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-
blende.
Kaolin (China Clay, Lithomarge). — Crys., in its purest form
kaolin appears as a white powder, usually amorphous, but
showing under the microscope six-sided scales, having a structure
similar to that of mica.14 H., 1-25. Sp. gr., 2'4-2-63. Feel.,
rather unctuous. Col., white when pure.14 Comp., H4Al2Si209.
Fus., infusible. Micr., silica. Cl. tube, water Ch., with cobalt
nitrate, a fine alumina reaction.15 Sol., insoluble in acids.
Occurrence. — The basis of all clay; it occurs more or less mixed
with water, ferric hydrates, quartz, and organic matter, forming
the variously coloured plastic clays.14 As regards its origin see
Chapter VII., Section IV., p. 128.
Labradorite, see Felspars ; Plagioclase.
Lepidolite or Lithia Mica, see Micas and Talcs ; Muscovite.
Leucite. — Crys., twenty-four faced trapezohedrons, generally
considered tetragonal, but resembling cubic.1 CL, imperfect. H.,
5-5-6. Sp. gr., 2-45-2'50. Fr., conchoidal. Tr., semi-trans-
parent. Col., white, ash-grey, yellowish and reddish white. Lus.,
vitreous to greasy. Comp., K20 21'53, A1203 23-50, Si02 54'97
per cent. Fus., infusible. Flame, alumina with cobalt. Bor.,
transparent glass. Sol., completely decomposed by HC1, with
separation of granular silica.14
Occurrence. — A characteristic constituent of lavas and some
varieties of basalt. By mere hydration and loss of potash it is
convertible into orthoclase and china clay.14
Limonite, see Iron.
Magnesite. — Crys., rhombohedral, but rare; usually granular,
crystalline, or massive. Cl., rhombohedral, perfect. H., 4-4'5.
Sp. gr., 2'9-3'l. Tr., Col., Lus., colourless and translucent, with
strong vitreous lustre in some crystallised kinds, but usually
opaque, white, or variously tinted with yellow, brown, or grey.
Comp., Mg.C03 or Mg 48'73, C02 51-27 per cent.14 Fus.,
infusible. Ch., with cobalt nitrate, fair magnesia reaction. Sol.,
effervesces fairly in hot HC1.15
Occurrence. — In crystals in talcose schist and occasionally in
beds.14
MANGANESE is, next to iron, the most common colouring in-
gredient of rocks, sands, and gravels. It also forms the dendritic,
moss-like markings so common on the surfaces of joints and planes
of bedding of some rocks. Its usual colour is black, but it is
also brown, reddish, and green, according (like iron) to its differ-
ent states of oxidisation and combination.4 It occurs in the
following forms : —
86 GEOLOGY FOR ENGINEERS. [PT. II.
MANGANESE (contd.) —
Pyrolusite (or the black peroxide known as Soft Manganese
Ore). — Crys., rhombic ; also massive and granular. H., 2,
when crystallised ; 1-1 -5 in fibrous and earthy kinds. Sp. gr.,
4 -8-4 '9. Lus., imperfectly metallic. Tr., opaque. Col.,
dark grey to black. Sir., black.14
Manganite (Grey Oxide). — Crys., rhombic. Cl., one per-
fect, one less perfect. H., 3-5-4. Sp. gr., 4-3 (fresh), 4-8
(weathered). Fr., uneven. Ten., rather brittle. TV-., opaque.
Col., dark grey to black, weathering greenish or brownish.
Sir., brown (fresh), black (weathered). Lus., semi-metallic.14
Psilomelane (Hard Manganese Ore). — Crys., amorphous;
in fibrous and other forms. H., 5-6. Sp. gr., 4'l-4-7. Fr.,
conchoidal, fibrous, or even. Tr., opaque. Col., black, bluish
or brownish black. Sir., brownish black. Lus., silky or
dull.14 Comp., hydrous oxide of Mn, Ba, and K.14
Wad or Bog Manganese Ore is a brown, earthy substance,
similar to psilomelane, but with more water.14
Dist. characters of manganese ores. — The black peroxide
of manganese is distinguished from iron oxide by having a
black streak. The presence of manganese may also be
detected by its producing with borax in the outer flame of
the blowpipe a violet bead, which in the inner flame becomes
colourless ; also a manganese mineral fused on platinum wire
with carbonate of soda imparts to it a fine, greenish-blue
colour, somewhat resembling turquoise.4
Marcasite, see Iron.
MICAS AND TALCS. — The talcs and micas include many species
which usually agree in dividing into thin laminae which are
sometimes more 01 less transparent. The talcs are softer than
the micas, may be bent, but will not spontaneously bend back
again, give a more or less greasy sensation when touched, and
are hydrous silicates of magnesia where part of the magnesia may
be replaced with iron, and are not acted on by acids. The micas
are usually in rhombic or hexagonal plates, are both flexible and
elastic, give a clean sensation when touched, are double silicates,
usually of alumina, magnesia, potash, and iron, and some species
are soluble in sulphuric acid. Talcs are often deposited from
water as pseudomorphs, in place of other magnesian minerals
which originally formed part of the rock ; but they cannot be
correctly described as hydrated micas, because micas contain
alumina, but may be formed in rocks which were previously in-
filtrated with magnesian silicates derived from decomposed mica,
hornblende, augite, and olivine.6
CH. V.J ROCK-FORMING MINERALS. 87
Micas.
Crys. and Cl. The minerals included under the general name
of Mica, though varying considerably in composition and in some
physical properties, are united by a marked common characteristic,
that is, one extremely perfect cleavage, parallel to the base of an
apparently hexagonal prism ; their crystals, which are often of
enormous size, being as a rule developed in the direction of this
plane, while their other faces are rough and imperfectly developed.
They are probably all monoclinic. H., the knife scratches micas
easily, producing a very characteristic grating sound ; the thumb-
nail scratches them with difficulty if at all. Sp. gr., see under
separate varieties. Col., silvery, bronze-coloured, green or black.15
Lus., semi-metallic to vitreous and pearly.14 Viewed from the
side the cleavage gives them a lamellar appearance and the
characteristic lustre is lost. Comp., two broad chemical groups
may be formed, the alkali micas and the magnesium-iron micas ;
writing the bases in descending order of importance, the micas of
the latter group are silicates of magnesia, alumina, iron, and
alkalies, while those of the former are silicates of alumina,
alkalies, iron, and magnesia.15
Biotite (Ferromagnesian or Black Mica). — Crys., most
frequently disseminated in scales. H., 2'5-3.7 Sp. gr.,
2'8-3'2. Col., various dark tints, from brown through
bottle-green to black.14 Fus., whitens and fuses on the
thin edges. Sol., completely decomposed by dilute HC1 or
H2S04, leaving a residue of glistening scales of silica.7
Occurrence. — Biotite is essentially the mica of modern
volcanic rocks, being found in the lavas of Vesuvius, etc.
In granites and the older crystalline rocks it is associated
with muscovite which is light-coloured.14
Phlogopite. — Crys., occurs not unfrequently in six-sided
prisms with a cross-section approximating to a regular
hexagon and irregular lateral faces ; also in small plates.
H., 2-5-S.7 Sp. gr., 2-75-2-97.14 Col, brownish red.14 Fus.,
whitens and fuses on the thin edges. Sol., attacked by
boiling dilute HC1 and H2S04, but very long boiling is
required for complete decomposition.7
Occurrence. — Essentially characteristic of the Archaean
crystalline limestones of North America, and occurs in
enormous crystals, up to 2J tons weight, with apatite.14
Also in metamorphic limestone and serpentine.6
Muscovite (Potash Mica). — Crys., frequently in rhombic or
hexagonal plates ; sometimes in irregularly shaped scales. H.,
2-2 -5.7 Sp. gr., 2 -83-2 '89. Col., colourless, grey, or light
88 GEOLOGY FOR ENGINEERS. [PT. II.
MICAS AND TALCS (contd.) —
brown.14 Fus., whitens and fuses on thin edges to a grey
or yellow glass. Sol., scarcely, if at all, attacked by acids.7
Occurrence. — The white mica of granite, gneiss, and the
older crystalline rocks generally ; the largest crystalline
plates, which are sometimes as much as 2 feet across, are
found in hollows in coarse granite or pegmatite veins.
Muscovite has been found in slags, and has been formed in
clayey sandstone walls of iron furnaces.6
Lepidolite or lithia mica is similar to muscovite, but con-
tains lithia, etc. It is usually red, pink, or violet, and occurs
in granite and gneiss.6
Distinguishing characters of micas. — Distinguished in rock
from hornblende by lustre, platy character, and hardness ; in
section by single cleavage, ragged fibrous edges, and the fact
that the basal sections are the darkest and show no cleavage.15
The only other two minerals that split up to the same extent
as micas are talc and selenite ; their laminae are flexible,
selenite imperfectly so, but neither of them are elastic,
whereas mica laminae are both flexible and elastic.7
Talcs.
