, as the volcano becomes extinct, is gradually filled up either with the fragmentary blocks, bombs, and ashy material of the final eruption — forming what is called agglomerate — or becomes plugged up by the cooled material of the final lava flow.3
Lava. — A modern coulee or lava-flow has a scoriaceous upper and under surface, and the vesicles are elongated in the line of flow. This fact enables us roughly to identify an effusive inter- bedded lava-sheet, and to distinguish it from a subsequent sheet of intrusive rock, which is usually more or less crystalline through- out. The vesicles of the ancient lava-flows are often filled up by a solid deposit carried in by infiltrating waters ; the amygdaloids (or almond-like inclusions) formed in this way often yielding agates or zeolites.3
Fragments. — The coarser materials ejected from volcanoes give origin to a volcanic breccia, or, when rounded by water, to a volcanic conglomerate ; the finer lapilli form beds of volcanic ashes. The ashes and tuffs being formed of fragments and deposited in layers are necessarily bedded or stratified, but are called pyro- clastic sediments to distinguish them from the ordinary aqueous deposits.3
SECT. I.] STRUCTURAL CHARACTERS OF ROCKS. 35
SUBSEQUENT OR INTRUSIVE ROCKS.
These are classified according to the form and position of the fissure in which they have consolidated into necks, veins, dykes, sills, laccolites, and bosses.
Necks are the filled-up throats of extinct volcanoes (see Chapter II., p. 28).
Veins and Dykes. — Intrusive veins are the narrow bands and strings of igneous rock which fill up irregular and narrow fissures and cracks. A dyke (fig. 8) is a wall -like mass of igneous rock filling up a more or less vertical fissure. Dykes differ from veins not only in their size, but also in the general parallelism of their sides, while they maintain an almost perfectly straight course for a long e
distance.3 FIG 8. -Volcanic dykes, a, b,
CI-TI A -77 7 beds of volcanic ashes, etc. ;
Sills.— A sill or sheet is a mass c% d> e> f> solid walis or' dyke^
of igneous rock which has made of stone, its way along the bedding plane
between two successive strata, forcing them apart and consolidat- ing in this intermediate position. At first sight a sill has the appearance of a contemporaneous lava-flow, but it can be distin- guished by noting that (1) it bakes and alters the beds both above and below ; (2) its upper and lower layers are rarely scoriaceous ; (3) when followed for some distance, it will be found to cut across the bedding, and to catch up fragments of the underlying and overlying rocks ; and (4) its edges, like those of dykes, frequently present selvages of more glassy material.3
Laccolites. — Sometimes the igneous material of an intrusive sheet has apparently forced up the overlying strata into a vast arch or anticlinal, and consolidated in the intervening space as a dome-like mass of crystalline rock. Such a mass is known as a laccolite or laccolith.3
Bosses. — The largest masses of igneous rock are known as bosses. They are usually composed of granite, and form broad, dome-like, heath-clad mountain areas often many miles across. The margins of each great boss are more or less irregular ; dykes, veins, and strings of granite, porphyry, etc., run out from the main granitic mass into the surrounding sedimentary rocks. These latter are intensely burnt and altered, and fragments and masses of them are often caught up and isolated in the granitic material of the boss and more or less metamorphosed (see Section III., p. 47).3
36 GEOLOGY FOR ENGINEERS. [PT. I. CH. III.
JOINTS.
Nature. — When igneous rocks cool they all contract, and thus fissures which are called joints appear in them. These joints run through the rock in different directions, according to its composition and the conditions under which it cooled ; and sometimes the same rock presents two or three kinds of joints, or it shows no joints at all. In granite the prevalent joints run in straight lines which cross each other at some angle ; and in basalt, phonolite, and some other rocks the joints often form six- sided columns, which may be straight or curved, and vary from an inch or two in diameter up to a width of many feet.6
Cause. — There is no doubt that some joints are a consequence of conditions under which the rock cools, but the forms and directions which they assume have always some predisposing cause, usually pressure or strain. The joints in granite could not be accounted for by cooling alone, unless it were supposed that cooling took place from opposite sides of the mass, so that the shrinkage planes formed on one side have intersected those formed on the other side. And it seems likely that jointing is primarily a consequence of the development of shrinkage planes in the direction of the predominant arrangement in the rock of its principal mineral constituent. Thus more than half of granite consists of orthoclase felspar, and if the majority of the felspar crystals have a prevalent direction, consequent either upon pressure or contraction, then there must have been a tendency for the rock in cooling to behave as though it consisted entirely of felspar, and to divide by joints which correspond more or less with the cleavage planes of orthoclase or with its crystalline faces. And when we bear in mind the circumstance that in granite the minerals have been arranged in at least two directions, it becomes probable that the felspar crystals should have more than one direction, so that a second set of cleavage planes may be produced running through the other minerals associated with the felspar ; and this may be the explanation of the fact that in most granite quarries the joints which correspond with orthoclase cleavage are crossed by others which, at first sight, seem to be inconsistent with it, and correspond better with the angular directions of the crystalline faces. In the same way the other kinds of joints might be regarded as consequences of the influence of the rate of cooling upon the mode of arrangement of the predominant mineral forming the rock.
The hexagonal structure of ice, kcematite, and quartz would seem to be connected with the fact that those substances crystallise in
SECT. II.] STRUCTURAL CHARACTERS OF ROCKS. 37
the hexagonal system, and circumstances have favoured their division into hexagonal prisms.
But the prevalent columnar structure of basalt (fig. 9) is of an altogether different nature. The surface of the floor of the lava- stream cooled uniformly, and therefore contracted, so that the cracks appeared near the surface or base, and penetrated deeper and deeper as the cooling progressed, sometimes leaving an undivided portion in the middle of a thick lava-flow.6
The jointing of granite is generally such that the mass is divided into numerous short prisms with a rectangular base. These, when exposed to the action of the atmosphere, or that of the sea on coasts, frequently present the appearance of some huge ruin (fig. 10).19
FIG. 9. — Columnar structure FIG. 10. — Jointed structure
of basalt. of granite.
Section II. — Aqueous Rocks.
Aqueous rocks are those which have been originally deposited in water. Their particles are usually smooth and rounded ; they contain fossils and are generally stratified, though some aqueous rocks are unstratified and some igneous rocks are stratified (see Chapter VII., Section I., p. 111). They are derived from other rocks.
After deposition various changes occur : —
1. They are consolidated and stratified.
2. The strata become inclined.
3. The strata are bent and sometimes inverted.
4. Joints are formed.
5. Fractures and movements cause dislocation.1
(i) STRATIFICATION.
The sediment carried off by the action of wind and water, as described in Chapter I., is laid down in lake and river bottoms or on the floor of the sea and consolidated into rocks, as described
38 GEOLOGY FOB ENGINEERS. [PT. i. CH. in.
in Chapter II., p. 32, in regular layers, strata, or tabular masses of various thicknesses. Stratified rocks are generally non-crystalline and fossiliferous, and the order of superposition is constant (see Chapter VIII., p. 137). This principle is our chief guide in tracing out geological formations.1
Forms of Bedding. — Laminae are the thinnest separable layers or sheets in the planes of deposition of stratified rocks. They may be parallel or oblique to the general stratification. They are generally found in fine-grained rocks.
The thicker layers of stratified rocks are usually spoken of as beds or strata. Single beds of rock are occasionally found to attain a thickness of 200 feet, but the average thickness is about 5 feet. There may be as many as thirty or forty laminae to the inch.
The lines of stratification must not be confused with those of
FIG. 11.— False-bedding.
lamination or of joints, cleavage, foliation, or flow-structure (see below).
False-bedding (fig. 11), also called Current-bedding, Cross-bedding, or Drift-bedding, is due to changes in the directions of the currents which laid down the deposits, and is characterised by laminae laid at various angles to the plane of the bed. It is a common feature among coarse sandstones, giving them a rough, uneven surface and a tendency to oblique fracture.
In the processes of stratification and consolidation concretions are formed, but as these are of the nature of an internal structure they are described in Chapter VI., p. 102.
Interposed Strata. — While it is true, as will be seen in Chapter VIII., p. 137, that the strata which cover extensive districts follow one another in strictly chronological order, still they are by no means uniform. The different strata frequently thin out in places so that they assume a wedge-shaped or lenticular section,
SECT. II.] STRUCTURAL CHARACTERS OF ROCKS. 39
and it not infrequently happens that, owing to local modifications, strata are interposed locally in various places l (fig. 12).
Character of Strata. — Fine-grained deposits, such as limestone and shale, havje a tendency to be more persistent and to cover larger areas than do conglomerates and sandstones. Groups and series may be composed of strata of every possible variety, but it more generally happens that certain varieties of rock are associated together ; thus fine-grained sandstone occurs with shale, conglomerate with grit, limestone with fine shales, etc.
Moreover, individual beds often are found to vary in composi- tion in different places. Conglomerate may pass into sandstone, sandstone may pass into shale, and shale into limestone.
The stratification, too, may in some places be very regular and in others very irregular, the thickness varying extremely and
Coralline Oolite.
ir^.,.,,,, ,,.
i*
Calcareout Grit.
FIG. 12. — Lenticular, interposed, FIG. 13. — Exchange or alterna-
and divided beds. tion of beds.
some beds dying out whilst others are interposed as above described. Careful observation is essential to enable the engineer to foretell what beds will be met with1 (see Part IV.).
Alternation of Beds. — When sets of strata are in contact —as, for instance, limestone lying upon sandstone, — it often happens that while the limestone above and the sandstone below are un- mixed with other matter, there is a middle class of beds composed of alternate layers of the sandstone and limestone. Thus in fig. 13 let a be the Coralline Oolite of England, and b calcareous sandstone beneath; the middle beds a a", b' b" are alternately oolite and sandstone.
In such a case, therefore, the two strata are said to exchange beds or to be subject to alternation at their junction, and the phenomenon seems to have been occasioned by temporary cessa- tions of the deposit of sandstone allowing the limestone which would normally have been only a cement to the sand to accumulate and form a limestone deposit.6
40
GEOLOGY FOR ENGINEERS.
[FT. I. CH. III.
(ii) INCLINATION OF ROCKS.
Dip and Strike. — Where strata have been tilted from a horizontal position their inclination to the horizon is called the dip. The amount of dip is expressed in degrees and measured by a clinometer; the direction of the dip is measured by a compass.
The line of direction followed by an inclined bed in crossing the country is known as its strike or level line. Strictly speak- ing, the strike is the intersection of the plane of the surface of the inclined bed with a horizontal plane. If a flat piece of cardboard is held in an inclined position in a trough of water, the horizontal
OUTLI ER 1
MASS OF THE FORMATION 1
FIG. 14.— Section of outlier.
FIG. 15.— Map of outlier.
FIG. 16.— Map of an inlier.
FIG. 17. — Section of inlier. A, Chalk ; B, Upper Green-sand.
line of intersection of the surface of the cardboard with the surface of the water answers to the line of strike ; and a drop of water placed on the cardboard, in air, will run down the steepest line upon the card and mark the line of dip. The direction of the strike is indicated by its compass-bearing, and is always at right angles to the direction of the dip. The strike of a bed is usually more or less straight, but if the bed is bent or folded the strike necessarily curves or changes from point to point.3 To find the amount and direction of dip, see Chapter X., p. 196.
Outcrop. — The area occupied by a stratum on the surface of a country is termed its outcrop. The line of outcrop or basset is the line where the bed comes to the surface from beneath an over- lying deposit. The line of outcrop of an inferior bed is the
SECT. II.]
STRUCTURAL CHARACTERS OF ROCKS.
41
denudation line, or limit of the outcrop of the stratum which rests upon it. In level country the outcrop usually runs straight, but every hill and valley, every variation in the texture of the stratum, tends to make its direction variable and sinuous, because outcrop lines are determined by the ways in which the overlying strata are removed by the action of frost, rain, and the sea, so as to uncover the layers beneath. The general direction of outcrop follows the direction of strike, but the details are the consequences of denudation.6
FIG. 18.— Unconformity of stratification.
Outliers and Inliers. — Two modifications of outcrop called " outlier " and " inlier " often occur. An outlier is a portion of a stratum which has become separated from the principal mass by denudation and remains isolated like an island. It is always newer than the formation around it (see figs. 14 and 15).
An inlier is an older deposit which is exposed by the removal of a portion of an overlying stratum, so that it lies within a girdle of the surface rock6 (see figs. 16 and 17).
CONFORMABLE
UNCONFORMABLE
FIG. 19. — Diagram of overlap.