Talc (Steatite). — Crys., probably oblique or rhombic,
similar to mica, being occasionally found in six-sided tabular
forms, having a very perfect basal and traces of a prismatic
cleavage. Usually in foliated, spheroidal, or radiated masses
(talc), also fine, scaly, or compact, with a schistose structure
(steatite or soap-stone). H., talc I;14 steatite up to 2'5.7
Ten., sectile and flexible in thin laminae, but not elastic,
the compact variety rather brittle. Feel., greasy or soapy, in
most cases, and sometimes making a white mark like chalk
upon a rough surface. Col., generally pale green, sometimes
silvery white, the compact varieties passing to dark green or
grey. Lus., pearly or greasy, Tr., transparent in very thin
laminae; imperfectly translucent.14 Comp., H2Mg3Si401215 or
Si02 63-49, MgO 31'75, H20 4'76. Cl. tube, usually gives
off water. Fus., fuses on edges of very thin laminae to white
enamel ; whitens, exfoliates, and becomes luminous. Flame,
with cobalt pale red of magnesia. Sol., not decomposed by
acids.14
Occurrence. — Talc occurs in many mountain districts,
notably in the Alps, both crystallised and forming part of
crystalline schistose masses, as talcose schist, or associated
with chlorite, serpentine, or dolomite. Steatite or soapstone
CH. V.] ROCK-FORMING MINERALS. 89
MICAS AND TALCS (contd.) —
is found in pseudomorphs after various silicates, and in beds
and masses.14 French chalk is a kind of steatite.
Dist. characters. — Its extreme softness ; 7 not as brilliant as
mica.15
Chlorite. — The name of a group of minerals, Pennine,
Clinochlore, Ripidolite, etc., composed of silicates of magnesia,
ferrous and ferric oxides, and alumina in various proportions
with much water. Probably all monoclinic, though many
approach the hexagonal system.15 Cl., basal, very perfect.14
H., 1-2-5. Sp. gr., 2'6-2*9.7 Ten., laminse flexible but not
elastic. Tr., transparent to translucent.14 Col., yellow-green
to blue-green.15 Lus., vitreous, sometimes pearly on cleavage
faces. Fus., whiten and exfoliate, but do not melt easily,
unless rich in iron, when a black slag is produced. Soda,
reaction of iron. Sol., partly decomposed by acids, and more
readily after heating, sulphuric acid being most efficacious.14
Occurrence. — In chlorite slate, protogine gneiss, diabase,
corresponding to mica as a rock constituent.6
Dist. characters. — Laminae are not elastic like mica ; differ
from talc in being more easily decomposed in H2S04 and less
greasy.
Micaceous Iron Ore, see Iron \ Haematite.
Microcline, see Felspars ; Plagioclase.
Muscovite, see Micas and Talcs.
Naphtha, see Asphalt.
Nepheline (Elseolite). — Crys., hexagonal. Brown or greenish,
greasy-looking masses in holocrystalline rocks, or colourless grains
and short hexagonal prisms in lavas. H., 5*5. Very easily
decomposed, and then produces soft, grey-brown areas and pseudo-
morphs. Flame, sodium. Fus., 3%5. Micr., silica. Sol., with
HC1 forms a strong silica jelly.15
Nephrite, see Augite-Hornblende group ; Hornblende.
Ochre, see Iron ; Limonite.
Oligoclase, see Felspars ; Plagioclase.
Olivine. — Crys., rhombic, in granules or approximately
rectangular crystals, somewhat conspicuously marked out from
their surroundings, which are commonly darker silicates.15 CL,
imperfect. Fr., conchoidal. H., 6-7. Sp. gr., 3-23-3-56. Str.,
white.14 Fus., infusible.15 Bor., iron and sometimes manganese.14
Micr., silica. Sol., most common varieties give a silica-jelly with
HC1.15
Occurrence. — Essentially characteristic of volcanic rocks, being
common in basalt, dolerite, and similar lavas. Very liable to
alteration by hydration, which changes it into serpentine.14
90 GEOLOGY FOR ENGINEERS. [PT. II.
Dist. characters. — Transparent, yellow-green appearance charac-
teristic j distinguished from quartz by its solubility.
Opal, see Silica Series.
Orthoclase, see Felspars.
Pearl-spar, see Dolomite.
Pennine, see Micas and Talcs ; Chlorite.
Petroleum, see Asphalt.
Phlogopite, see Micas and Talcs.
Phosphorite, see Apatite.
Plumbago, see Graphite.
Potash Mica or Muscovite, see Micas and Talcs.
Psilomelane, see Manganese.
Puddler's Ore, see Iron ; Haematite.
Pyrite, see Iron ; Pyrites.
Pyrolusite, see Manganese.
Quartz, see Silica Series.
Red Ochre, see Iron ; Haematite.
Bipidolite, see Micas and Talcs ; Chlorite.
Rock-Salt. — Crys., cubic, or as an efflorescence in fibrous
masses and in thin beds of a fibrous structure, like gypsum.
CL, perfect. H., 2-2 '5. Sp. gr., 2 -25. Fr., conchoidal. Ten.,
brittle, but cuts toughly. Col., colourless when pure or with a
slight blue or green tinge. Lus., vitreous. Comp., NaCl or Na
39, Cl 61 per cent.14 Flame, intense sodium. Fus., about 1.
Micr., with copper oxide, strong chlorine reaction. Sol., soluble
in water. Taste, characteristic.15
Occurrence. — With other salts of the same class, gypsum and
anhydrite, in beds and masses of considerable extent in many
geological formations.14
Dist. characters. — Sodium colour in flame.
Ruddle, see Iron ; Haematite.
Sanidine, see Felspars ; Orthoclase.
Satin-spar, see Gypsum.
Schorl, see Tourmaline.
Selenite, see Gypsum.
Siderite, see Iron ; Spathic Iron Ore.
Sienna Earth, see Iron ; Limonite.
SILICA SERIES. — Silica, dioxide of silicon, or anhydride of silicic
acid, occurs in at least three different conditions, each of which is
marked by distinct physical and crystallographic characters.
These are : —
(1) Hexagonal tetartohedral in quartz.
(2) Rhombic or asymmetric in asmanite and tridymite.
(3) Amorphous in hyalite and opal.14
Quartz. — Crys., hexagonal; or, as indicated by the occasion-
CH. V ] ROCK-FORMING MINERALS. 91
ally recurring tetartohedral faces, rhombohedral. The usual
forms are either hexagonal pyramids or combinations of the
hexagonal pyramid and hexagonal prism. Twinning is
common.16 Aggregates radiated in druses with the points
free are common, also fibrous, granular, cryptocrystalline,
massive, and compact aggregates. Cl., rare. H.,1. Sp.gr.,
2'65-2'66. Fr., conchoidal, parallel, splintery in crystals of
lamellar structure. Tr., transparent and colourless in the
purest varieties, translucent in various degrees, opaque in
quartzite. Col., white, grey, yellow, brown, red, blue, violet,
green, and black. Lus., vitreous. Comp., Si02, Si 46'67,
0 53 '33 per cent.14 Fus., infusible. Micr., undissolved.
Ch., fuses readily with soda; cobalt nitrate added to the
glass produces a deep blue glass, as in ordinary fusible
silicates. Sol., insoluble in all acids except hydrofluoric.15
Tridymite. — Cry*., triclinic 14 or hexagonal.15 CL, imper-
fect. Fr., conchoidal. H., 7. Sp. gr., 2'28-2'33.14 May be
seen as thin, transparent hexagonal plates, several being
grouped together, in the cavities of some highly siliceous
lavas. Brittle, and difficult to extract.15
Asmanite is very similar to tridymite.
Opal and Hyalite are amorphous forms of hydrated silica.
Occurrence. — Silica in its various forms is the most
abundant of all minerals ; quartz forms one of the principal
components of granite, gneiss, and mica schist, an accessory
component of many other rocks, and the mass of all quartzites
and sandstones. The finest crystals are usually found in
hollows in granitic rocks where the component minerals have
had room to develop. Quartz is very common in mineral
veins associated with galena, blende, pyrites, and other
metallic minerals. It also occurs in pseudomorphs of many
minerals.
Crystals of quartz, when perfectly limpid and colourless, are
known as rock-crystal, or popularly as Cornish, Bristol, or
Irish diamonds.14
Hornstone or chert, lydian-stone, quartzite, and flint are rocks
chiefly composed of silica, and are described in Chapter VII.
Chalcedony is essentially a mixture of quartz and amor-
phous hydrated silica.
Dist. characters. — The low specific gravity distinguishes
quartz from many colourless gems.15 The hardness is a
distinguishing feature, and tourmaline, the hardness of which
is 7-7'5, is distinguished from it by the habit of its crystals
and fusibility ; epidote, whose hardness is 6-7, has a marked
cleavage and crystals totally unlike quartz.7
92 GEOLOGY FOR ENGINEERS. [PT. II.
Smaragdite, see Augite-Hornblende group ; Hornblende.
Spathic Iron Ore, see Iron.
Specular Iron Ore, see Iron ; Haematite.
Sphserosiderite, see Iron ; Spathic Iron Ore.
Sphene (Titanite). — Crys., oblique, twins common. CL, im-
perfect. If., 5-5-5. Sp. gr., 3-4-3'6. Tr., imperfectly
transparent to translucent. Col., green, yellow, or brown, rarely
red. Lus., adamantine, vitreous.14 Comp., CaSiTi05. Fus.,
fairly easy. Bor. and Micr., titanium reaction, silica in latter.
Ch., the soda residue boiled with tin in HC1 gives a clear
titanium reaction.15
Occurrence. — In granite, crystalline schists, and limestone,
magnetic iron ore, and certain volcanic rocks.14
Dist. characters. — The particular wedge-shaped form of its
crystals, its strong resinous lustre, and its hardness often enable
us to identify sphene without any further tests.7
Steatite, see Micas and Talcs ; Talc.