Unconformability. — When there is a break in the succession of strata and the surface of the older strata becomes denuded and the strata disturbed and inclined before the next strata are laid down, the new strata are said to rest unconformably on the old strata (see fig. 18).1
Overlap. — Strata are sometimes conformable in one section and yet when traced to a distance are found to be unconform- able to the deposits on which they rest. This condition is termed overlap or transgression, because the overlying deposit extending
42
GEOLOGY FOR ENGINEERS.
[FT. i. CH. in.
beyond the beds previously deposited, overlaps and covers them up. Overlap occurs whenever the level of land is depressed over a wide area, so as to allow the sea to extend inland and throw down a stratum upon ground where the series had necessarily been interrupted6 (fig. 19).
(iii) CURVATURE OR FLEXURE.
Owing to the action of the forces referred to in Chapter II., strata have frequently been displaced from their horizontal
ENE
FIG. 20.— Anticlinal dip.
position and bent or folded in various directions.1 Dip, no matter how simple it may appear in a single section, is always a part of a fold of the earth's crust.6
Plication or Folds. — When geological folds are broad and gentle they are called Undulations ; when sharp and compressed they are known as Contortions. Sometimes they are even pushed over the vertical, and the strata are bent underneath those which
FIG. 21. — Synclinal dip.
were originally below them, when they are called Over/olds or Inversions?
When strata are inclined in two opposite directions so that the dips converge upward, and a ridge is formed, it is called an Anticlinal or saddle (see fig. 20). When the dips converge downward, the trough so formed is called a Synclinal (Hg. 21). When the dip is in only one direction it is called Monoclinal flexure.
If the beds dip away in all directions from a centre, they are said to have a periclinal or qua-qua-versal dip, and the structure is called a Dome. If they dip everywhere toward a
SECT. II.] STRUCTURAL CHARACTERS OP ROCKS. 43
centre, they have a centroclinal dip, or form a basin.1 Overthrust occurs when the upper or arch limb has been pushed over the lower or trough limb ; underthrust when the lower or trough limb has been pushed under the upper or arch limb.10
(iv) JOINTS.
Nature. — All water-formed rocks, after being upheaved, dry and shrink. The superficial beds in any quarry may be seen to be divided more perfectly and into smaller pieces than the masses, which are deeper seated and moist. This shrinkage is riot merely lateral, but to some extent vertical also, and these shrinkage planes are the beginnings of joints. Afterwards, when the strata became strained and bent during the changes of level in land, these planes became extended and systematised in definite and parallel directions.6
In sedimentary rocks the joints traverse, as a rule, only a single bed or stratum, fresh joints occurring in the strata above and below.3 Some rocks have very numerous, approximate, and closed joints, as shale, some kinds of slate, and laminated sand- stones ; in others, as limestones, the joints are less frequent and more open.
In coarse sandstones the joints are very irregular, so that quarries of this rock produce blocks of all sizes and forms. From this cause coarse sandstone rocks show themselves against or facing the sea, in- precipitous valleys, or on the brow of hills, in rude and romantic grandeur.
In clay vertical joints are numerous, but small and confused, whereas in indurated shale they are of extraordinary length, very straight and parallel, dividing the rock into rhomboidal masses. Rhomboidal joints are frequent and very regular in coal.
In limestone the vertical joints are generally regular, and arranged in two sets, which cross at nearly equal distances, and split the beds into equal-sized cuboidal blocks ; and thus the mountain limestone is found to be divided into vast pillars which range in long perpendicular scars down the mining dales of the north of England.6
Master Joints. — In examining with attention a considerable surface of rock, it will be found that amongst the joints are some more open, regular, and continuous than the others, which occasionally altogether stop the cross joints, themselves ranging uninterruptedly for some hundreds of yards, or even for greater distances. There may be more than one such set of long joints, and, indeed, this is commonly the case ; yet, generally, there is one set more commanding than the others, more regular and
44
GEOLOGY FOR ENGINEERS.
[FT. I. CH. III.
determined in its direction, more completely dividing the strata from top to bottom, even through very great thicknesses and through several alternations of rock.6 These joints are called Master joints or stines, backs, bords, etc.
(v) DISLOCATION.
Faults are the result of vertical movements by which whole masses of strata, either horizontal or inclined, being too rigid to bend under flexure, are dislocated so that on one side of the line of fracture the corresponding rocks are much higher than on the other. This difference of level in places sometimes amounts to hundreds or even thousands of yards. The succession of strata is on each side the same, their thickness and qualities are the same, and it seems impossible to doubt that they were once
FIG. 22.— Breadth and throw of a fault.
FIG. 23. — Dislocation of strata.
connected in continuous planes, and have been forcibly and violently broken asunder.6
The actual plane of fracture and slipping along which the strata have given way is known as the .^Fault-^lane. and the line of outcrop of this plane of fracture upon the surface^of the ground as the Fault-line. That side of the fault-plane upon which the beds have been relatively depressed is known as the downthrow side, and the opposite as the upthrow side.3 The tferow is the perpendicular distance between lihe two portions of any dislocated stratum * (d b' in fig. 22).
Hade. — The plane of separation between the elevated and depressed portions of the strata is sometimes vertical, but generally sloping a little. The direction of inclination of the plane of a fault is termed its hade, and is measured from the vertical (c bf in fig. 22). In this case a peculiar general relation is observed between the inclination of this plane and the effect of the disloca- tion. In fig. 23, for instance, the plane of separation z z slopes
SECT. II.] STRUCTURAL CHARACTERS OF ROCKS. 45
under the depressed and over the elevated portions of the disrupted strata, making the alternate outer angles zzb, z z b' acute.
In several hundred examples of such dislocations which have come under notice an exception to this rule is rarely found. The direction of the hade is almost invariably towards the down- throw. A similar law is found to prevail very generally in the crossing of nearly vertical mineral veins ; for instance, in fig. 24 a a are two portions of a metallic vein dislocated by another vein b b. In this case the relation of the line b b to the lines a a is the same as that of z z to the lines b b' in fig. 23.
The contrary appearances, had they occurred, would have been as represented in fig. 25, and such occur in the mining district of Cornwall ; they are termed upthrow or reversed faults. When faults are parallel to each other, and the throw is always in the same direction, the strata descend like steps, and the faults are
FIG. 24. — Dislocation of vein. FIG. 25. — Reversed fault.
known as Step-faults^ When faults cross each other they produce the phenomena termed Trough-faults or Cross-faults.6
Shift. — The breadth or shift of a fauTt~~is~ the perpendicular distance between the planes perpendicular to the beds at their fractured ends9 (b d in fig. 22).
Fault-line. — The line in which a fault extends is always sinuous, and, owing to displacement, faults always include many pockets in which minerals may accumulate. The line of dislocation is generally distinguished by a fissure which is filled by fragments of the neighbouring rocks or by basalt, and then is called a Dyke, or by various sparry and metallic minerals, and is then called a Mineral vein (see Chapter II.). The faulted surfaces which have been compressed against each other are hardened, striated, and often polished, when they are termed Slickensides.6
Section III. — Altered and Metamorphic Kocks.
Nature of Alteration (see Chapter II., Changes in Rocks). — The newest water-formed rocks are similar in appearance to
46 GEOLOGY FOB ENGINEERS. [PT. I. CH. III.
deposits which are now being laid down ; but the older strata have often undergone changes which have obliterated some of their original features which were due to deposition, and have imparted characters which sometimes make it difficult or impossible to discover from observation that they were ever deposited in water at all. Thus clays have been changed into slates, sandy clays into schists (see Foliation, p. 49), certain sandstones into quartzites, and ordinary limestones into crystalline or statuary marble. Rocks so changed are sometimes included under the generic term Metamorphic, but it is more usual now to class rocks which still retain traces of bedding and other obvious proofs of their originally derivative condition as Altered, and to reserve the term metarnorphic for rocks which have been more highly altered and have acquired a foliated or schistose character (see Foliation, p. 49), as when clay-slate, which is itself an altered rock, has been metamorphosed into a garnetiferous mica schist. The still more highly metamorphosed massive crystalline rocks, such as granitoid gneiss, bedded granite, and felsitic schist or Halleflinta, are also classed as metamorphic.1
Causes. — These changes are due partly to the action of slowly infiltrating water by which rocks became modified in composition, which is known as Hydro-metamorphism ; partly to the action of heat, by which rocks became modified in structure (see Chapter VI., p. 95), which is known as Thermo-metamorphism, or, as the altera- tion effected by heat is restricted to the rocks in contact with the intrusive masses, as Contact Metamorphism ; and partly to the action of crust pressure, by which rocks become modified in structure, which is known as Dynamo-metamorphism, or, as the alteration by pressure is usually widespread, as Regional Metamorphism. 1
HYDRO-METAMORPHISM.
Action. — Water infiltrating through the pores and fissures of rocks, either alone or in combination with various gases, desposits carbonate of lime, silica, or salts of iron in the interstices of the rocks, or dissolves and removes some of the soluble parts of their component minerals.1
Results. — Impure limestone may lose its carbonate of lime and become rotten-stone ; silica may be deposited in the interstices of loose sandstones and form quartzites ; open rock-fissures become filled up by crystallised deposits of quartz, calc-spar, and other minerals, forming what are known as Mineral veins ; * and the metals themselves may be thus carried off and redeposited in faults and fissures in association with quartz and other minerals, forming valuable lodes or metalliferous veins.3
SECT. III.J STRUCTURAL CHARACTERS OF ROCKS. 47
THERMO- OR CONTACT METAMORPHISM.
Action. — Where great masses of igneous material, in a molten or intensely heated state, force their way into fissures in the earth's crust (as in the case of dykes and bosses), they bake, harden, and occasionally even crystallise the rocks into which they are injected.3
Effects. — Earthy and clayey rocks are changed into porcellanite and lydian-stone ; loose sandstones are altered to semi-crystalline quartzites ; limestones into marbles; and, in extreme cases, ordinary detrital sediments become metamorphosed into crystalline and gneissoid rocks. The metamorphic action due to heat is best seen around any granite boss.1
DYNAMO- OR REGIONAL METAMORPHISM.
The irresistible crushing forces generated in the earth-crust by the lateral pressure effect the most startling changes not only in the original texture, but in the original structure of rocks subjected to their influence. Soft clays and shales become crushed and compacted into hard slates, the original bedding becomes obliterated, and the rock now opens in parallel sheets, the surfaces of which have little or no relation to the original layers of sedimentation (see Cleavage).
Finally, where the pressure has been most intense, even the massive igneous rocks have been forced to assume a platey structure (Foliation} splitting into irregular leaves or folia of various degrees of thinness, and their very minerals themselves have been com- pelled to recrystallise in new and different forms.3
Cleavage. — In the case of rock-masses composed of homo- geneous and comparatively soft material, crust-pressure frequently produces the structure called Cleavage.1 This consists in a peculiar fissility of the rocks which are affected by it, parallel to a certain plane, which almost always cuts at a considerable angle the plane or curved surfaces of the stratification. In fig. 26, which represents a mass of rocks in which this definite quality of splitting is developed, BB is the surface (curved in this instance) of one bed of the stratification ; J is on the plane, here supposed vertical of a joint; C is one of the planes of cleavage, cutting the surface of stratification BB in ss. Parallel to this plane C, the mass of rock here represented is cleavable by art, and is often actually cleft by nature into very thin and numerous plates which, when of suitable quality and reduced to proper size, constitute the roofing-slates of our European houses. The edges of these plates may be traced with care on the vertical
48
GEOLOGY FOR ENGINEERS.
[PT. I. CH. III.
FIG. 26. — Showing that cleavage does not pass through a bed of sand- stone^).
surface of the joint J and the sloping surface of the bed B, and are represented in the figure by fine lines.
It will be observed that these lines do not cross the bed marked g. This is supposed to be a hard grit or conglomerate, and such rocks are sometimes only in a slight degree affected by the cleavage which, however, is perfect above and below them in fine- grained and more argillaceous strata. Certain small joints, however, and numer- ous cleavage planes often cross sandstone beds, and then the cleavage and joint planes in those beds are not parallel to the general cleavage, but meet the surfaces of stratification as in fig. 26, at angles more nearly approaching to a right angle. At I the cleavage crosses nodular limestone or ironstone, and in these irregular layers becomes irregular, curved, and confused.
On the surfaces of stratification the cleavage structure is frequently traced in narrow, interrupted hollows and ridges ;
these surfaces have in fact been folded, or plaited, or puckered by the force which occasioned the cleavage ; and the little folds thus occasioned are traceable across shells, trilobites, etc., which are thus more or less distorted in figure.
Stratification and cleavage. — One general relation appears between the stratification and the cleavage — a relation arising from the displacement of the strata by axes of elevation and depression. Parallel to these axes is the "strike" or horizontal line on the surface of the strata ; if this be taken on a great scale and the strike of the cleavage (similarly defined) be compared with it, the direction of each is found to be the same, or nearly so ; in other words, the cleavage edges on the surface of the strata are horizontal lines (ss in fig. 26). The direction, then, of the cleavage in a given district is dependent in a general sense on that of the axes of earth-flexure in that district ; but the inclination of the cleavage has no necessary known relation to that of the strata (fig. 27); beyond this, that the dip of the strata being moderate, that of the cleav- age is usually greater. In a country where the strata are much undulated, the cleavage may be, and mostly is, in parallel planes.6
FIG. 27. — Parallel cleavage in contorted strata of North Devon.
SECT. III.] STRUCTURAL CHARACTERS OF ROCKS. 49
Joints. — In slate districts, the joints, more numerous and more regular than in any other known rock, have almost universally a tendency to intersect one another at acute and obtuse angles, and thus to dissect whole mountains into a multitude of angular solids, with rhomboidal or triangular faces, which strongly im- press upon the beholder the notion of an imperfect crystallisation, produced in these argillaceous rocks since their deposition and consolidation by some agency, such as heat or pressure, capable of partially or wholly obliterating the original marks of stratifica- tion ; but we may with more probability here also appeal to tension in successively different directions as the true cause of these phenomena.6
Foliation. — This term is denned as "a crystalline segregation of certain minerals in a rock, in dominant planes, which may be those of stratification, of joints, of shearing, or of fracture under the strain of flexure " ; 10 but it is more ordinarily used as a synonym for schistosity or the quality of being schistose, a schist being l a rock which has had a parallel or foliated struc- ture secondarily developed in it by shearing, a process generally accompanied by more or less recrystallisation of the constituents in layers parallel to the cleavage. The secondary foliation or schistosity may be, but generally is not, parallel to the bedding.1 Foliation is, in fact, only an intense form of cleavage, or is due to the same cause when the forces producing it are more powerful.6
The dominant and characteristic rocks of areas of regional metamorphism are the foliated rocks or crystalline gneisses and schists. These are normally divided into lens-like layers or folia alternately of different texture or mineralogical composition, and the plane of easiest division between the folia is known as the plane of schistosity? The distinctive feature in foliation is the crystallisation of the mineral flakes which produce the tendency to split into layers along the plane of schistosity which is characteristic of foliated rock.
The schistose rocks are always crumpled and contorted, and commonly occur on the flanks of the older mountain ranges.6
Relation between Igneous, Aqueous, and Metamorphic Rocks. — The central cores of many volcanoes are found to be of granite ; and when this rock cools more rapidly, as at the earth's surface under the pressure of the atmosphere, the minerals no longer form separately, but constitute rock consisting more or less obviously of a felspathic matrix in which crystals may occur. When poured out in a lava stream these rocks are called felstones, and when they assume a looser texture became scoriae or ashes. If now we suppose the rocks over a central granite mass to become
4
50
GEOLOGY FOR ENGINEERS.
[PT. I. CH. III.
fractured through their thickness so as to allow water to penetrate down to the heated mass and form a funnel or vent out of which the heated materials may escape, it is obvious that the central crystalline rocks will throw out lavas and ashes which may build up a volcano. Thus it follows that clay, slate, gneiss, granite, felstone, rhyolite, may all exist simultaneously as different con-
BEDDING
CLEAVAGE
FOLIATION
CLAY
SLATE GNEISS GRANITE
FIG. 28. — Ideal section.
ditions of the same rock, which have been produced in sequence to each other by the pressure which also brings mountains into existence, and changes the outlines of land and water. The ideal section (fig. 28) will illustrate the relations of the several kinds of rocks to each other, and show the order in which the several classes of rocks may succeed each other on the flanks of a mountain range.6
PART II.
ROCKS AND MINERALS.
THE terms Petrology, Petrography, and Lithology are frequently used indiscriminately to denote the science of rocks, but Petrology is more generally used to denote microscopic characters and Lithology to denote macroscopic characters.
The engineer can best study the nature and effects of geological forces after he has acquired some knowledge of the constituents of the earth's crust.1
Minerals are either the uncombined chemical elements in a native state or compounds of these elements formed in accordance with chemical laws.10
Rock is a solid mineral product which is at once of considerable extent and presents a general similarity of characters throughout 10 (see Introductory Remarks, Chapter VI.).
PT. II. CH. IV.
CHAPTER IV. THE STUDY OF MINERALS.
THE first mineral product met with in the examination of the solid portion of the earth is usually a loose soil, beneath which is a firmer material to which the term Rock is applied.
On inspection the soil is found to be a mixture of fragments of substances of different kinds, and in most rocks the unaided eye is able to detect different kinds of matter. In granite, for instance, mere inspection shows us that there are at least three different kinds of matter, which are distinct from each other not only in outward appearance, but in all their manifold properties. It will be found, moreover, that by no amount of mechanical division can any of these three substances be reduced to others having different characters. They are therefore called Minerals. l
The distinguishing characteristics of minerals are : —
1. Chemical composition.
2. Form.
3. Physical characters.1
Section I. — Mineral Chemistry. DEFINITIONS.
It is desirable that the student of geology should possess, at least, an elementary knowledge of chemistry, but to make these notes more complete a few definitions are given.1
Element. — That form of matter which cannot be decomposed by any means known to science.
Compound. — The union of any two elements forms a binary compound, as H20, hydrogen oxide or water; Si02, silicon dioxide or silica. A ternary compound consists of three constituents.
Compound Radicle. — A group of different atoms acting as a single element in a compound and incapable of independent
52
SECT. I.] THE STUDY OF MINERALS 53
existence, as NH4, ammonium, in NH4C1, ammonium chloride.8 The elementary atoms are simple radicles.
Acid. — A compound containing hydrogen the whole or part of which is displaceable by a metal.8 The union of a non-metal with hydrogen, or with hydrogen and oxygen, usually produces an acid, as HC1, hydrochloric acid ; H2S04, sulphuric acid ; H2Si02, silicic acid.2
The most important acids which affect rocks are silicic acid, carbonic acid, and sulphuric acid.1
Base. — A compound body capable of neutralising an acid, either partly or entirely. An alkali is only a base which is very soluble in water.8 The union of a metal with oxygen usually produces a base, as A1203, alumina ; CaO, lime.2
Salt. — A compound derived from an acid by the displacement of its hydrogen by a metal.8 The action of an acid on a base produces a salt, as NaCl, sodium chloride (common salt) ; CaC03, carbonate of lime or calcium carbonate, the principal component of limestone.
Most minerals are salts, by far the greater number which form rocks being silicates of one of the bases, or mixtures of them ; a few are carbonates, sulphates, sulphides, chlorides, etc.2
Oxide. — Any binary compound of oxygen either with an element or with an organic radicle. Monoxide, an oxide containing a single atom of oxygen in combination with a basic radicle ; Sesquioxide, an oxide in which two basic radicles, usually metals, are combined with three atoms of oxygen; Dioxide or Binoxide, an oxide containing two atoms of oxygen to the molecule ; Protoxide, an oxide containing only one atom of oxygen. This term is used in comparison with Peroxide, an oxide having a larger proportion of oxygen than any other oxide of the same series.8
Terminations. — The endings -ous and -ic distinguish between two compounds formed by oxygen with the same element, -ous implying the smaller proportion of oxygen ; e.g. ferrous oxide = protoxide of iron; ferric oxide = peroxide of iron.8 The ending -ate is used for the salt of an acid ; e.g. sulphate = a salt of sulphuric acid, carbonate = a salt of carbonic acid, etc. The ending -ide denotes a compound of an element or radicle with another ; e.g. sulphide (also known as sulphuret) = a compound of sulphur, arsenide = a compound of arsenic.
An earth is an earth-like metallic oxide, as alumina, etc. Alkaline earths are barium hydrate, strontium hydrate, and calcium hydrate.
A metal is an element capable of forming a base by combining with oxygen.8
54 GEOLOGY FOR ENGINEERS. [PT. II. CH. IV.
Metalloid. — This term was formerly used to denote all non- metals, but is now restricted to those which resemble metals, as arsenic and antimony.10
Quantivalence. — The elements are divided into groups ac- cording to their power of combining with or replacing different quantities of hydrogen. Those which are equivalent in combining or displacing power to a single atom of hydrogen are said to be univalent or monad elements, e.g. chlorine, bromine, iodine, fluorine ; those equivalent to two atoms of hydrogen are termed bivalent or dyad elements, e.g. oxygen, sulphur, selenium ; those equivalent to three atoms are termed triad elements, e.g. nitrogen, phosphorus, and arsenic ; those equivalent to four atoms are termed tetrad elements, e.g. carbon and silicon.
Not only can the elements thus be considered as possessing varying quantivalence, but also those groups of elementary atoms which act collectively as elements and to which the name of compound radicle is given (see above).
Thus the atom of hydrogen is a monad simple radicle, the atom of oxygen a dyad simple radicle, whilst the group OH is a monad compound radicle.
Radicles play the same part among carbon compounds, where they are called organic radicles.1
Anhydride is a compound which produces an acid when brought into contact with water.8
CONSTITUENTS OF EARTH.
Elements. — Chemists have classified the constituents of the earth into elementary bodies, or elements, which no analysis has yet been able to further subdivide.6 The most important of these, from a geological point of view, are given in the accompany- ing table, the most abundant of each group being placed first : —
,T , , Atomic nr <. i Atomic
Non-metals. weight Metals.
Oxygen . . . 16'0 Aluminium . . 27'0
Silicon . . .28-4 Calcium . . 40 '0
Carbon . . . 12'0 Magnesium . . 24 '3
Sulphur . . 32-06 Potassium . . 39-11
Hydrogen . v. 1*008 Sodium . . 23-05
Chlorine . . 35 -45 Iron . . .56*0
Phosphorus . . 31'0 Manganese . . 55 '0
Fluorine . . 19'0 Barium. . . 137'0
Lithium . 7 '02
Chromium 52' 1 8
SECT. I.] THE STUDY OP MINERALS. 55
A few elements preponderate very greatly in the earth's crust, notably oxygen. It has been estimated that within 60 miles of the earth's surface the percentages are as follows : —
Oxygen . . . . . . . 50
Silicon . . -. . , '. 25
Aluminium . . . ... . ' , 10
Calcium . . 4J
Magnesium . . . . . 4j
Sodium . . . . •-.'•. 2
Potassium . . . ... 1 £
The remainder . . . . . 3J
1005
Oxygen (0) forms by weight about one-half of the mineral world. It unites with all elements except fluorine and bromine, forming with some acids, with others alkalies, and with others neutral substances.
Silicon (Si), though very abundant in nature, is never found in the free state, but always in combination, either with oxygen alone, as silica (see Compounds, below), or with oxygen and metals forming silicates.10
Carbon (C) is especially remarkable for its uniform presence in organic substances. Free carbon occurs in the form of diamond, graphite, and anthracite. Carbon is capable of combining with oxygen in two proportions, forming the compounds known as carbon monoxide (CO) and carbon dioxide (C02). Carbonates are the salts of carbon dioxide.
Sulphur (S) is remarkable for its abundant occurrence in nature in the uncombined state in many volcanic districts.8 Sulphur and oxygen, though very dissimilar in their physical characters, correspond very closely in the nature of the compounds which they form, and in the properties they exhibit, when both are in the gaseous state.10 It is found as sulphuretted hydrogen in many mineral waters, and very abundantly in com- bination with metals forming sulphides and in combination with oxygen and in metal-forming sulphates.
Chlorine (Cl) is never found in the uncombined state, but is very abundant in the mineral world in the form of chlorides. Chlorates are the salts of chloric acid (HC103).8
Fluorine (Fl) is always found in combination. It does not combine with oxygen, and chiefly occurs combined with calcium, as fluor-spar.
Hydrogen (H) is very abundant in nature, occurring as a constituent of water and in organic compounds.
56 GEOLOGY FOR ENGINEERS. [PT. II. CH. IV.
Phosphorus (P) occurs combined with oxygen, chiefly in calcic phosphate.
The metals chiefly occur in the form of oxides or combined with sulphur.