Sulphur. — Crys., rhombic, pyramidal in habit, twins common ;
also in stalactitic, globular, reniform, and irregular masses, and
in powdery incrustations, the latter known as flour or flowers of
sulphur. Cl., imperfect. H., 1 -5-2-5. Sp. gr., 207. Ten.,
brittle, somewhat sectile. Tr., transparent, translucent, opaque.
Col., sulphur-yellow, passing through orange to brown, and
through primrose- and straw-yellow to white. Lus., adamantine,
resinous. Comp., almost chemically pure in lighter-coloured
crystals ; orange or darker tints often contain selenium or arsenic ;
compact varieties usually mixed with clay, bitumen, gypsum, or
celestine. flame, native sulphur gives a blue flame, but this is not
seen in the heating of sulphides and sulphates. Cl. tube, yellow
sublimate from many minerals, the colour most noticeable when
hot. 0. tube, sulphurous anhydride is often evolved. Ch.,
blackens silver coin after fusion in R. F. with soda and addition
of water to the slag.15
Occurrence. — Common in volcanic districts as a product of
solfataras, as the emanations of steam in the vicinity of volcanoes
are termed.14
Dist. characters. — Colour and smell.
Talc, see Micas and Talcs.
Titanic Iron Ore, see Iron ; Ilmenite.
Titanite, see Sphene.
Tourmaline (Schorl) — Crys., hexagonal; the crystals are
prominently dissimilarly ended, faces of prisms usually striated
vertically ; large crystals often in parallel columnar groups ; also
fibrous in radiated and plumose forms. CL, imperfect. Fr.,
conchoidal, uneven. H., 7-7'5. Sp. gr., 2 '94-3-24. Tr.,
CH. V.] ROCK-FORMING MINERALS. 93
transparent to translucent; opaque in black varieties. Col.,
sometimes colourless, usually black ; less common colours are
green, brown, blue, and red, the latter the rarest. Lus., vitreous.14
Comp., borosilicate of various bases. Flame, some specimens
give boron flame when fused with fluor-spar and bisulphate of
potash. Fus., various, but often easy. Micr., silica.
Occurrence. — In granite and other crystalline rocks.14
Dist. characters. — Distinguished from hornblende by more
resinous fracture and absence of cleavage.
Tremolite, see Augite- Hornblende group ; Hornblende.
Tridymite, see Silica Series.
Umber, see Iron ; Limonite.
White Iron Pyrites, see Iron ; Marcasite.
Zeolites are hydrated silicates of alumina, lime, potash, and
soda, often in fibrous aggregates, usually with a perfect cleavage,
hardness varying from 3'5 to 6. Sp. gr., 2'2-2'4. Col.,
generally milky white, some reddish. They occur filling up
cracks or hollows among lavas or other minerals. The various
species are known as Apophyllite, Prehnite, Thomsonite, Chabasite,
Stilbite, Natrolite, Analcime. Fus., they intumesce and melt.
Sol., generally gelatinise with HC1.15
Zinc-blende (Blende). — Crys., cubic, twins common ; also in
cleavable crystalline masses of various kinds, and massive, of a
compact or finely granular texture in columnar, reniform, and
other concretionary shapes. Cl., very perfect. H., 3 '5-4.
Sp.gr., 3'7-4'2. Ten., brittle. Tr., transparent and translucent
when light-coloured, opaque in dark and compact varieties. Col.,
usually yellow, brown, or black ; compact varieties lighter. Str.,
white. Lus., adamantine or resinous Comp., ZnS, zinc 67,
sulphur 33 per cent.14 Fus., about 6. Cl. tube, thin sulphur.
Ch., zinc incrustation, at times excellent with cobalt nitrate,
poor in other examples ; best produced when specimen is in R. F.
Some varieties give cadmium incrustation ; often magnetic
residue. Soda, sulphur reaction. Sol., effervesces in hot
HC1, sulphuretted hydrogen being evolved.1
[PT. II. CH. VI.
CHAPTER VI.
THE STUDY OF KOCKS.
THE term Rock is applied to any bed, layer, or mass of the earth's
crust whether consolidated or not, not excluding beds of clay and
sand. A rock may consist of one mineral species, as limestone,
or of several intermingled, as granite.10 The minerals may be
either loose, incoherent grains, e.g. blown sand, or coherent
crystals or grains, angular or rounded, cemented by crystalline
or by amorphous matter. The usual cement is either silica,
felspathic matter, carbonate of lime, carbonate of iron, or peroxide
of iron.2
Classification. — Rocks may be named and classified accord-
ing to —
I. Their mode of origin, viz. : —
1. Igneous.
2. Aqueous or sedimentary or derivative.
3. Metamorphic and altered.
II. Their chemical and mineralogical composition.
III. Their structure.
The division according to mode of origin into igneous, aqueous,
and metamorphic rocks, which has been already adopted in
Chapter III., Structural Characters of Rocks, will be followed in
Chapter VII., in which the characteristics of rocks are described.
In the first three sections of this chapter the mode of origin,
chemical composition and mineral constituents, and structure
are treatedly separately, the subject-matter of each section being
subdivided under the heads of Igneous, Aqueous, and Metamorphic
Rocks. The physical characters of rocks are described in a
separate section.1
Section I. — Mode of Origin.
IGNEOUS ROCKS.
Plutonic or abyssal rocks are those which consolidated at
considerable depth within the earth's crust.
94
SECT. I.] THE STUDY OF ROCKS. 95
Volcanic rocks are those which consolidated from fusion under
superficial conditions. A plutonic rock may have exactly the
same mineralogical composition as a volcanic rock (see Section
II., Mineral Constituents}; but, owing to the different conditions
under which it solidified, it will differ in the following points : —
(1) It will contain no vesicular (p. 98), slaggy, or glassy portions.
(2) It will generally be more coarsely and completely crystalline.
(3) It will not be stratified.
(4) The crystals will probably contain water-cavities.
The term Hypabyssal is used by petrologists for rocks filling
necks, dykes, etc., and so forming a connecting link between
volcanic and plutonic rocks, but the two main divisions are
sufficient for the purposes of the engineer.1
AQUEOUS ROCKS.
Arenaceous or sand rocks are typically fragmented or clastic
in character, viz. composed of grains, derived from the waste of
igneous rocks, held together by a cement or base.
Argillaceous or clay rocks similarly consist of derived
elements held together by a fine textured base or paste and
retaining enough moisture to be plastic.
Calcareous or lime rocks are chiefly of organic origin.1
ALTERED AND METAMORPHIC ROCKS.
The mode of origin of these rocks has already been described
in Chapter III., but some of the chief effects of metamorphism
may be noted here.
Igneous rocks are frequently altered by thermal metamor-
phism, the principal changes being the replacement of one or
more minerals by others in the vicinity of the region of thermal
activity, e.g. intruded granite. The acid rocks are less liable
to thermal metamorphism than the intermediate and basic rocks.
In Arenaceous rocks the effects of thermal metamorphism
depend on the nature of the deposits. A pure quartz sandstone or
quartzose grit will be changed into a homogeneous quartzite, while
if the original rock was impure and contained other substances,
silicates of alumina, garnet, micas, etc., may be produced and the
rock may assume a gneissose character.
Among Argillaceous rocks clays are altered into slates and
shales, and when more highly metamorphosed the whole body of
the rock becomes altered into schists or compact masses like
hornstone.
Calcareous rocks are altered into marbles' and crystalline
limestones, etc.1
96 GEOLOGY FOR ENGINEERS. [FT. II. CH. VI.
Section II. — Chemical and Mineralogical Composition.
The percentage chemical composition of a fragment of rock
depends on the chemical composition of the various mineral and
chemical substances of which the rock is composed, and is only of
service in so far as it affords an indication of the nature of the
various substances.
The mineralogical composition is of great importance to the
engineer, to enable him to ascertain the comparative durability of
his materials.
General Terms. — The following terms are used to denote the
composition of rocks : —
Felspathic, consisting of, containing, or resembling felspar.
Siliceous, composed of or containing silica.
Quartzose, composed of or containing quartz.
Gypseous, having the properties of or containing gypsum.
Pyritous, having the property of one of the native metallic
sulphides known as pyrites, though the term is often restricted to
iron pyrites.
Carbonaceous, pertaining to or yielding carbon.
Saliferous, containing a considerable proportion of salt in
beds.
Micaceous, composed of or containing layers or flakes of
mica.1
IGNEOUS ROCKS.
The magma or ground mass is invariably composed of silica,
combined with the bases iron, alumina, lime, potash, and soda.
When the silica is in excess of the bases, the rock is said to be
acid or acidic ; when the percentage of silica is low, the rock is
said to be basic.1
Groups. — Igneous rocks may therefore be divided into groups,
according to their percentage of silica, as follows : —
Acid group with 65 to 80 per cent, of silica. Sp. gr., below
2 '7 5. Granites, el vans, rhyolites, felsites.
Intermediate group with 55 to 70 per cent, of silica. Sp. gr.,
2 '70 to 2 '80. Syenites, diorites, trachytes, andesites, porphyrites.
Basic group with 45 to 60 per cent of silica. Sp. gr., 2*80 to
3 '00. Gabbros, dolerites, basalts.