Compounds.— The main bulk of the crust of the earth is formed of a few predominate compounds.
Silica or silicon-dioxide (Si02) constitutes more than half of the known portion of the crust of the earth. It occurs in abundance as the mineral quartz, and also forms many silicates in combination with metallic bases.1
Alumina or aluminium oxide (A1203) occurs chiefly in union with silica. It is the most abundant of all the earths, and, being a common constituent of the silicate minerals, forms the basis of many rocks and soils. When crystallised it is intensely hard, and is found nearly pure as sapphire, corundum, and emery. In its amorphous form it is a soft white insoluble powder.8
Calcium (Ca) occurs in combination with silica and various silicates as a rock-builder, but it is most abundant in union with C02 as calcite (CaC03) or limestone rock. Combined with sulphuric acid calcium forms gypsum and anhydrite.
Magnesium (Mg), though not as abundant as calcium, is an important constituent of rocks. Talc, steatite, and asbestos are silicates of magnesia (MgO) ; magnesite is a carbonate, and dolomite is formed by a combination of calcium carbonate with magnesium carbonate. Epsom salts is the common name for sulphate of magnesia.
Potassium (K) or sodium (Na), in combination with silica, is found in most silicates in small quantities. Sodium combined with chlorine forms common salt (NaCl).
Iron (Fe) is the principal colouring agent in rocks (see Chapter VI., Section IV.). Its peroxide (ferric oxide) forms large masses, and with the protoxide (ferrous oxide) occurs in most crystalline rocks.
Manganese (Mn) is often found with iron among aqueous rocks.1
The combinations of water (H20) with other substances are generally called Hydrates. When water acts upon a compound body it may either effect a simple solution or enter into chemical combination with it.8
Most bases are capable of combining with water to form hydrates, as exemplified in the slaking of lime.
The hydrate of a metal is defined as a compound formed by the replacement of a part of the hydrogen in water by a metal ; thus potassium hydrate KHO is formed from water H20 by the replace- ment of H by K.8
SECT. II.] THE STUDY OF MINERALS. 57
CHEMICAL CHARACTERS.
The chemical composition of a mineral can only be ascertained by exact analysis, which is beyond the scope of this book. It is, however, given in some cases in the list of minerals in Chapter V. as a guide to some of their properties.
Solubility in Acids. — This test has been very freely applied to minerals, though with results varying according to the strength of the acid, the temperature employed, and the time allowed for the attack. Hydrochloric and sulphuric acids are those most commonly required ; nitric acid may be useful if to hand. Organic acids, such as citric, tartaric, and oxalic acids, may also be used.
Acids are chiefly used in the examination of carbonates.15
See Chapter XL, Section III., for the methods of testing with these reagents.
Odour is not possessed by any minerals in a dry, unchanged state ; but it may be obtained from several by moistening with the breath, by friction, by heat, or by the application of acid. Amongst the most remarkable varieties are the following : — Argillaceous, the odour of moistened clay, obtained from ser- pentine, chlorite, and some allied minerals by breathing upon them. Fetid, the odour of sulphuretted hydrogen, obtained from some varieties of quartz and limestone by friction or a blow with a hammer. Sulphurous odour, obtained by friction from pyrites, and by heat from most of the sulphurets. Horse-radish odour, perceived when the ores of selenium are heated. Alliaceous or garlic odour, obtained by friction from some, and by heat from most, of the arsenical salts and ores.13
Taste is a means of distinguishing many of the soluble minerals. Many decomposed minerals, although they have no sensible taste, adhere more or less strongly to the tongue, and thus affect that organ. The tastes of minerals are thus described : — (1) astringent, having the taste of vitriol ; (2) sweetish-astringent, taste of alum ; (3) saline, taste of common salt; (4) alkaline, taste of soda; (5) cooling, taste of saltpetre; (6) bitter, taste of Epsom salts; (7) sour, taste of sulphuric acid.13
Behaviour before the Blowpipe, e.g. flame-coloration, fusi- bility, reactions, etc., is described in Chapter XL, Section IV.
Section II. — Mineral Forms.
MODE OF OCCURRENCE.
Minerals occur in two conditions : —
(1) Amorphous.— They are without any definite geometrical form ; they break in all directions with equal facility ; when
58 GEOLOGY FOR ENGINEERS. [PT. II. CH. IV.
broken, they exhibit a conchoidal or earthy surface (see Section III., p. 65), and they are equally hard and equally clastic in all directions.
(2) Crystalline. — They have a definite geometrical form ; they possess the property of cleavage (see Section III., p. 62) ; they are neither equally hard nor equally clastic in all directions.
The same mineral may occur in both conditions, and when no indications of crystalline structure are apparent in a mineral aggregate it is said to be massive.12
Amorphous minerals occur in the following states : —
(a) Colloidal, viz. resembling jelly or glue. The substance has no power to crystallise, and, if soluble in water, is held in solution very feebly and is easily precipitated. It is, however, often insoluble in water.
(b) Glassy or vitreous — more common in rocks than in minerals. The glass may consist of several minerals fused into one homo- geneous substance. The same substance is often capable of assuming both the crystalline and glassy state. Glassy bodies occasionally become stony by the formation of minute crystals within them ; the glass is then said to be devitrified.
CRYSTAL FORMS.
The Crystal. — The term crystal is applied to natural and artificial substances which, in solidifying from a state whether of solution or fusion, assume definite polyhedral forms which are constant for the same substance.12
The surfaces of a crystal are called planes or faces • they inter- sect in edges and angles. A crystal edge is the line of intersection of two crystal planes ; the angle which such an edge encloses is called an interfacial angle. By the term crystal angle is meant the solid angle in which three or more crystal faces meet.
Every plane in a crystal has a definite inclination or slope in relation to every other plane, except such as may be parallel to it. These mutual inclinations are quite independent of the size or general form of the crystals, and they are constant for similar planes even in different crystals of the same mineral, as is shown by measurement with the goniometer 39 (see Chapter XL, p. 219).
Crystallography. — To understand the higher branches of this subject, a good deal of mathematical knowledge and skill is required, and the engineer will probably be content to refer to specialists in all difficult cases. A brief description of the various crystal systems may, however, assist him in identifying many of the more important rock-forming minerals.1
SECT. II.] THE STUDY OF MINERALS. 59
Axes. — The planes of all crystals are referred to certain imaginary lines termed "axes," which are supposed to exist within the crystal. These axes cross each other at a certain point within the crystal, and each axis terminates on the surface at similar and opposite angles, or edges, or faces.5 That part of each axis extending from the centre to the surface of a crystal, or, in other words, the axial intercept, is called its parameter.1
The axes may cut each other at right angles or at any other angles. The number and relative situation of the axes, and the ratios of the parameters, together constitute what are called the elements of a crystal.39
Crystal Systems. — There are six of these.
Cubic (regular or monometric). — Three axes at right angles to each other, the axes (consequently the parameters or semi-axes) equal in length. As the axes are equal to each other, and similarly related, the " elements " are said to be fixed.39
FIG. 29. — Cubic system, a, octahedron ; b, dodecahedron ; c, tetrahedron ; dy combination of cube and octahedron.
Important forms are the octahedron (magnetic iron ore), fig. 29, a ; hexahedron or cube (fluorspar, rock-salt) ; dodecahedron (garnet), fig. 29, b • hemi-octahedron or tetrahedron (grey copper, blende), fig. 29, c ; also combinations as in fig. 29, d.
Tetragonal (dimetric). — Three axes at right angles, two equal to each other, called lateral. The third or principal axis is variable ; in some pyramidal minerals it is longer, in others shorter than the laterals. There is consequently one " variable element" in this system, viz. the proportion existing between the length of the principal and lateral axes.39 Principal forms are : tetragonal pyramid and tetragonal prism of the first order (fig. 30, a) ; the same of the second order, differing only in the position of the lateral axes (fig. 30, b) ; and ditetragonal pyramid and prism (fig. 30, c).
Rhombic. — Three axes at right angles. All unequal in length, and the relative lengths varying in different minerals. One is selected as the principal, the others are called lateral. The
60
GEOLOGY FOR ENGINEERS.
[PT. II. CH. IV.
longer lateral axis is the "macrodiagonal," the shorter the " brachydiagonal." Thus there are two variable elements in this system, viz. the ratios respectively of the "macro" and "brachy" diagonals to the principal.39
The most perfect form is a double pyramid on a rhombic base.
Various combinations are shown in fig. 31.
FIG. 30. — Tetragonal system, a, pyramid and prism of first order ; b, pyramid and prism of second order ; c, ditetragonal pyramid and prism.
Oblique (monoclinic). — Three axes, two at right angles, the third inclined at different angles in different systems ; relative lengths variable in different minerals, and usually all unequal. One of the two which are at right angles is taken for "principal," that at right angles with it is termed the "orthodiagonal," that
CL b rt e
FIG. 31. — Rhombic system, a, b, c, d, e, various combinations.
which is inclined is the "clinodiagonal." There are consequently three variable " elements " in this system, viz., the ratios of two axes to the third, -and the inclination of the " clinodiagonal " to the "principal."39 The ideal form is the oblique rhombic octahedron (fig. 32, a). Various combinations are shown in fig. 32, 6 and c.
SECT. II.]
THE STUDY OF MINERALS.
61
Anorthic (triclinic, doubly oblique). — Three axes, all variable in length and usually unequal; all inclined at different angles. Thus there are four variable " elements " in this system, viz. the
FIG. 32. — Oblique system, a, oblique rhombic octahedron ; b, c, combinations ; b, gypsum ; c, pyroxene.
ratios of two axes to the third, and their inclinations to each other. Either of the axes may be taken as principal, when the other two will be lateral. The longer lateral may be termed "macro- diagonal," the shorter " brachydiagonal," as in'the rhombic system.39
Very few forms occur in nature. The doubly oblique prism (fig. 33) is the ideal or FlQ> 33> _ Anorthic fundamental form of sulphate of copper.13 system, a, doubly
Hexagonal (rhombohedral). — Four axes, oblique prism (sul- three lateral — equal — lying in one plane, phate of copper), and inclined to each other 60° ; the fourth is principal, at right angles to the three lateral, of different
FIG. 34. — Hexagonal system, a, hexagonal dodecahedron ; b, rhombohedron ; c, d, combinations (d, quartz).
length ; sometimes longer, sometimes shorter. This is the only variable element in the system.39
The principal simple form is the hexagonal dodecahedron
62 GEOLOGY FOR ENGINEERS. [PT. II. CH. IV.
(fig. 34, a) ; the rhombohedron (fig. 34, b) is a common type ; and various combinations are shown in fig. 34, c, d. Fig. 34, d, is the most usual form of quartz.
Modified Forms. — The perfectly developed crystal is very rare. When all the faces are exhibited a crystal is said to be holohedral ; when half of the faces are suppressed it is hemihedral. A crystal is often twinned,1 that is, formed apparently by the outgrowth of two similar crystals from a medial line.5 The term truncation denotes that an edge is replaced by a surface, which may be either parallel to it or placed obliquely ; and bevelment, that the edge is replaced by two planes placed parallel to it. The faces and edges of crystals will often vary, but the angles remain unchanged.1
Irregular Grouping of Crystals. — Masses of crystals, when not arranged as symmetrically twinned forms, are spoken of as groups or crystalline aggregates. These are commonly found in hollow spaces or druses in the containing rock, attached at one end, with the faces terminating the opposite end freely developed, the individuals of the group having a more or less radial arrangement diverging from the point of attachment. This, in general terms, may be considered as the most typical kind of grouping of well-individualised crystals. When the aggregates are of a more compact kind, the individuals are rarely recognisable with anything like their full number of faces, but appear, as a rule, as columnar or fibrous masses (see Structure, Section III., pp. 63- 64) arranged in parallel or divergent forms. The latter, when in sufficient numbers, make up more or less spheroidal masses which, according to the size of the spheroids,12 assume imitative shapes, as described under Structure in Section III., pp. 63-64.
Pseudomorphism. — A crystal is called a pseudomorph when it has the crystalline form characteristic of a mineral different from it in chemical composition.7
Section III.— Physical Characters of Minerals.
The most important of these are, cleavage, structure, fracture, tenacity, hardness, touch, specific gravity, translucency, colour, streak, lustre.
For other properties, e.g. refraction, polarisation, pleochroism, fluorescence, etc., as well as for thermal and electrical properties, the reader is referred to text-books of mineralogy.
CLEAVAGE.