Ultra-basic group with 35 to 50 per cent, of silica. Sp gr., 2'85
to 3-4. Peridolites.2
The Acid group is distinguished by the presence of free silica
or quartz in more or less abundance. The chief felspar is ortho-
clase, but plagioclase also occurs.
SECT. II.] THE STUDY OF ROCKS. 97
The Intermediate group is characterised by rocks containing
little or no quartz and more plagioclase felspar than ortho-
clase.
In the Basic group the rocks usually contain no quartz and
very little orthoclase, but olivine is very often present.
In the Ultra-basic group the rocks are largely composed of
olivine combined with other ferro-magnesian minerals and iron
ores.1
Chemical Constituents. — The oxides of iron and magnesium
are of considerable importance, especially in the basic rocks.
The alkalies, potash and soda, are, however, the most important
constituents of rocks, notably in felspars, micas, amphiboles, and
pyroxenes. Phosphoric acid and titanic acid are present in most
basic rocks in the shape of phosphate of lime (apatite) or titani-
ferous iron ore (ilmenite) and sphene (titanosilicate of lime).
Fluorine, chlorine, and sulphur also occur.1
Mineral Constituents. — Plutonic and volcanic rocks are,
speaking generally, composed of the same minerals, felspars, micas,
hornblende, augite, and other common silicates being their
principal components. Sometimes plutonic and volcanic rocks
are even composed of the same minerals mixed in the same
proportions.1
AQUEOUS ROCKS.
Arenaceous Rocks. — The commonest constituents of sands are
minerals, such as white mica and quartz, which are least liable to
chemical change, as the materials which formed the rocks from
which the sands were derived have probably been subjected to
chemical action during the processes of disintegration, transporta-
tion, and deposition.
Other constituents may be found locally, such as garnet, flint,
tourmaline, or ilmenite. The cement may be calcareous, ferruginous,
or siliceous.1
In Argillaceous Rocks the constituents cannot easily be
identified owing to their minuteness. The derived portions may
be quartz, felspars, or micas ; carbonates, pyrites, and glauconite
also occur. The base, which is of exceedingly fine texture, is prob-
ably often of micaceous origin, though formerly it was supposed
to be kaolin.1
Calcareous Rocks. — These are composed, as a rule, of calcareous
organisms, the hard parts of which consist chiefly of calcite or
aragonite (see Chapter VII. , Section II., p. 116). Impure calcareous
rocks contain sand and fine detritus, etc. In dolomitic limestones
and dolomites a portion or the whole of the calcite is replaced by
dolomite.1
7
98 GEOLOGY FOR ENGINEERS. [PT. II. CH. VI.
ALTERED AND METAMORPHIC ROCKS.
The principal change in composition is due to recrystallisation,
and while the chief original minerals are not much altered,
accessory minerals are developed during the process of
alteration.1
Section III. — Structure.
As in the case of minerals (vide Chapter IV., Section III., p. 63)
the terms " structure " and " texture " are often used indiscrimin-
ately, but it is preferable to limit the use of the latter term to
the nature of the surface, as rough, even-grained, etc., while
the former term is used to denote the method in which
the component parts of a solid are built up.1
. General Terms. — The various kinds of structure of rocks are
described below according to their classification as Igneous,
Aqueous, or Metamorphic, but the following terms are used in a
general sense : —
Crystalline, composed of angular grains or particles more or
less crystallised in place, and not of rounded fragments of pre-
existent masses. For "Holocrystalline," "Hemicrystalline," "Micro-
crystalline," see under Igneous Rocks, groups 1 and 2.
Cryptocrystalline, composed of minute crystals invisible to the
naked eye.
Granular, composed of approximately equal grains, either
crystalline in outline or rounded by attrition.
Cellular or Vesicular, containing small spherical or bubble-
shaped cavities. For the "Pumiceous," " Scoriaceous," and
" Amygdaloidal " varieties of this structure see under Glassy and
Lithoidal Rocks.
Compact, so closely grained that no component particles or
crystals can be recognised by the eye — a term used in field
observation (see Chapter X., Section III., p. 201).
Massive, without definite crystalline form.1
IGNEOUS ROCKS.
For the purpose of studying their structure, igneous rocks may
be divided into three groups: — (1) holocrystalline rocks, (2)
lithoidal rocks, (3) glassy rocks. The first of these groups
consists generally of plutonic rocks (see Mode of Origin, ante),
and the third of volcanic rocks, whilst the second group comprises
both of these varieties.
SECT. III.] THE STUDY OF ROCKS. 99
The following kinds of structure are common to all three
groups : —
Granitic, resembling granite.
Porphyritic. — In most igneous rocks there are two phases of
crystallisation. In the first phase, well-defined, relatively large
crystals, known as Porphyritic^ float in a molten base or magma.
In the second phase the magma consolidates and forms the ground-
mass in which the porphyritic crystals are embedded. When the
crystallisation is complete the ground-mass is crystalline, but
usually there is a glassy base.
The porphyritic and granitic are the two principal structures
found among plutonic rocks.1
Felsitic Matter. — When devitrification takes place owing to
meteoric influence the glassy base is replaced by a cryptocrystal-
line aggregate of quartz and felspar known as Felsitic Matter.
This may also be developed during consolidation.1
Columnar. — Showing a tendency to cleave into columns, as in
the basaltic columns of the Giant's Causeway, or in the irregular
columns seen in many granitic rocks.1
Spheroidal. — The rock breaks up into roughly or regularly
concentric coats. The coarser type of this structure may be seen
in granites, and its most delicate type as the perlitic structure of
glassy rocks.
Drusy. — Crystals, often of great beauty, are developed in the
walls of cavities in the mass.
Banded Structure. — The crystals or masses of differing com-
position are carried out by flow into separate bands. Common in
glassy rocks.15
Group 1. — Distinctly Holocrystalline Rocks.
The term " holocrystalline " is used to denote rocks which
are completely crystalline without admixture of amorphous
material.
Pegmatitic or Graphic. — Two constituents, most commonly
quartz and felspar, have developed simultaneously in large
crystals mutually intergrown. The felspar being predominant,
the quartz appears as hook-shaped and irregular forms apparently
disconnected from one another. Graphic granite provides the
best and almost only type.15
Fluidal Gneissic.— The banded or foliated structure of many
holocrystalline rocks arises in some cases during their original
flow, and may be designated as above, to distinguish it from the
metamorphic gneissic structure (see under Metamorphic Rocks).
100 GEOLOGY FOR ENGINEERS. [PT. II. CH. VI.
The smaller constituents flow round " eyes " formed by the larger
ones, and sometimes the intrusion of a non-homogeneous magma
produces a banded structure on a handsome scale.15
Ophitic. — Often with the eye the crystals of one constituent
will be seen to have developed freely, while another constituent
has settled down in large crystals round them, so that the inter-
spaces of the former are filled over considerable areas by material
having parallel cleavage-surfaces or crystal-faces. On turning the
rock specimen in the hand, the light will glance from some such
surface and show the real continuity of areas that appear distinct
from one another on the broken surface of the rock. This structure
is common in dolerites and diabases.15
Orbicular. — A rare structure in which the crystals are grouped
so as to form spheroidal aggregates with or without radial or
concentric arrangement. A fine example is the orbicular diorite
("Corsite") of Corsica.15
Group 2. — Lithoidal Rocks.
This group includes rocks of dull, very close-grained texture,
giving them a " stony " appearance, such as the " lithoidal lavas " of
old Continental writers, which may, or may not, contain some
glassy matter. All the common lavas and most porphyries are
included.1
Hemicrystalline Structure. — The matrix is compact and often
almost vitreous to the eye. The lens will sometimes show
spherulites aggregated together, and banded and fluidal structures
may appear. Such rocks, consisting of a close admixture of
crystallites, crystals, and glass, are often called Crypto-
crystalline.1^
Microcrystalline Structure. — The individual constituents
become fairly distinct with the lens, though very possibly not
specifically determinable by this means ; the microscope reveals
no glass.
Scoriaceous Structure occurs commonly in the rocks of this
group 15 (see under group 3).
Horny. — Hornlike and slightly lustrous and translucent, like
flint and chert.
Group 3. — Glassy Rocks.
Perlitic Structure is a microscopic form of spheroidal structure.
It consists in the presence of cracks having approximately spherical
forms, caused by the contraction of the rock as it cooled.
Spherulitic Structure is characterised by the presence of
SECT. III.] THE STUDY OF ROCXS, . 101
spherules or globules, each of which is generally composed of
fibrous crystals, which radiate from a centre. It is distinguish-
able from the surrounding glass by its different colour and
appearance.1
Lithophyse Structure. — A comparatively rare form of spheru-
litic structure. The lithophyses (stone-bladders) were so named
from the supposition that their hollows were caused by the
expansion of vapours in the interior. The lithophyse looks like
a large spherulite, the concentric coats of which are separated
from one another by interspaces in which minute crystals have
commonly been developed. In older examples these hollows have
been filled up.15
Fluidal Structure. — Though commonly associated with banding,
this texture may occur in a simple form when, owing to the
motion of the rock, all the crystallites and crystals are carried
along with their longer axes parallel to one another.15
Pumiceous and Scoriaceous. — The rock may be completely
glassy, with numerous elongated steam- vesicles, as in pumice ; or
more lithoidal and less completely vesicular, as in common
scoriae (see "Cellular," under General Terms, p. 98). Such
rocks often become amygdaloidal when the cavities are filled
with alteration products ; so called from the supposed almond-
like shape of the cavities.
Group 4. — Volcanic Fragmental Rocks.
The finer portions of the fragmentary materials thrown up by
volcanoes (see Chapter II.) are soon converted into mud by the
action of steam, and form a cement to weld together the larger
fragments.
Agglomerate. — Composed of an unstratified mass of fragments
of compacted volcanic debris.
Brecciated. — Composed of breccia or angular fragments.1
AQUEOUS ROCKS.
These may be divided into (1) coarsely fragmental rocks, (2)
ordinary stratified rocks, which will form groups 5 and 6 of the
whole series.
Group 5. — Coarsely Fragmental Rocks.
Fragmental or Clastic rocks are composed of fragments of pre-
existent rocks which have become cemented together.
Brecciated (see group 4, Volcanic Fragmental Rocks). — A
breccia may also be produced by the crushing of a rock owing
102 'GEOLOGY FOR ENGINEERS. [PT. II. CH. VI.
to earth-movements, the fragments being afterwards cemented
together : in such a breccia parts recognisable as having belonged
to the same fragment of the original mass may be expected to
be occasionally found in close proximity to one another.1
Conglomerate Structure. — Composed of fragments the edges
of which are rounded. The same is generally applied to rocks
the fragments of which are of considerable size.1
Group 6. — Ordinary Stratified Rocks.
The points to be noted are the character of the bedding and the
degree of coarseness of the constituents.15
Laminated Structure (see Chapter III., p. 38).— Divisible into
thin layers. Usually the rock splits easily along the planes of
lamination, but sometimes the laminae cohere so firmly that
the rock will break more easily in some other direction.1
Oolitic. — Formed of egg-like granules with concentric coats
and often a central nucleus of some fossil or mineral fragment.
Common among limestones.15
Pisolitic. — A coarse development of oolitic, with grains as large
as peas.
Concretionary. — Composed of inorganic matter which has been
aggregated in nodules or lumps round some central point or
nucleus.1 On being broken open they often show shrinkage-
cracks filled with products of infiltration, giving rise to a septarian
structure.15
Pebbly. — Containing small, water- worn fragments or pebbles, as
in the case of coarse sandstones, which thus pass into conglomerates.
Psammitic or resembling sandstone.1
ALTERED AND METAMORPHIC ROCKS.
These rocks fall naturally into three groups, forming groups
7 to 9 of the whole series : —
(1) Those which retain traces of bedding.
(2) Foliated and schistose rocks.
(3) Amorphous rocks.
Group 7. — Rocks retaining Traces of Bedding.
The structural characters are partly those due to the original
rocks and partly those set up by the action of heat and pressure
during alteration.
Crystallisation, as in the case of quartzite and crystalline
SECT. III.] THE STUDY OF ROCKS. 103
limestone, is usually incomplete, but is evidenced by the additional
hardness and frangibility.1
Cleavage (see Chapter III., Section III., p. 47) is a fissile structure
brought about by heat and pressure, and is best seen in slates.1
Cleavage must be distinguished from lamination, hand specimens
at times leaving this point unsettled. Traces of the original
bedding must be keenly looked for, and hard, resisting bands or
coloured stripes at an angle to the cleavage-planes often afford the
necessary evidence. Fossils will sometimes be found distorted on
the cleavage-planes. A rippled, wavy structure, the herald of
foliation, often causes the cleavage to become imperfect.15
The fluidal structure referred to under Igneous Rocks, group 3,
in this section is seen in vitrified sandstones.1
Group 8. — Foliated or Schistose Rocks.
Foliation (see Chapter III., Section III., p. 49) consists in the
grouping of the mineral constituents along surfaces that are
parallel to, or follow, the curvature of one another. Although
the development of minerals, notably mica, along some cleavage-
planes connects cleavage and foliation, in many cases the latter
structure is due to the rolling out, as in a mill, of previously
crystalline materials, so that each fragment assumes the form of
a much extended lenticle. Hence it is important to trace, if
possible, the passage of a foliated rock into one with normal
structure, whether igneous or sedimentary, and too much care
cannot be devoted to the question as to what minerals in the
schistose product are deformed primary substances, and what
have, on the other hand, been developed during the period of
crush and pressure.15
The resistance of large, pre-existing crystals produces the
eye-structure of many gneisses, the smaller constituents flowing
round the larger ones and tailing out in streams on either side.
This structure is best seen on surfaces perpendicular to the planes
of foliation.15
For Mylonitic and Granulitic structures see Chapter VII.,
Section III., Distinctly Foliated Rocks, p. 127.
Group 9. — Amorphous Metamorphic Rocks.
These occur under their most typical form in masses, and then
pass gradually along their margins into some form of foliated
rock, which in its turn shades away into less highly metamorphosed
beds, and so on till unaltered clastic strata are reached.7
104 GEOLOGY FOR ENGINEERS. [FT. II. CH. VI.
Section IV. — Physical Characters.
Hardness is a character of the immediate constituents or
minerals of which rock is composed, and is only important as a
rock-character when the rock is so fine in grain that the hardness
of the individual constituent cannot be separately determined or
when the adhesion of the different constituents to each other is
of appreciable importance as compared with the cohesion of the
parts of a constituent.
For determination of hardness see Chapter X., Section III.,
p. 201, and Chapter XL, Section I., p. 207. The scale of hardness
is given in Chapter IV., Section III., p. 66. l
Fracture. --The character of the surface of fracture of a rock
depends on the kind of fracture of each of the constituents, on the
sizes and arrangement of the constituents, on their modes of
union, and on their cohesive and adhesive power. The terms
used are the same as in the case of minerals (see Chapter IV.,
Section III.), and the following are typical examples :—
Conchoidal . . . Flint.
Even .... Chert.
Uneven . . . Basalt.
Splintery . . . Cast Iron.
Earthy . . . Chalk.1
Colour and Lustre. — Owing to the varieties of colour and
lustre met with in one and the same rock, they are comparatively
unimportant, but some indication of the nature of a rock may be
obtained from them if due caution is observed.1
Iron is one of the most important colouring agents. Scarcely
any rock is free from iron. In many it is present as ferrous
carbonate, which is white when pure and therefore imparts no
colour to the rock. Rocks which contain iron under this form
are usually bluish or greyish, the colour being due sometimes to
organic matter, sometimes to various inorganic substances.
Rocks, however, seldom show this bluish or greyish hue except at
some depth below the surface, or where they have been otherwise
shielded from the action of the air. Where they have been
exposed they are commonly red, brown, or yellow.
Ferrous carbonate is an unstable compound, and under the
oxidising influence of the atmosphere and of water becomes con-
verted either into ferric oxide (2FeC03 + 0 = Fe203 + 2C02) or one
of the ferric hydrates, and the colours given by these compounds
are strong enough to overpower the original grey hue of the rock.
Ferric oxide colours red ; ferric hydrate generally produces some
SECT. IV.] THE STUDY OF ROCKS. 105
tint of brown or yellow, the exact shade depending perhaps on
the degree of hydration.
The student may observe instances of this change of colour in
the sinking of shafts or wells : the sandstones brought up from
any depth are almost invariably blue or grey; the same beds
when quarried at the surface are brown or yellow. The same
difference may be noticed between the top and bottom beds of a
deep quarry. It is not uncommon, too, to come across blocks of
stone which are blue inside, "blue-hearted," and have a brown or
yellow outside crust. This change has naturally gone on to a
larger extent in porous rocks, like sandstone, than in impervious
clayey rocks.
The blue colour of rocks is caused by finely disseminated iron
pyrites in some cases, in others perhaps by ferrosoferric phosphate ;
the latter salt may also be the cause of the green colour of certain
rocks, while in other cases this colour may be due to a silicate of
iron, and sometimes perhaps to a ferric hydrate, or a ferrosoferric
hydrate.7
A white colour may be due to the absence of metallic oxides, or
to weathering or bleaching (see Chapter VII., Section IV., p. 132).
Organic matter will colour clays and other rocks from light grey
to black ; and in some sandstones black patches of colour are due to
the presence of peroxide of manganese. Carbonaceous matter, of
course, usually gives a black colour, and so at times does iron in
the form of ilmenite or magnetite.1
Lustre. — The terms used for minerals (see Chapter IV., Section
III., p. 67) apply equally to rocks, but this quality is not of the
same value in the latter case.1
Streak. — While the hardness is being tried the colour and
lustre of the streak or mark left on paper by the abraded powder
(cf. Chapter IV., Section III., in the case of minerals) should also
be observed.1
Feel and Smell are distinctive in the case of certain rocks, e.g.
talcose and other magnesian rocks often have a soapy or greasy
feel, and trachyte is notably rough. Some rocks have a distinct
bituminous odour.1
Specific Gravity and Fusibility (see Chapter XI., Section I.).
Magnetism is important in the cases of rocks containing
magnetite, etc. (see Chapter XL, Section I.).1
[PT. II. CH. VII.
CHAPTER VII.
KOCKS.
IN this chapter an attempt has been made to describe the more
important rocks in such a way that the engineer may be able
to distinguish them with comparative ease. The science of
petrology, has, however, advanced considerably of late years
and the various types have been found to slide into one another,
so that, especially among igneous rocks, the nomenclature is almost
bewildering and differs, moreover, in the various text-books. For
more detailed descriptions, therefore, especially as regards micro-
scopic characters, reference should be made to text-books on
petrology.1
Section I. — Igneous Rocks.