This is the property possessed by many crystals, of splitting in certain directions more readily than in others. It is peculiar to
SECT. III.] THE STUDY OF MINERALS. 63
crystals. The surfaces of separation are called cleavage planes, and are usually parallel to the faces of one of the principal crystal forms of the mineral.1 When the direction of such surfaces is known, a comparatively slight cutting or wedging strain will be sufficient to produce a separation, While the resist- ance in the other directions may be considerably greater.12 Cleavage is therefore directly related to crystalline structure, but has no relation to tenacity or hardness.
Laws of Cleavage. — (1) It is uniform in all the varieties of the same mineral.
(2) It occurs parallel to the faces of a fundamental form, or along the diagonals.
(3) It is always the same in character parallel to similar faces of a crystal, being obtained with equal ease and affording planes of like lustre ; and, conversely, it is dissimilar parallel to dissimilar planes.
(4) All simple minerals do not submit to cleavage with the same readiness, and in some the difficulty of effecting it is almost insuperable. Quartz, for example, cannot be cleaved by the knife and hammer ; but it may sometimes be made to exhibit the property by plunging it into cold water while very hot.
(5) Some minerals present peculiar cleavages of subordinate character, independent of the principal cleavage ; thus calc-spar has sometimes a cleavage parallel to the longer diagonal of its faces.
(6) Cleavage extends to rock-masses where it is observed, as in slate, chiefly with reference to one set of planes. The jointed structure of many rocks is another result of the same property13 (see Chapter III., Section III.).
Quality of Cleavage. — The terms used to denote the quality of cleavage are highly perfect, as in mica ; very perfect, as in fluor- spar, barytes, and hornblende ; perfect, as in augite and chrysolite ; imperfect, as in garnet and quartz ; and very imperfect, when only traces of cleavage can be obtained.12
STRUCTURE.
The term "structure" is often reserved for the larger and coarser features, while "texture " is used for the smaller and finer ones, but it is preferable to use the latter term to describe the nature of the surface of a mineral or rock.
The most important kinds of mineral structure are as follows J : — Columnar. — Made up of minute fibres or prisms, closely com- pacted together. It is common in the seams of rocks, and sometimes in incrustations. It may be of the following kinds : —
64
GEOLOGY FOR ENGINEERS. [FT. II. CH. IV.
(1) Fibrous, or with delicate parallel fibres. Ex., gypsum and asbestos. (2) Reticulated, the fibres crossing and resembling a net. (3) Stellated, fibres radiating from a centre and producing a star- like appearance. Ex., stilbite, wavellite. (4) Radiated and
FIG. 35. — Imitative shapes of minerals, a, globular ; b, reniform ; c, botryoidal ; d, mammillary ; e, stalactitic.
divergent, fibres radiating but not stellar. Ex., quartz, grey antimony.
Lamellar. — Exhibits laminae or leaves (parallel plates), either thick or thin, separating easily or with difficulty. (1) Foliaceous, leaves thin and separating easily. Ex., mica, whence this variety is sometimes called micaceous. (2) Tabular, laminae thick. Ex., quartz, heavy-spar. The laminae may be clastic, as in mica ;
SECT. III.] THE STUDY OF MINERALS. 65
flexible, as in talc or graphite ; or brittle, as in diallage. They are also sometimes arranged in stellar shapes, as in mica.
Granular. — This term explains itself, and admits of the follow- ing varieties : — (1) coarse granular, as granular marble ; (2) fine granular, as granular quartz, specular iron ; (3) impalpable, as chalcedony, opal ; (4) friable, or easily crumbled by the fingers.
Imitative Shapes. — Massive and imperfectly crystallised minerals (see Section II., p. 62) sometimes take the following shapes (see fig. 35) : — globular, when the shape is spherical, and the structure either radiating or concentric ; reniform, or kidney- shaped ; botryoidal, when a mass consists of a number of rounded prominences like a bunch of grapes ; mammillary, resembling the former, but consisting of larger prominences ; filiform, like a thread ; acicular, slender, like a needle ; stalactitic, cylindrical or conical, hanging from the roof of a cavern or cavity : carbonate of lime, brown iron ore, malachite, and chalcedony are the chief minerals found in a stalactitic form. Drusy — a cavity is said to be drusy when it is lined with distinct crystals. A mineral having a drusy cavity is sometimes called a geode.13
FRACTURE.
The following terms are used to describe the surfaces of minerals broken in directions which are not cleavage planes : —
Form of Surface. — Conchoidal (shell-like), having curved mark- ings like those seen on the inside of many bivalve shells, as in flint and opal.
Even, a surface free from marked depressions or elevations.
Uneven, a surface having irregular depressions or elevations.39
Nature of Surface. — Smooth, as in lithomarge ; splintery, as in serpentine and fibrous haematite; hackly, or covered with sharp, wire-like points, as in native copper; earthy, when the mineral breaks like a piece of dried clay.
TENACITY.
Frangibility or resistance to crushing. Minerals may be tough, or only broken with difficulty, as hornblende ; brittle, or very easily broken with a blow, as tourmaline.39 Others are friable and pulverulent, or easily crushed, between the fingers, to a powder.1
Sectility is the property of being smoothly cut with a knife, as in the case of mica.
Ductility is the property of being drawn out, as into wire or threads.
5
66 GEOLOGY FOR ENGINEERS. [PT. II. CH. IV.
Malleability is the property of being hammered without breaking or cracking, e.g. gold, silver, copper, etc.10
Rigidity. — A substance is said to be flexible when a thin plate can be bent and remains so without breaking, as talc ; and elastic when, after being bent, it springs back to its original form.12
HARDNESS.
The hardness of minerals may be compared by trying to scratch
them with a knife or a file. Moh's scale of hardness is as follows : —
1. Talc. 6. Orthoclase.
2. Selenite. 7. Quartz.
3. Calcite. 8. Topaz.
4. Fluor-spar. 9. Sapphire.
5. Apatite. 10. Diamond.
If a mineral will scratch talc with the same ease with which selenite scratches it, its hardness will be 1'5; if it only just scratches talc, its hardness will be I'l or T2; and if selenite only just scratches it, its hardness will be 1-8 or 1-9.1
TOUCH.
The feel or touch of some minerals is characteristic. The following terms are used : —
Soapy, or unctuous, as talc and other magnesian minerals.
Meagre, or moistureless : dry and rough to the touch, as chalk and magnesite.
Harsh, or unpleasantly rough, as actinolite. Some minerals adhere to the tongue?
SPECIFIC GRAVITY.
Definitions. — The density of a substance is the mass or quantity of matter per unit of its volume. It is proportional to the specific gravity, since mass is proportional to weight. The specific gravity of a substance is the weight of any volume of it compared with that of an equal volume of water.10
TRANSLUCENCY.
In systematic mineralogy, minerals are classed as transparent, semi-transparent, translucent in various degrees, and opaque, according to their power of transmitting light throughout their
SECT. III.] THE STUDY OF MINERALS. 67
mass ; these terms being used in the popular sense, without reference to the homogeneity or colour of the substance. The test of transparency is the power of discerning an object through a parallel-sided plate or crystal of a certain thickness. Rock- crystal, calcite, gypsum, and barytes, and, among the ores of the heavy metals, zincblende in its lighter varieties, are among the most transparent substances known. When the object is only imperfectly seen, the substance is semi-transparent ; when only a cloudy light like that seen through oiled paper or ground glass is transmitted, it is translucent ; when no light is transmitted, it is opaque. These terms are to a certain degree relative, particu- larly in the lower degrees, where the thickness of the substance must be considered, especially when it is dark-coloured. Flint and obsidian, for example, are said to be translucent at the edges, or in thin splinters, while in thicker masses they are apparently opaque. Ferric oxide and its hydrates are also fairly translucent in minute, microscopic crystals, but opaque when sufficiently large to be apparent without magnifying. Magnetite, on the other hand, does not appear to be susceptible of transmitting light under any condition, and is therefore opaque, as are also the native metals and most of the heavy metallic sulphides.12
COLOUR.
Owing to the frequent admixture of foreign substances which cause the same mineral to assume varying tints, colour is often of very little use for distinguishing minerals. Some metallic colours are, however, easily distinguishable, e.g. copper red of metallic copper ; bronze red and bronze yellow of magnetic pyrites; brass yellow of copper pyrites; lead grey of galena, and iron black of magnetite and graphite.12
STREAK.
The colour of the powder of a mineral produced by drawing it over a file or piece of unglazed porcelain is, however, a better guide, as it is usually constant for the same mineral.1
LUSTRE.
This is the quality of the surface of a mineral as regards the kind and intensity of the light it reflects. The chief kinds are : —
Metallic, the brilliancy of polished metals, characteristic of native metals and heavy metallic sulphides.
Adamantine, the brilliancy of the diamond.
68 GEOLOGY FOR ENGINEERS. [FT. II. CH. IV.
Vitreous or glassy, characteristic of quartz, etc.
Resinous or waxy.
Fatty or greasy, the brilliancy of a freshly oiled reflecting surface ; characteristic of slightly transparent minerals such as serpentine, nepheline, and sulphur.
Nacreous, like mother-of-pearl, characteristic of minerals with perfect cleavage, like gypsum.
Silky, characteristic of fibrous aggregates, such as satin-spar.
Intensity of lustre is denoted by the terms splendent, shining, glistening, or glimmering, but these are used very loosely.12
CH. V.]
CHAPTER V. ROCK-FORMING MINERALS.
CLASSIFICATION.
IN this chapter the most important rock-forming minerals are, for convenient reference, described in alphabetical order of single minerals and important groups, such as augite-hornblende, felspar, iron, manganese, micas and talcs, and silica series. In works on mineralogy they are, however, usually classified in chemical groups, as shown in the following list : —
Native Elements. — Graphite, sulphur.
Sulphides. — Iron pyrites, marcasite, copper pyrites, galena, zincblende.
Fluorides. — Fluor-spar.
Chlorides. —Rock-salt.
Anhydrous Oxides. — Quartz, haematite, ilmenite, magnetite.
Hydrous Oxides. — Limonite, psilomelane.
Anhydrous Silicates. — Felspars, garnet, nepheline, epidote, micas, chiastolite, tourmaline, augite-hornblende group, leucite, olivine.
Hydrous Silicates. — Zeolites, kaolin, talc, chlorite, glauconite, serpentine.
Carbonates. — Calcite, aragonite, dolomite, siderite.
Sulphates. — Barytes, celestine, anhydrite, gypsum.
Phosphates. — Apatite.
Titanate. — Sphene.
Hydrocarbons. — Asphalt.1
ABBREVIATIONS.
Crys. = Crystallographic and other forms. Cl. = Cleavage. H. — Hardness. jSp. gr. = Specific gravity. Fr. = Fracture. Ten. = Tenacity. Feel. — Feeling to the touch. Tr. = Trans- lucency. Col.— Colour. Sir. = Streak. Lus. = Lustre. Comp. = Chemical composition. Flame — Flame-coloration. Fus.=
69
70 GEOLOGY FOR ENGINEERS. [pi. II.
Fusibility. Bor. = Borax bead. Micr. = Microcosmic salt bead. Cl. tube = Closed tube. 0. tube = open tube. Ch. =0n charcoal. Soda = Sodium carbonate. HCl = Hydrochloric acid. 7/2$04 = Sulphuric acid. Sol. — Solubility in acids.
Testing Minerals. — Chapter XL must be read in conjunction with Chapter IV.
LIST OF MINERALS.
Actinolite, see Augite -Hornblende group ; Hornblende.
Adularia, see Felspars ; Orthoclase.
Alabaster, see Gypsum.
Albite, see Felspars ; Plagioclase.
Amphibole, see Augite-Hornblende group ; Hornblende.
Analcime, see Zeolites.
Andalusite (Chiastolite). — Crys., rhombic, usually elongated prisms. Cl.t imperfect. H., 7-7'5. Sp. gr., 3'15-3'35. Fr., uneven, splintery. Tr., transparent to opaque. Co/., white, grey, reddish brown, olive-green or violet. Str.t white. Lus., vitreous. Comp., silicate of alumina, Si02 36 '90, A1203 63'10 per cent.14 Flame, with cobalt, alumina blue. FUB., infusible. Soda, swells up to porous mass, but does not fuse. Sol., not affected by acids ; decomposed by fusion with caustic alkalies.