PLUTONIC ROCKS.
Granites. — Belong to the Acid group; specific gravity about
2-65.15 The typical granitic structure is holocrystalline, with no
paste or matrix, the crystals or grains all touching one another.
They are usually compact, but sometimes porous, and the crystals
vary in size from a mustard seed to the size of a closed fist.6
Varieties of granite due to structural differences are porphyritic
granite, gneissose granite, graphic granite, pegmatite, and
eurite ; see below.
Granites are aggregates of quartz and felspar with mica, horn-
blende, or augite as accessories. Varieties due to differences in
composition are described below.
The quartz usually occurs in more or less angular grains, but
not often with the crystalline faces perfectly developed.6 It is
recognisable by its vitreous lustre, conchoidal fracture, and
absence of cleavage, and is either colourless or has a smoky
tinge.
The felspar is usually orthoclase, but plagioclase (oligoclase or
albite) occurs ; orthoclase is generally the predominating
mineral.1 It usually occurs in twin crystals, cleaves with a
106
SECT. I.] ROCKS. 107
pearly fracture; and gives granite its characteristic colour, being
pink, red, or brownish red, reddish brown, white, yellowish grey,
green or reddish grey, and even blue in Connecticut and the
Pyrenees. The oligoclase is less transparent, contains more soda
than orthoclase, is more fusible, and has a grey or greenish tinge.6
Albite and labradorite also occur.7
The mica generally occurs in thin plates which are often
hexagonal. Crystals are rare. It varies in colour, being silvery
white, brown, or black. The white potash mica (muscovite) is
rarer and more diffused than the black magnesian mica (biotite).
Both kinds often occur together.6 The dull edges of biotite
crystals often resemble fibrous hornblende, but the lustre of the
basal planes will easily serve to identify them.15 Certain large-
grained granites contain lithia mica.6
The hornblende crystals are irregular. They show a prismatic
cleavage and are green or brownish green.
Granite proper contains both light and dark micas, and the
quartz and felspar are in approximately equal proportions.
Muscovite granite has white mica only.1
Biotite granite or granitite has only dark mica, oligoclase pre-
dominates, and quartz is of reduced importance.
Hornblende granite or syenitic granite is intermediate between
typical granite and syenite. It contains less quartz than granite,
and hornblende to a large extent replaces the mica, which is
always dark.6
Augite granite is of rare occurrence.1
Protogenic or talc granite of the Alps has the same composition
as granite, but contains in addition a pale green, talc-like
mineral. Its quartz is easily broken. The oligoclase has a
greenish tinge, while the orthoclase is grey. The mica is usually
in six-sided plates. The talc is only freely developed when the
rock becomes schistose.
Gneissose granite is granite which has a schistose character.
Graphic granite is also schistose, but consists of orthoclase and
quartz so arranged in parallel layers that a transverse fracture
exhibits the quartz in forms suggesting letters of an Oriental
language. It occurs near Ilmenau, and by Limoges, etc.
Pegmatite is a kind of giant granite in which the crystals of
orthoclase are sometimes a foot long, and the white mica occurs
in large flakes. It is only known in other granite, and generally
contains tourmaline, garnet, topaz, etc. It is seen near Penig in
Saxony. Sometimes the greater part of the rock is formed in a
milk-white quartz. It occurs in Ireland, and is frequently
cavernous, with the walls of the cavities covered with crystals.
Tourmaline granite is granite in which the mica is partly
108 GEOLOGY FOR ENGINEERS. [PT. II. OH. VII.
replaced by schorl. The felspar is flesh-coloured, and there is
very little quartz.6
Porphyritic granite contains large porphyritic crystals of ortho-
clase. The ground-mass is composed of the same constituents as
ordinary granite.
Eurite is the name given by some authors to a white micro-
granite which is also called Aplite and Granulite.1 Under the
name Eurite some writers also include quartz and felspar
porphyries, elvans, and felsites.15
Elvan is the Cornish name for certain 'granitic and porphyritic
rocks.
Granite occurs in large masses or bosses with veins and dykes
(see Chapter XIII. for further details). It has also remarkable
weathering properties J (see Chapter VII., Section IV.).
Syenites belong to the Intermediate group. Specific gravity
about 2'75.15 The structure is like that of granite, and the texture
is even-grained. It consists essentially of orthoclase felspar with
one of the ferromagnesian group. According to the nature of
the latter we have hornblende syenite or syenite proper, augite
syenite, and mica syenite? Syenites are typically without quartz.6
The felspars resemble those of granite.1
The hornblende is usually green, but sometimes a deep brown.
It commonly occurs in lamellar and columnar crystals, and
encloses magnetite, apatite, brown mica, and titanite. The augite
is usually colourless or very pale green. The mica (biotite) is
brown or green ; both colours may occur together.6
Nepheline Syenite. — The nepheline in the coarse elaeolite form
resembles brownish or greenish quartz, but may be distinguished
by the knife 15 and by its characteristic greasy lustre.1 The
varieties with hornblende have been called Foyaite, from Foya in
Algarve, and those with mica Miascite, from Miask in the Urals.15
Compact Syenite. — The fine-grained form corresponding to the
eurite form of granite sometimes has porphyritic orthoclase,
whence came the old name " orthoclase-porphyry " (see under
Porphyry, below).1
Porphyry. — A general term denoting rocks which contain an
alkali felspar and occupy a position structurally between porphy-
ritic granites and rhyolite.10
Granite Porphyry is similar to porphyritic granite, but the
matrix is finer and more compact than that of a granite.
Quartz-Porphyry (see under Eurite).1 — Specific gravity about
2'65.15 Belongs to Acid group. Composed of colourless, white, or
smoky porphyritic crystals of quartz and felspar in a ground-mass
of the same minerals. Is compact in structure, the ground-mass
being microcrystalline or felsitic.
SECT. I.] ROCKS. 109
Felspar-Porphyry, also called Orthoclase-Porphyry, Syenite-
Porphyry^ or Orthophyre, contains little or no free quartz and
belongs to the Basic group. It bears the same relation to
syenite that quartz-porphyry does to granite, and includes many
so-called Felstones.1
Diorite. — Intermediate group. Specific gravity, 2*85 to 3-0.
Like syenite it has a holocrystalline granitoid structure, while
ophitic structure is found in the more basic varieties. Differs
from syenite in having a soda-lime felspar instead of orthoclase
as one of its principal constituents. - The other principal con-
stituent is usually green hornblende, but mica, augite, and
enstatite also occur. Quartz may be present, and when in
considerable quantity the rock is termed Quartz- Diorite.1
G-abbro. — A granitoid rock, used to some extent for building
under the commercial name of granite. Belongs to both the
Intermediate and the Basic groups. Specific gravity, 2-9 to 3 -02.
Holocrystalline and granitic in structure, the texture varying from
medium to coarse grain. Consists essentially of a lime-soda felspar,
usually labradorite, but sometimes anorthite, and a pyroxene,
usually diallage, which often fills up the spaces between the felspar,
its cleavage surfaces having a marked metallic or pearly lustre.
In Gabbro proper diallage or augite predominates ; Hornblende
Gabbro contains hornblende in addition, which is green or brown ;
Olivine Gabbro, in addition to hornblende and augite, contains
olivine which, when fresh, is colourless, but is often stained with
limonite ; in Norite diallage is replaced by hypersthene which
has a coppery lustre.1
The name gabbro is sometimes restricted to varieties contain-
ing olivine, which are more basic ; while the intermediate types,
in which no olivine is present, are known as Pyroxene Diorites.15
VOLCANIC ROCKS.
Ehyolite. — The volcanic equivalent of granite, and corresponds
with quartz-porphyry. It is also called Quartz- Trachyte and
Liparite.1 Specific gravity about 2'5.15 It is compact, lithoidal,
or porphyritic, often with marked fluidal, perlitic, and spherulitic
structure, and has a glassy ground-mass. Highly acidic.
Consists of quartz and orthoclase with either mica, hornblende, or
pyroxene in a light-coloured ground-mass, like that found in
trachyte, chiefly composed of microliths of felspar.
Nevadite is a crystalline and granitoid variety of rhyolite.
Obsidian is a term which includes both rhyolite glass and
trachyte glass.1 These glasses have a low specific gravity, a
marked conchoidal fracture, and a high fusibility.15
110 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
Felsite. — This term was formerly used to describe many rocks
which are now known as rhyolites, but its use is now restricted
to the felsitic structure described in Chapter VI., p. 99.
Pitchstone is almost identical with obsidian, but is less glassy
and has a greasy or pitch-like lustre and a fracture more or less
conchoidal and at times rather splintery. It contains more
water than obsidian and is generally dark green or brown in
colour, but sometimes dark yellow or red.
Trachyte. — The volcanic equivalent of syenite, and corresponds
with felspar-porphyry.1 Intermediate group.1 Specific gravity
abont 2 '5. Compact and lithoidal in structure and very often
scoriaceous, causing the rough texture from which the name is
derived. Usually pale in colour, but reddish, yellowish, or even
black trachytes are found.15 Constituents are the same as those
of syenite, the orthoclase being usually sanidine in large, plate-
like crystals which are porphyritic, the ground-mass being
felsitic.
Phonolite or Nepheline-Trachyte is the volcanic equivalent of
nepheline-syenite and is commonly known as Clinkstone, as it
emits a ringing sound when struck.1 Intermediate group.
Specific gravity about 2*55.15 Compact, lithoidal, or glassy in
structure, of greyish-green colour and spotted appearance.
Sometimes has a fissile character and splits into slabs which can
be used for roofing. The fissile character is intensified by
weathering, which also includes a spheroidal or onion-like
structure in the decomposing rock.
It consists of sanidine and nepheline with a ferromagnesian
constituent ; sometimes leucite is present in combination with or
replacing nepheline.1
Andesites. — Dark-coloured lavas prevalent in the Andes ; the
volcanic equivalent of diorite. They belong to the Intermediate
group and consist chiefly of a glassy, plagioclase felspar with
mica, hornblende, or pyroxene, and a lithoidal to glassy ground-
mass. The absence of orthoclase is characteristic.1 They vary
in colour from grey to dark green, and when hornblende abounds
may be dark brown or black.6 They occupy an intermediate
position between trachyte and basalt.
Trachytic Andesite (Mica or Hornblende Andesite) has a
structure like that of trachyte, and is commonly porphyritic.
The ground-mass is characteristically trachytic, and the colour is
usually darker than that of trachyte. Specific gravity i
about 2-75.15
Basaltic Andesites (Pyroxene Andesites). — Structure lithoidal,
sometimes with glassy interspaces between the crystals. They
are darker than the trachytic andesites, and approach basalts in
SECT. I.] ROCKS. Ill
texture, becoming even black and notably heavy. The fracture
is conchoidal. Specific gravity, 2'75 to 2-9.15
Qiiartz-Andesite, or Dacite, contains a considerable proportion
of quartz, but otherwise resembles the trachytic andesites,
though it has some features in common with rhy elites.1 Specific
gravity about 2'65.15
Altered Andesites, in which the glassy matrix is replaced by a
brown earthy base, are sometimes called Porphyrites, but this
term is now used for rocks resembling porphyry, but having a
soda-lime felspar; they are the plutonic equivalents of andesite.1
Basalt Rocks. — Very compact, black, dark brown or greenish
rocks varying in structure from holocrystalline to semi-vitreous
and sometimes porphyritic or ophitic.1 They often form immense
dykes with a tendency to cleave into hexagonal columns as on
the Giant's Causeway.10 They consist of plagioclase and augite
with olivine in the most basic varieties, and magnetite, ilmenite,
and apatite. Specific gravity, 2 '9. They belong to the Basic
group, and are the volcanic equivalents of gabbro.
Basalt proper consists of augite and plagioclase felspar ; when
olivine or hornblende occurs the rock is called Olivine basalt or
Hornblende basalt. Leucite and nepbeline also occur.
Dolerite is sometimes classed with gabbro as a plutonic rock,
but it is usually considered to be a coarse variety of basalt.
Diabase is a name given to a doleritic rock in which a greenish
chloritic colour has been given by the alteration of the olivine or
augite.1
Peridotite. — A name used for a basalt or dolerite rich in olivine,
chiefly noticeable for their alteration into serpentine (see
Section III., p. 124).
VOLCANIC FRAGMENTAL ROCKS.
As these rocks are bedded, they are often included with other
bedded rocks under the general term "Sedimentary Rocks," but
it seems better to class them according to their mode of origin
among the igneous rocks.
As regards volcanic ejecta see Chapter II., p. 27.1
Volcanic Sands are mere water-worn deposits the materials of
which have been derived from some neighbouring volcanic area.15
Volcanic Agglomerates or Coarse Tuffs. — The constituents are
blocks of volcanic or more deeply seated rocks, angular and often
of considerable size. These are frequently scoriaceous and
amygdaloidal, and represent the more vitreous parts of lavas.
Spheroidal, bomb-like forms may be looked for as well as twisted,
ropy types coloured externally a rusty brown. The ground-mass
is formed of similar smaller fragments and fine dust, and the
112 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
whole becomes in older examples as firmly cemented together as a
conglomerate, the joint-planes traversing the included blocks and
the binding material alike.15
Tuffs aud Ashes. — The tuffs are so often altered soon after
deposition, owing to the attacks of volcanic vapours, that their
former character is lost, and they appear compact and even
uniform on newly fractured surfaces. Weathering, however,
reveals the coarsely fragmental structure, and develops again the
scoriaceous character of many of the included blocks. Examples
of the weathered surface should always be collected. The beds
will be found, on tracing them out, to vary considerably and
rather rapidly, and to present, if deposited on land, marked
variations in thickness.
The loose tuffs of Late Tertiary volcanoes are readily recognised.
The embedded crystals, such as augite or felspar, and the blocks
of lava, will enable one to ascertain the character of the materials
that rose in the volcanic vent. Earlier and consolidated beds
will, however, be sometimes blown to pieces and mingled with
these fresher layers.
The finer ashes form very compact beds that require the
microscope for their determination.15
Section II. — Aqueous Rocks.
These consist of the stratified rocks which have been formed
by deposition in water. The pyroclastic sedimentary rocks
which have been formed from volcanic fragments have been
already described in Section I.
Aqueous rocks may be divided into (1) fragmental or clastic
rocks, viz. formed from materials derived from older rocks ;
(2) rocks formed by chemical or organic agencies.1
FRAGMENTAL OR CLASTIC ROCKS.
These consist of pebbles, sand, or mud which have become
hardened by various natural cements into solid beds or strata.
The pebbles then become a conglomerate, the sand a sandstone,
and the mud a clay.6 They are all mechanical deposits, and vary
indefinitely in composition according to the nature of the sources
from which they were derived. They may be divided into
arenaceous or sandy rocks and argillaceous or clayey rocks.1
(i) Arenaceous Rocks.
Sand. — By this term we understand the materials constituting
the fine-grained siliceous rocks called sandstones. This sand has
SECT. II.] BOCKS. 113
in every case been derived from the destruction of igneous or
metamorphic rocks, and in some cases of cherts or flints. The
quartz from granite consists of separate grains which often have
an irregular and complex form, but the quartz from felsite is
much more truly crystalline, and the planes of the crystals are
frequently perfect, though the angles are more rounded than in
the quartz from granite. Sometimes the grains are corroded as
though partly dissolved by the action of the alkalies liberated
when the associated felspar was decomposed. The quartz derived
from gneiss and mica schist, especially when those rocks have a
thin foliation, is remarkable for being flattened in the plane of
foliation, and consists of numerous small crystals dovetailed
together, so that when broken up it gives rise to a fine-grained
sand, or a sand containing grains which show a compound
structure ; and if the parent rock contained mica, thin plates of
mica are found between the parallel grains of quartz.
The grains of sand are rarely obtained direct from the rock
which yields them without experiencing a large amount of wear.
This attrition is due to transport of the material by rivers, and
grinding by the waves on the seashore. Some ancient sand-
beds are made up of grains which are unworn and practically
new, while the grains on many a modern sea-beach are of vast
antiquity, and have formed part of several geological formations,
in each of which they have been worn. When we examine some
of the modern sands in process of formation, the amount of wear
is found to be unexpectedly small ; thus the sand of the river-
terraces at Dunkeld is almost entirely angular, and presents the
features characteristic of sand derived from schists. The sands
of the Arabian, Egyptian, and great African deserts, on the other
hand, are exceptionally worn, every grain presenting the char-
acters of a miniature pebble, a feature resulting from the agency
of wind in rubbing the grains against each other (cf. p. 7).6
Sandstone consists of grains of sand compacted by some
cementing medium, which may be calcareous, ferruginous,
siliceous, or a mixture of some of these. The calcareous cement
has probably been originally deposited in the form of mud, etc.,
at the same time as the sand grains, but has had no binding
effect until it has been dissolved and redeposited with a more or
less crystalline texture. Ferruginous cement may occur alone or
associated with calcareous matter. The red oxide of iron and
brown hydrated oxide both occur and often form a thin coat
round each grain. When the cement is siliceous it is often
deposited in crystalline continuity with the quartz grains.
Argillaceous cement also occurs formed by the decomposition of
felspars, etc.1
8
114 GEOLOGY FOR ENGINEERS. [FT. II. CH. VII.
When sandstone is capable of being easily dressed by the
hammer for building purposes it is denominated freestone, and
when capable of being split up into large sheets for paving, etc.,
it is known as a flagstone?
Quartzite (see also under Altered and Metamorphic Rocks,
Section III., p. 123). — When a deposit of quartz-sand has become
completely compacted by a cement of quartz, the result is a
quartzite, but one in which the original grains and cementing
material are clearly distinguishable.
Grit is a hard and firm sandstone formed of coarse, sharp grains.
Sandstones are described as Micaceous, Felspathic, Quartzose,
Glauconitic (cf. p. 80), according to the nature of their materials.1
Conglomerate. — A coarse, clastic deposit composed of pebbles
or fragments of pre-existent rocks (cf. p. 102). When the pebbles
are rounded the rock is called Pudding stone ; when they are
angular it is called a Breccia (cf. p. 101).1
Greywacke is an old, somewhat vague term now used for a
hard, compact, greenish-grey felspathic sandstone.1
Sandstones formed close against a mass of granite, or similar
plutonic rock, may sometimes closely imitate the igneous mass,
especially when seen in section. The fine-grained Arkose
produced under such conditions contains all the minerals of the
igneous rock, closely set, and fitted into one another by the
pressure of overlying strata.15
Bluestone is a bluish, fine-grained, argillaceous sandstone used
for flagging and building. The term is also used locally for any
stone of a blue-grey colour.10
(ii) Argillaceous Rocks.