Occurrence. — The variety called Chiastolite shows a cuneiform or tessellated pattern in the cross-section of the prism. It is found in argillaceous schists, mica schist, gneiss, and similar rocks.14
Anhydrite (Anhydrous Calcium Sulphate). — Crys., rhombic ; crystals uncommon, usually massive ; also in fibrous, lamellar, and granular aggregates, the former being often curved. Cl., three rectangular cleavages. H., 3-3'5. Sp. gr., 2*89-3. Fr., uneven or splintery. Tr., transparent to translucent. Col., usually white or blue, sometimes red. Str.t white. Lus., vitreous on basal cleavage, pearly on the others. Comp., CaS04. or CaO 41-18, H2S04 58'82 per cent.14 Flame, calcium with HCl. Fus., about 2 '5. Cl. tube, no water. Ch., with soda, sulphur reaction. Sol. in HCl.15
Occurrence. — Essentially an associate of rock-salt, and generally of gypsum. When exposed for a long period to the air it becomes partially hydrated, or changes into gypsum 14 (see Chapter XIX.).
Dist. characters. — The three cleavages, hardness greater than gypsum, does not split into laminae like gypsum.1
Anorthite, see Felspars.
Apatite (Phosphate of Lime). — Crys., hexagonal, pyramidally hemihedral. Crystals, when of large size, are usually columnar
CH. V.] ROCK-FORMING MINERALS. 71
and moderately broad, but, when microscopic, are often acicular. Also massive and in botryoidal and reniform aggregates, with a radiated fibrous composition. CL, none, H., 4*5-5. Sp. gr., 3-05-3-25. Fr., conchoidal. Tr., transparent, translucent, or opaque. Col., variable, bluish green and greenish yellow most common; sometimes colourless ; also, pink, violet, blue, or grey. Lus., vitreous on crystal faces, resinous on fractures.14 Comp., 3Ca3P208 + Ca(Cl2Fl2). Flame, with H2S04, green (phosphorus). Fus., near 5. Cl tube, with magnesium, phosphorus reaction. Sol., soluble in strong HC1. A drop of H2S04 added to the solution precipitates microscopic crystals of gypsum. Treated with nitric acid and ammonium molybdate solution, gives strong yellow precipitate. Small fragments may thus be dealt with on a glass slip.15
Occurrence. — Principally in veins or interspersed in irregular crystals, often of very considerable size, in crystalline limestones.14 Sometimes visible as yellowish-white streaks in metamorphic rocks, scratchable with the knife ; but, despite its abundance, commonly too small for detection with^the eye.15
Dist. characters. — When in crystals apatite may be often identified by its shape. Its inferior hardness prevents its being mistaken for beryl. Hexagonal crystals of calcite are softer, differently terminated, and effervesce with acids.7
Varieties. — Those containing little or no fluorine are called Chlorapatite, and those with little or no chlorine Fluorapatite.7
Phosphorite is a massive, amorphous, concretionary, and mam- millated variety, often with a fibrous structure.4
Coprolites and guano are impure varieties of phosphorite (see Chapter VIZ., Section II., Phosphatic Rocks, p. 121). Apophyllite, see Zeolites.
Aragonite. — Crys., rhombic, twins common ; crystals usually short, prismatic, pointed, somewhat resembling calcite : also in fibrous, radiated, granular, stalactitic, spheroidal, and curved coral or plant-like forms (flosferri). CL, imperfect. H., 3 '5-4. Sp. gr., 2'9-3. Fr., conchoidal, uneven.14 Col., colourless. Coinp., CaC03, same as calcite. Flame, with HC1, strong calcium. Fus., infusible. Sol., effervesces freely in cold HC1.15
Occurrence. — Principally in hollows and druses in marls, lime- stones, basalts, or other rocks, and in mineral veins, especially those of iron ore.14 Common as a constituent of the shells of many genera. Forms also radial groups in the cavities of altered rocks.15
Dist. characters. — Aragonite is harder than calcite, but this test cannot be applied to the imperfectly crystallised varieties. The absence of the marked cleavage of calcite is a good distinguishing
72 GEOLOGY FOR ENGINEERS. [PT. II.
test. But nothing is so characteristic as the way in which it falls into powder before the blowpipe.7 Its specific gravity is higher than that of calcite.15
Asbestos, see Augite- Hornblende group ; Hornblende. Asphalt (Bitumen). — Crys., amorphous, filling cavities and veins in rocks encrusting other minerals, also in drops and stalactitic. H., 2. Sp. gr., 1 '0-1*8. Fr., conchoidal, sometimes vesicular. Col. and Lus., black and lustrous like pitch. Comp., contains carbon, hydrogen, and oxygen, but not in any very well-defined proportions. Fus.j at 90° to 100° C., mostly with a strong bituminous odour.14 Occurrence. — A number of natural inflammable pitchy or oily substances are included under the general term Bitumen. They consist of various hydrocarbons with variable quantities of oxygen and nitrogen. The solid varieties go by the general name of Asphalt or mineral pitch. The liquid forms are called Naphtha when they are thin and slightly coloured, Maltha or mineral tar when they are very viscid, and Petroleum when they are inter- mediate between these extremes.7
AUGITE - HORNBLENDE GROUP. — Augite and hornblende are usually dark green or black minerals which belong to the monoclinic system, and are commonly a little more easily scratched than the felspars with which they always occur. They are probably different forms of the same mineral which assumes the form of hornblende on cooling slowly, and that of augite on cooling in lava streams.6
Augite (Pyroxene). — Crys., oblique, crystals mostly short or long columnar, rarely tabular; prism-angle 87°; also in granular irregular masses ; twins common. C7., one, perfect. H., 5*6. Sp. gr., 2'9-3'5. Tr., translucent to opaque. Col., white, grey, green, brown, or black. Sir., white or grey. Lus., vitreous to pearly.14 Comp., approxi- mately (Ca, Mg, Fe)Si03 with some A1203 and Fe203. Fus., about 3*5. Micr., silica.15
Occurrence. — In basalt and dolerite, diabase, and modern lavas. The pale varieties are chiefly found in altered lime- stones.6
Diallage. — A thin, foliated variety, occurs in gabbros of Cornwall and gneiss in Spain.6
Rhombic Pyroxenes. — Enstatite, bronzite, and hypersthene resemble augite, but crystallise in rhombic system. They are found in gabbros and serpentine.1
Hornblende (Amphibole). — Crys., oblique, like augite, but prisms often longer and of more fibrous aspect ; prism angle 124°;15 also in acicular forms and fibrous and granular aggregates. Cl., one, perfect. H., 5-6. Sp. gr., 2'9-3'5.
CH. V.] ROCK-FORMING MINERALS. 73
Fr., subconchoidal, uneven. Ten., rather brittle. Tr. translucent, opaque. Col., white, passing by various shades of green to black. Lus., vitreous, pearly (cleavages), silky (fibres).14 Comp., approximately (Mg, Ca, Fe)Si03 with often much A1203 and Fe208. Fus., 3-5. Micr., silica.15
Occurrence. — Hornblende is a common constituent of crystalline rocks, being more particularly associated with the more highly silicated felspars, quartz, chlorite, magnetite, and pyrites. Tremolite, a white or colourless variety, affect- ing fibrous or columnar forms, is essentially an associate of crystalline limestones and dolomites. Nephrite or jade is probably a compact variety of tremolite. Asbestos, a fibrous or felted variety, is commonly associated with serpentine.14 Actinolite and smaragdite are greenish varieties. Barytes (Heavy- spar).— Oys., rhombic, crystals usually thin, in prisms or domes, and often large up to 18 inches long, but generally in parallel or divergent groups : also in spheroidal aggregates, lamellar, cleavable, massive, and in stalactitic forms with a fibrous structure.14 Cl., parallel to the base and lateral faces of the unit prism.7 ff., 3-3'5. Sp. gr., 4'3~4'72. Tr., transparent to translucent. Col., white, grey, yellowish, or brown, rarely blue. Sir., white. Lus., transparent crystals, vitreous ; translucent ores, nacreous.14 Comp., BaS04 or BaO 65'68, S03 34*32 per cent, often with impurities. Flame, barium, green. Fus., about 3, commonly decrepitates. Ch., with soda, sulphur reaction.15 Sol., insoluble.14
Occurrence. — A very common vein mineral, especially accompany- ing lead ores.14 Gawk is a w&te, massive, or cryptocrystalline variety which is ground up and used for the adulteration of white lead.
Dist. characters. — The high specific gravity, cleavages differing from calcite in two of the planes, being at right angles to the third, colour, and presence of sulphur.1 Biotite, see Micas and Talcs. Bitter-spar, see Dolomite. Bitumen, see Asphalt.
Blackband Ironstone, see Iron ; Spathic Iron Ore. Black Lead, see Graphite. Black Mica, see Micas and Talcs ; Biotite. Blende, see ZincUende.
Bog or Brown Iron Ore, see Iron ; Limonite. Bog Manganese Ore, see Manganese.
Bronzite, see Augite- Hornblende group • Rhombic Pyroxenes. Brown-spar, see Dolomite. Calcite. — Crys., rhombohedral. The habit of the crystals may
74 GEOLOGY FOR ENGINEERS. [PT. IT.
be either columnar, tabular in various degrees down to the thinnest hexagonal scales, rhombohedral or scalenohedral, the last two kinds being most common. Twins are common. Crystalline aggregates of various kinds are abundant, especially in stalactitic and radiated forms, and in finely granular masses and pulverulent crusts. In all cases the crystalline structure is recognisable.14 CL, most perfect parallel to the faces of a rhom- bohedron.7 H., 3. Sp. gr., 2 '6-2 '8. Tr., transparent to trans- lucent and opaque. Col., colourless, or grey, bluish or greenish, or white. Lus., vitreous, pearly on opaque varieties. Comp., CaC03 or CaO 56, C02 44 per cent.14 Flame, Fus., and Sol., like aragonite.15 Soluble in acetic acid.14
Occurrence. — Abundant in all limestone regions, being especially common as a deposit from water in caverns and veins.14
Dist. character. — The marked cleavage.
Gawk, see Barytes.
Celestine (Strontium Sulphate). — Crys., rhombic, same as barytes ; also in fibrous, columnar, radiated, or spheroidal forms. CL, same as barytes. H., 3-3'5. Sp. gr., 3'92-3'98. FT., conchoidal to uneven. Tr., transparent to imperfectly translucent. Col., colourless, white or pale blue, sometimes reddish. Lus., vitreous or pearly in crystals, silky in fibrous variety.1 Comp., SrS04 or SrO 56-52, H2S04 4348 per cent.14 Flame, strontium. Ch., with soda, sulphur reaction.15
Occurrence. — Found principally in marls and limestones.14
Dist. characters. — Distinguished from gypsum, should the flame be doubtful, by hardness, specific gravity, absence of water, and insolubility in HC1. The latter character distinguishes it from anhydrite.15
Chabasite, see Zeolites.
Chalcedony, see Silica Series ; Quartz.
Chalcopyrite, see Copper Pyrites.
Chiastolite, see Andalusite.
China Clay, see Kaolin.
Chlor-apatite, see Apatite.
Chlorite, see Micas and Talcs.
Clay Ironstone, see Iron ; Carbonates.
Clinochlore, see Micas and Talcs ; Chlorite.
Copper Pyrites (Yellow Copper Ore, Chalcopyrite). — Crys. tetragonal, usually compact, interspersed in granules or in reniform or botryoidal masses. Cl., not very distinct. H., 3'5-4. Sp. gr., 4-1-4-3. Fr. conchoidal. Ten., brittle, slightly sectile. Tr., opaque. Col., brass to gold-yellow ; when tarnished irised in various colours. Sir., greenish black, shining. Lus., sub-metallic. Comp., CuFeS2 or Cu 34'57, Fe 30-54, S 34 "89 per cent.14
CH. V.] ROCK-FORMING MINERALS. 75
Flame, copper colours with HC1. Fus., easy. Bor. and Micr., copper reactions ; green in 0. F. when hot, owing to presence of iron. CL tube, decrepitates, and some sulphur. Ch., fuses, with intumescence and scintillation, to a magnetic globule. Roast in 0. F., and then reduce ; a copper bead separates in the mass. Soda only obscures the reaction. Sol., slowly soluble in nitric acid with separation of sulphur.15
Occurrence. — The standard ore of most copper-mining districts ; u occasionally met with in rocks, such as diabase, some granites, gneiss, argillaceous schists, etc.16
Dist. characters. — Easily distinguished by hardness from iron pyrites which cannot be scratched by the knife.15
Coprolites, see Apatite.
Diallage, see Augite- Hornblende group ; Augite.
Dolomite (Bitter Spar). — Crys., rhombohedral, with curved faces, often of considerable size; also in druses and irregular aggregates, arid in crystalline concretions of stalactitic, spheroidal, and other forms. Compact in rock and masses, sometimes slaty or finely granular. CL, cleavable, cleavage planes usually curved. H., 3'5-4'5. Sp. <jr., 2-85-2'95. Col., white, or some pale shade of yellow or brown ; blue, green, or red less common. Lus., nacreous, translucent.14 Comp., (CaMg)C03. Flame, with HC1, calcium. Fus., infusible. Sol., effervesces in hot HC1 ; insoluble in acetic acid.15
Occurrence. — Abundant in mineral veins, especially with copper and lead ores ; and also forming rock-masses often of consider- able extent. Pearl spar or brown spar is a dolomite containing more or less iron, which is usually light grey or white, with a pearly lustre when fresh, but by exposure to the air turns brown.14
Dist. characters. — Sp. gr. of calcite is less ; the latter is soluble in acetic acid and in cold HC1.
Enstatite, see Augite- Hornblende group ; Rhombic Pyroxenes.
Epidote. — Crys., oblique ; crystals usually much elongated, with faces striated ; also fibrous, granular, massive, and in pseudo- morphs. Cl., one, perfect, ff., 6-7. Sp. gr.™ 3'32-3'49. Ten., brittle.7 Tr., translucent to opaque. Col., yellowish to oil-green, brownish grey, or black. Lus., vitreous.14 Comp., H2Ca4(AlFe)6Si6026. Fus., slightly more fusible than actinolite ; intumesces somewhat. Micr., silica.15
Occurrence. — In many granites and in crystalline schists and near to the contact with intrusive rocks in sandstones ; also in dolerites and other lavas.14
Dist. characters. — Peculiar colour and brittleness ; 7 its hard- ness distinguishes it from hornblende.15
76 GEOLOGY FOR ENGINEERS. [PT. II.
FELSPARS are the most abundant minerals in igneous rocks. They can be just scratched with a knife, being softer than quartz, harder than apatite, and much harder than carbonate of lime. The colour is often milky-white, sometimes bright red owing to the presence of oxides of iron, and occasionally grey or black, or even green. All felspars consist chemically of silicates of alumina combined with some other silicate, which is usually silicate of potash, or soda, or lime, or some combination of lime and soda ; and, according to variations in chemical composition, the different varieties or species of felspar are identified and named. With these chemical differences are associated differences of crystalline form. When a typical felspar contains potash, it crystallises in prisms in the oblique or monoclinic system, and is recognised by fracturing at right angles to the side of the prism ; but when the crystal con- tains soda or lime it crystallises in the doubly oblique or triclinic system, and the cleavage is then at an oblique angle. For most purposes it is sufficient to identify these two groups, known as Orthoclase and Plagioclase, but the most important varieties are briefly described below.
Felspars are also classed as potash, soda, or lime felspars, orthoclase being the typical potash felspar, while the remainder are plagioclase.6
The composition of all the felspars is liable to vary, by the partial replacement of the alkaline bases by one another, They all weather under the action of the air and rain, decomposing and losing their colour (often forming a white coating) ; but albite is less liable to this change than the other varieties of felspar.4
Distinguishing characters. — The felspars may be distinguished from quartz (1) by their cleavage : even small grains show, when broken, bright cleavage faces, while quartz breaks, like glass, with an uneven or conchoidal fracture ; (2) by their fusibility, if the student is sure of his power to produce a steady hot flame ; (3) generally by their inferior hardness.
To tell one felspar from another is by no means an easy matter. The more massive cleavable varieties of orthoclase have a characteristic look, and though mere appearance is a very dangerous test to trust to in determining a mineral, a fairly experienced eye can often be pretty sure of such forms as orthoclase by their look alone. Again, a felspar is known to belong to the triclinic group if it shows the characteristic striation either to the naked eye or by the aid of a pocket-lens. Whenever this striation is visible, we may be sure that the felspar is not orthoclase ; the absence of striation, however, does not prove that the felspar is not triclinic. To detect the striation the crystal should be held so that a good light falls on the basal plane, and
CH. V.] ROCK-FORMING MINERALS. 77
turned backwards and forwards till the light falls at the right angle to show the marking distinctly."
The lime felspars are soluble in heated HC1, whereas the soda and potash felspars are insoluble. The soda felspars colour the blowpipe flame yellow, and are more fusible than the potash felspars.4
Oblique (Monoclinic) Felspar.
Orthoclase. — (Potash felspar). — Cry 8., oblique, prismatic and granular.13 CL, two, at right angles. H., 6. Sp. gr., 2-53-2-62. Fr., conchoidal, splintery, or uneven. Tr., translucent to opaque. Col., colourless, white, flesh-red, pink, brick-red, smoky grey, pale green, bright green. Str., white. Lus., vitreous, pearly on cleavage. Comp., Si02 64 '68, A1203 18-43, K20 16-89 per cent. Potash is generally partly replaced by soda.14 Flame, potassium fair, with blue glass ; often much sodium (soda-orthoclase). Fus., 5, forming a cloudy glass, coloured varieties becoming white before fusion. Micr., silica.15 Sol., not affected by acids, but partially decomposed by caustic soda lye; dissolves very slowly in salt of phosphorus, leaving a siliceous skeleton.
Occurrence. — The typical constituent of granite, syenite, gneiss, and trachyte, usually in association with quartz.14 Is green from containing copper in some of the rocks of South America and Colorado.
Sanidine is a grey and glassy variety of orthoclase, usually with a little lime and magnesia; occurring in trachytes, phonolites, obsidian, and pitchstone.6
Adularia is a nearly transparent variety of orthoclase with a little lime ; occurring in the granite of St Gothard.6
Tridinic Felspar or Plagioclase.
Microcline — A felspar with the composition of orthoclase, but triclinic. In a very large number of cases microcline has been found to contain included bands and portions of orthoclase and albite.7 Sp. gr., 2'57-2-60. Col., flesh-red, yellowish, or green. Lus., vitreous.
Occurrence. — The common felspar of graphic granite.15 Albite (the typical soda felspar). — Crys., triclinic, rhomboidal prism ; 4 crystals rarely simple, being almost invariably twinned. Cl., basal, and parallel to brachy- pinacoid; perfect. H., 6-6-5. Sp. gr., 2'59-2'65. Tr., transparent to translucent. Col., colourless, white or some very pale tint of red, yellow, green, or grey. Lus., vitreous, pearly on principal cleavage face, which is usually finely
78 GEOLOGY FOR ENGINEERS. [PT. II.
FELSPARS (contd.) —
striated. Comp., Na2AlPSi6016, corresponding to Si02 68-62, A1203 19-56, Na26 11-82 per cent. Fus., rather more readily than orthoclase, colouring the flame yellow. Sol., not acted on by acids.
Occurrence. — As a constituent of granite and other crystalline rocks, but usually in subordinate quantity to orthoclase ; in crystals, or fibrous, lamellar, or globular aggregates on veins.14
Oligoclase (the commonest form of soda felspar.) — Crys., similar to albite. CL, one perfect, one tolerably perfect; basal cleavage surface usually finely striated, generally in cleavable masses. H., 6'7. Sp. gr., 2-56-2-72. Tr., usually opaque or translucent at the edges. Col., white or variously tinted, yellowish grey, bluish, green, or red ; mostly very pale in tint. Lus., greasy on cleavage faces, vitreous or subvitreous on others.14 Comp., Si02 61*9, A1203 24'1, Na20 8-8, CaO 5-2 per cent.4 Flame, sodium. Micr., silica. Fus., 3-5. Sol., not decomposed by HC1.15
Occurrence. — As a constituent of igneous rocks, either as the sole felspar, or in association with orthoclase and albite as in granite, or with labradorite in basalt and dolerite.14
Anorthite (the typical form of lirne felspar).4 — Crys., triclinic, also massive in granular or lamellar aggregates. CL, two, both perfect. H., 6. Sp. gr., 2-66-2'78. Fr., conchoidal, brittle. Tr., transparent to translucent. Col., colourless, white, pale grey or reddish. Lus., vitreous, pearly on cleavages. Comp., Si02 43-08, A1203 36-82, CaO 20"10 per cent.14 Flame, calcium, on decomposition with HC1. Fus., nearly as high as orthoclase. Micr., silica. Sol., decomposed by HC1.15
Occurrence. — Comparatively rare ; found in old lavas, diorite, etc.6
Labradorite. — Crys., triclinic, mostly in cleavable masses, repeatedly twinned like albite. CL, two, perfect; cleavage faces generally striated. H., 6. Sp. gr., 2'68-2-82. Tr., translucent to nearly opaque. Col., colourless, but more generally of a bluish or brownish grey, at times nearly black. Lus., vitreous, pearly or greasy on cleavage faces.14 Comp., frequently (Na2Al2Si6016)2(CaAl2Si208). Flame, calcium and sodium, the former often overpowered by the latter. Fus., 3-5. Micr., silica. Sol., slowly decomposed by HC1.1
Occurrence. — The common felspar of basalt and dolerite, but generally not recognisable except by the microscope.14 Fluor-apatite, see Apatite.
CH. V.] ROCK-FORMING MINERALS. 79
Fluor-spar. — Crys., cubic, crystals either cubic or octahedral ; also in fibrous, radiated, or agate-like masses, and compact or earthy. CL, very perfect octahedral, except in the compact varieties, which are uncleavable. H., 4. Sp. gr., 3 '16-3 '19. Fr., subconchoidal or splintery in massive varieties, but rarely observable in crystals owing to cleavage. Tr., transparent to sub translucent ; the compact variety opaque. Col., very variable, rarely colourless, and transparent ; generally purple or pale green, dark green, yellow ; deep blue less common, pink or rose colour the rarest. Sir., white. Lus., vitreous.14 Comp., CaFl3. Flame, calcium, fairly good. Fus., decrepitates much, but finally fuses at 2 '5-3 with ebullition. Cl. tube, fluorine reactions well given; sometimes phosphorescent. Fused with micr. on glass bead ; etches the glass.15
Occurrence. — Essentially a vein mineral, being found with tin and copper ores in Cornwall and Saxony, and much more abundantly with lead and silver ores. In veins in granitic and crystalline rocks the crystals are usually small, but in those traversing sedimentary strata, as in the clay slates of Cornwall, and more especially in the carboniferous limestone districts of Northumberland and Durham, they are often of great size.14
Dist. characters. — Distinguished from calcite by its superior hardness and specific gravity.15
G-alena. — Crys., cubic, twins common ; also massive in aggre- gates, with a distinct crystalline structure, or finely granular. CL, one highly perfect. Fr., conchoidal, but obtainable with difficulty owing to perfection of cleavage. Ten., brittle, slightly sectile. Tr., opaque. Col., lead-grey, tarnishing to a darker tint. Str., similar to colour. Lus., metallic, very brilliant when fresh. Comp., PbS, or Pb 86'6, S 13'4 per cent.14 Flame, lead. Fus., very easy. Cl. tube, thin, white-yellow sulphur sublimate. 0. tube, after strong heating, a distinct and characteristic heavy sublimate of lead sulphate forms as a white streak on the under side of the tube. Ch., lead incrustation fringed with lead sulphate.15 Sol., partly soluble in nitric acid, depositing sulphur and lead sulphate ; soluble in HC1 when hot, depositing chloride of lead on cooling.14
Occurrence. — The most abundant lead ore ; widely distributed both in stratified deposits and veins, but principally in the latter.
Dist. characters. — Colour and cubic cleavage are characteristic.15
Garnet. — Crys., cubic, crystals often completely developed, and included in rocks ; also grouped in druses, in rounded masses and grains, lamellar and massive aggregates. CL, imperfect. #.,7-7'5. £p.#r.,3'16-4-38.14 Fr., subconchoidal or uneven.16 Tr., transparent to opaque. Col., usually red, but very variable. Sir.,
80 GEOLOGY FOR ENGINEERS. [PT. II.
white. Lus., crystals vitreous; surf aces of fracture resinous.14 Comp., common varieties represented by (Ca, Fe, MgMn)3(Al2Fe2Cr2)Si3012. Fus., the common iron-alumina and lime-iron garnets fuse at 3. Micr., silica.
Occurrence. — Very widely distributed, being found in granites, gneiss, and other schistose rocks, crystalline limestone, magnetite, and chromic iron ore. The massive variety sometimes occurs in bands of considerable thickness, as in the gneiss of Bengal.14
Dist. characters. — The crystalline forms, rhombic dodecahedron, etc., are characteristic and can be traced even in worn specimens. Low fusibility distinguishes red garnet from ruby, etc.15
Glauconite (Greensand). — A silicate of alumina, iron, potassium, etc., usually impure, amorphous, or earthy, yellowish to dark green ; opaque ; granular.
Graphite (Plumbago, Black Lead). — Crys., in six-sided prisms with flat ends and modified basal edges, which may be hexagonal or oblique ; crystals usually short, columnar, or tabular ; also in columnar, fibrous, and radiated aggregates, plates, scales, and compact masses. Cl., basal, very perfect, ff., 1-2. Sp. gr., 2-2 '6. Ten., sectile, flexible in thin laminae. Feel., unctuous and cold in the hand. Tr., opaque. Col. and Str., iron-grey, black. Lus., metallic. Comp., carbon, with variable amounts of ash, mostly iron, silica, and earthy matters. Purest varieties contain 94 to 96 per cent, of carbon, while in those of inferior quality it may be as low as 35 per cent.14 Fus., infusible. Bor., in R. F. gives dusky bead full of black flecks.15
Occurrence. — Chiefly interspersed in grains, scales, or small fragments in granite, gneiss, and crystalline limestones, and in larger irregular masses, which are more or less lenticular in shape.
Dist. characters. — Molybdenite and micaceous heematite are very similar in appearance to graphite : the former is distinguished from it by the slightly green colour of its streak, and by giving the reaction of sulphur in the open tube ; the latter is distinguished by its red streak and by its giving reactions of iron with fluxes.14
Molybdenite has sp. gr. of 4*5, that of graphite being only 2 ; graphite is also blacker in colour.15
Guano, see Apatite.
Gypsum (Selenite). — Crys., oblique ; crystals mostly stout, columnar, or tabular ; twins of two kinds common ; more com- plicated groups are stellate or spheroidal, with parallel or curved planes ; aggregates also common, and massive, earthy, or granular. CL, one highly perfect, one less perfect. H., 1*5-2. Sp. gr., 2-2'4. Ten., flexible in thin laminae. Tr., transparent or translucent. Col., colourless, snowy white, grey, reddish, or brown. Lus., vitreous, nacreous on the best-developed cleavage planes, and
OH. V.] ROCK-FORMING MINERALS. 81
silky on those of the pyramid. Comp., CaS04+2H20 or CaO 32-54, H2S04 46-51, H20 20'95 per cent.14 Flame, calcium with HC1. Fus., about 2*5. Cl. tube, becomes white and opaque; much water. Ch., with soda, sulphur reaction. Sol., in HC1.15
Occurrence. — The term Selenite is confined to the crystallised varieties. The finely grained cryptocrystalline varieties are called Gypsum. When very finely grained and mottled by coloured impurities, so as to be available for ornamental purposes, the mineral is called Alabaster. Intermediate between the largely crystalline and the cryptocrystalline forms are fibrous varieties which, when the fibres have a silky lustre, are called Satin-spar.1
Gypsum is very abundant in certain sedimentary formations and as a deposit from water.14
Dist. characters. — Selenite can seldom be mistaken ; its foliation is most pronounced, and the laminae are neither elastic like those of mica, nor greasy and difficult of fusion like those of talc. The mere look of gypsum, taken in conjunction with its softness, usually enables us to recognise it with certainty.7
Haematite, see Iron ; Oxides.
Heavy-spar, see Barytes.
Hornblende, see Augite- Hornblende group.
Hyalite, see Silica Series.
Hypersthene, see Augite- Hornblende group ; Rhombic Pyroxenes.
Ilmenite, see Iron ; Oxides.
IRON is found chiefly in the form of oxides, carbonates, and sulphides, native iron being of very rare occurrence except in meteorites.15
Oxides of Iron.
There are three oxides of iron : —
Percentage of metallic iron.
Monoxide or ferrous oxide, FeO . . . . 7 7 -7 Sesquioxide, peroxide, or ferric oxide, Fe203 . . 70 -0 Magnetic oxide or ferrosoferric oxide, Fe304 . . 7 2 '4
The first is an unstable compound, and whenever it is produced is converted into a higher oxide, a carbonate, or some other com- pound. The other two occur as minerals.7
Magnetite (Magnetic Iron Ore), a Ferrosoferric Oxide.— Crys., cubic ; crystals are sometimes found completely developed, embedded in slaty or aqueous rocks, but more usually grouped ; also compact, massive, granular, and earthy, often in veins and beds of great size. Cl., octahedral. If., 5"5-6'5.14 Sp. gr., 4'9-5'2.7 Fr., conchoidal or granular.
6
82 GEOLOGY FOR ENGINEERS. [PT. II.
IRON (contd.) —
Ten., rather brittle. Tr., usually opaque. Col., black. Str., black. Lus., metallic.14 Comp., Feg04. Fus., 6. EOT. and Micr ., iron reactions. Mag., magnetic before reduction, attracting its own powder ; many masses show polar magnet- ism of opposite kinds.15
Occurrence. — Abundant in the older crystalline rocks of Norway, Sweden, and Russia, the larger deposits being usually found in crystalline limestone, chlorite schist, horn- blende schist, serpentine, and less commonly in quartzite or mica schist which, under similar conditions, usually carry deposits of specular or micaceous haematite. As a constant, though not very large, constituent it appears in igneous rocks, particularly those of a low percentage of silica, such as basalt, diorite, etc., being usually interspersed in minute crystals or granular masses : these are often titaniferous and vitreous or slaggy in aspect. These fine grains or crystals, when set free by the disintegration of the rocks containing them, form the black magnetic sands with which gold and other heavy minerals are associated in alluvial deposits.14 "
Dist. characters. — Its strong magnetism, black streak, and very common occurrence in regular octahedrons.7
Haematite (Specular Iron Ore). — Crys., hexagonal, rhombo- hedral ; 14 most commonly in clusters of very flat, knife-edged crystals ; 7 also massive, and in radiated fibrous aggregates forming spheroidal, reniform, and botryoidal masses, very common ; also pseudomorphous. Cl., imperfect, ff., 5*5-6 '5 in specular iron; 3-5 in haematite. Sp. gr., 4 '5-5*3, the purest being the densest. Fr., conchoidal, fibrous, uneven. Ten., brittle. Tr., opaque. Col., bluish iron-black in crystals ; fibrous and earthy varieties, various shades of brown and bronze-red, and when wet often nearly vermilion-red. Str., purplish to brown-red. Lus., crystals, metallic ; fractured surfaces dull.14 Comp., Fe203 or ferric oxide. Fus., infusible. Bor. and Micr., iron reactions Cl. tube, generally a trace of water, but far less than limonite. Ch., in R. F., magnetic residue. Sol., soluble in HC1 after some time.15
Occurrence. — The hard, brilliant, well-crystallised forms are known as Specular Iron', the fibrous and dense crystalline varieties as Haematite, Red Hcematite ; and the softer kinds as Micaceous Iron Ore, Puddler's Ore, and Ruddle or Red Ochre. u Haematite occurs in large deposits, both in beds and veins.
Dist. characters. — The red streak is characteristic.
Ilmenite (Titaniferous Iron Ore). — Crys., hexagonal; crystals generally tabular, and at times aggregated in
CH. V.] ROCK-FORMING MINERALS. 83
IRON (contd.) —
rosette-like groups forming the so-called iron roses; also massive, and in loose blocks and grains. CL, imperfect. H., 5-6. Sp. gr., 4*30 -5'21. Fr., conchoidal, uneven. Tr., opaque. Col., black, inclining to brown, or dark grey. Sir., black. Lus., semi-metallic. Mag., sometimes magnetic. Comp., contains iron, magnesium, titanium, and oxygen in variable proportions.14 Fus., practically infusible. Bor., iron reactions. Micr., iron and titanium. Ch., in R. F., magnetic residue. The soda residue, boiled with tin in HC1, gives a satisfactory titanium reaction.15
Occurrence. — Common as a constituent of crystalline and igneous rocks in many parts of the world, and occasionally in large deposits with quartz, rutile, felspar, garnet, and other silicates.14
Dist. characters. — Presence of titanium.
Limonite (Brown Iron Ore, Brown Haematite, Bog Iron Ore). — Crys., amorphous, or in undefined cryptocrystalline forms ; in fibrous, granular, compact and earthy masses, and in concretionary forms of all kinds; also pseudomorphous after pyrites, siderite, etc.14 H., 5-5 '5 in purer forms ; earthy forms often softer. Sp. gr., 3-6-4. 7 Col., brown in all shades, from nearly black to yellow. Sir., yellowish brown. Lus., silky in fibrous kinds ; nearly glassy or resinous when compact, and dull and earthy in granular or pulverulent kinds. Comp., H5Fe409 or HgO 14'4, Fe203 85'6 per cent. ; 14 hydrated ferric oxide, or ferric hydrate giving 60 per cent, metallic iron. Fus., about 5. Bor. and Micr., iron reactions. Cl. tube, water. Ch., in R. F., magnetic residue. Sol., in HC1 after some time.15
Occurrence. — A common product of the alteration of minerals containing iron or ferrous oxide, such as pyrites, siderite, ferrous sulplates, and silicates, etc.
Ochre, Umber, and Sienna Earth are intimate mixtures of limonite and clay.14
Carbonates of Iron (Ferrous Carbonates).
Spathic Iron Ore (Siderite, Chalybite, Sphserosiderite, Clay Ironstone). — Crys., rhombohedral, crystals often with strongly curved faces ; usually found in crystalline aggregates coarsely foliated, radiated, or finely granular in structure or in apparently amorphous nodules known as clay ironstone or spheerosiderite. CL, rhombohedral, perfect. ff., 3-5-4-5. Sp. gr., 37-3'9. Tr., slightly translucent.
84 GEOLOGY FOR ENGINEERS. [FT. II.
IRON (contd.)—
Col., pale yellowish grey, or bluish when fresh, but becoming darker or brown by exposure. Lus., pearly. Comp., FeC03 or FeO 62, C02 38 per cent. ; the corresponding amount of metallic iron being 48 '2 per cent.14 Fus., infusible, de- crepitates when heated and is converted into magnetic oxide. Bor., reaction of iron with soda', manganese. Sol., slowly soluble in HC1, with effervescence.15
Occurrence. — The purer varieties of spathic iron ore and those rich in manganese are especially valued for the production of the highest classes of malleable iron and steel and ferro- manganese. Clay iron ores are found in spheroidal or flattened nodules, occasionally united into irregular beds in the shales of the coal measures. Black-band ironstone is a variety of compact ferrous carbonate, mixed with sufficient carbonaceous matter to burn readily when ignited, so that it can be calcined without additional fuel.14
Sulphides of Iron (Ferrous Sulphides}.
Iron Pyrites (Pyrites). — Crys., cubic, crystals often large (3 inches across); also massive, and in various crystalline aggregates, stalactitic, globular, botryoidal, reniform ; usually of a radiated structure, and interspersed in dendritic patches and grains on rocks and fossils ; also in pseudomorphs. Cl., cubic, very imperfect. H., 6-6-5. Sp. gr., 4'9-5'2. Fr., conchoidal. Ten., brittle. Tr., opaque. Col., pale to full brass-yellow, passing into gold-yellow and brown. Str., black. Lus., metallic. Comp., FeS2 or Fe 46'7, S 53'3 per cent., often containing some copper, cobalt, or arsenic.14 Fus., about 2. Bor. and Micr , iron reactions. Cl. tube, abundant sulphur. Ch., magnetic after reduction. Sol., insoluble in HC1, decomposed by nitric acid.15
Occurrence. — The most abundant of metallic sulphides. It is found in rocks of all ages, variously interspersed from isolated crystals and grains to rock-masses ; more common in rocks that are impermeable to water, or contain carbon- aceous substances, such as clay, slate, and coal, than in those that are freely permeable, like sandstone.14
Dist. characters. — Brass-yellow colour and hardness such that it cannot be touched by the knife.
Marcasite (White Iron Pyrites). — -Very similar to pyrites, but sp. gr. is 4'65-4'88 ; colour brass-yellow, but lighter than pyrites ; crystals rhombic ;14 is readily decomposed on exposure to the atmosphere. Occurs often as concretions in the Chalk. 15
CH. V.] ROCK-FORMING MINERALS. 85
Iron Pyrites, see Iron ; Sulphides.
Jade or Nephrite, see Augite- Hornblende group ; Horn-