Clay. — Clay consists chemically chiefly of silicate of alumina,
and has very nearly the same composition as the mineral felspar,
which makes up so large a part of fire-formed rocks. Sometimes,
when hardened by pressure, and by containing other minerals,
the clay is called shale ; it then splits into thin layers in the
direction in which it was deposited. Clay consists of extremely
fine particles which can easily be transported by moving water,
as may be seen by the muddy state of the rivers after rain in
clayey districts. The colour of clay is generally due to some
oxide of iron ; it is usually grey or blue, sometimes brown,
occasionally white, yellow, red, crimson, purple, or black.6
Any change of colour should be noted in a clay-pit as deeper
beds are approached, for the suspicion of alteration hangs over
most brown clays. The irregular greenish or red streaks of the
" mottled clays" impart a characteristic effect to many fresh-
SECT. II.] BOCKS. 115
water deposits.15 Clay generally occurs in valleys and low
lands ; it does not easily allow water to pass through it, but
always holds a good deal of water suspended in its substance ;
and when this has evaporated, large and deep surface-cracks
and fissures are formed which may be enlarged by rain into
gullies.
No such careful and detailed examination has been made
of existing mud and clay as of sand or limestone. The subject is
much more difficult, and, as a rule, nothing can be distinguished
by the microscope but more or less irregular granules, minute
flakes of mica, and sometimes needle-like prisms, with variable
amounts of calcareous granules and sand. There is necessarily
every gradation between sands and clays on the one hand, and
limestones and clays on the other ; and the observations on the
deposits now forming are too few to .completely demonstrate the
conditions under which many of the newer clay-beds were formed.
It may, however, be regarded as certain that when the quartz
grains in clays are coarse the clays are derived from granite,
while when fine they are due to the destruction of schists. The
newer clays, as a rule, give no indications of pumice or volcanic
dust, but many of the older muds now changed into slate rocks
appear to be entirely of volcanic origin.
Clays may originate in many ways; the red earth found in
caves, and washed in by the streams flowing through them, is
obtained from the destruction of the neighbouring limestone
rocks ; for, after the carbonic acid gas dissolved in water has
carried away the whole of the carbonate of lime, there remains an
insoluble residue of silicate of alumina and oxide of iron which,
although forming but a small percentage of the limestone, yet
has often contributed to the accumulation of small deposits, such
as those in caves.
Clays when sandy are termed loam ; and when calcareous are
Shale.— The beautiful laminated structure of some clays
becomes more apparent where the materials are more consolidated
and the rock passes into shale. On the surfaces of such beds
delicate fossils must be looked for, the leaves of Tertiary deposits,
the Wealden entomostraca, the plant remains of the Coal Measures,
and the impressions of the graptolites being familiar examples (see
Chapter IX., pp. 181, 184). Very fine calcareous beds, like parts
of the Solenhofen "slate," resemble some pale shales, but can at
once be distinguished chemically with acid. Among the older rocks
there is a tendency for shales to become darker than the corre-
sponding modern stratified clays, and graphitic matter becomes
finely disseminated by organic decay. The fissility of the layers, due
116 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
to shrinkage and pressure of upper deposits, is the essential
character of these shaly forms.15
Mudstone or clay rock is hard and compact without any
tendency to split.
Marl is a loose appellation for all friable compounds of clay
and lime.3
ROCKS FORMED BY CHEMICAL OR ORGANIC AGENCIES.
These may be divided into calcareous, siliceous, phosphatic,
carbonaceous, and ferruginous.
(i) Calcareous Rocks.
Limestone. — Limestone may be formed from the waste of
older limestones, from remains of mollusca and other organisms,
or by chemical precipitation and deposition. It consists of pure
carbonate of lime or of carbonate of lime mixed with silica,
alumina, and iron, etc. When any of these occurs in excess the
rock is known as a siliceous, argillaceous, arenaceous, or carbon-
aceous, etc., limestone.1
The carbonate of lime sometimes exists in the crystalline form
of calcite, sometimes in the form of aragonite, and many shells
have one layer of calcite and the other layer of aragonite. There
is no means known by which calcite can be changed into aragonite,
the former being a remarkably stable substance, but aragonite is
as strikingly unstable. When its temperature is raised it passes
into a mass of crystals of calcite ; it is also easily dissolved, and
since calcite is usually deposited from cold solutions of carbonate
of lime, it happens that organisms formed of aragonite are often
removed entirely from a deposit, or replaced by structureless
calcite. This difference explains not only the circumstance of
preservation of many groups of fossils, but also important points
in the general structure of limestones.6
Physical characters. — The colours of limestones are very various ;
but the hardness, about 3, helps greatly in the detection of
these rocks. While at times finely granular limestones resemble
quartzites, and dark varieties even imitate compact basaltic lavas,
the knife readily settles the question, and leaves a well-marked
scratch, filled with white powder, across the limestone.
The specific gravity is generally rather under that of calcite,
probably owing to organic impurities. Some compact varieties
give only 2*6, while the dolomites (see below) run up to about 2-85.
Varieties with much aragonite will give 2*8. With hot acid all
varieties effervesce freely. The ordinary limestones do so when
SECT. II.] BOCKS. 117
a drop of cold acid is laid upon them ; but the dolomitic lime-
stones show a less rapid effervescence, and true dolomite gives
barely a trace until heated in the acid.
We may note that fissile limestones are rare, and that planes of
lamination, though they may be quite apparent, as in some Tyrol
dolomites, do not necessarily form easy planes of separation. The
distinct vertical joints, passing down through many feet of strata,
give, with the bedding-planes, the well-known block-like character
to exposed limestone surfaces, and tend to perpetuate the terraced
cliffs so familiar in the field. In the hand, compact limestones
break through with a clean fracture in almost any direction, the
surfaces produced by trimming being conchoidal in those of the
finest grain.
Concretions of silica (flint and chert, see (ii) Siliceous Rocks, in
this subsection), and the replacement of whole beds by pseudo-
morphic action, are common features of limestones of every age.
The faces of cracks in limestones, and the surfaces of hollows
and caves, will be commonly found coated with stalactitic crusts,
often of great delicacy. Similar deposition upon leaves, twigs,
etc., from springs containing carbonate of lime, gives rise to
travertine or "calcareous tufa," the interspaces becoming finally
filled up with calcite and the whole mass consolidated into a lime-
stone showing vegetable impressions.15
Chalk is a white, fine-grained limestone containing at times as
much as 94 to 98 per cent, of carbonate of lime. It may be quite
soft and earthy or harder and more compact, and frequently con-
tains nodules of flint and iron pyrites.
Chalk marl is chalk mixed with clay.
Oolite or oolitic limestone is composed of grains like the roe Qf a
fish, and in pisolite or pisolitic limestone the grains are as large as
peas. The grains have several concentric coats, and may be
hollow or may enclose a minute grain of some mineral substance.
Oolite is usually a dull yellow colour, but grey oolite is found.
Its peculiar structure makes it a "freestone," or one which can be
cut in any given direction. Bath stone, Portland stone, etc., are
oolitic limestones. Pisolitic limestone is 'sometimes known as
" pea grit."
Crystalline limestone has a coarse or fine crystalline structure
which may be due to alteration (see Section III., p. 124) or to
original structure, each crystal being a fragment of a fossil.1
Brecciated limestones. — Owing to the yielding nature of the
rock, these types are fairly common where earth-movements have
taken place. The cracks become filled with calcite. By develop-
ment of mica along surfaces of movement they pass over into the
metamorphic "calc schists" (see Section III., p. 127). The defor-
118 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
mation of fossils in such rocks, or their reduction to mere mineral
fragments, affords a most interesting field for observation.15
Stalactites, or the root-like pendants from the roofs of limestone
caverns, etc., and stalagmites, or the lumps and bosses which rise
from the floors, are formed by deposition from water which has
passed through calcareous rocks, and commonly show a crystalline
structure to the eye.1 The successive layers in some stalactites,
and in most stalagmites, are well marked on broken surfaces, and
the mode of deposition can be clearly appreciated from this
structure, from the form of stalactites, and from the characteristic
mammillated surfaces of stalagmites.
Travertine, consisting of carbonate of lime deposited upon
twigs, leaves, etc., in streams, often contains relics of vegetable
matter, or casts of such materials appear when the consolidated
mass is broken open. Travertines are characteristically pale in
colour, being opaque, white, brownish grey, or slightly tinged with
orange where iron oxides are more abundant.15
Rottenstone is a name given to the siliceous skeleton formed
from siliceous limestone by the weathering out or decomposition
of the calcareous part of the rock.
Cornstone is an arenaceous limestone in which the carbonate of
lime is sufficiently predominant to enable it to be burnt for lime
when better stone is not available. Arenaceous limestones pass
into calcareous sandstones.
Carbonaceous or bituminous limestone obtains its dark colour
from the decomposition of vegetable or animal matter, and fetid
limestone owes its smell to the same cause.1
Coral limestones. — Scattered corals occur in many shelly lime-
stones ; but occasionally the branching or astrsean types build up
reef-like masses among ordinary sediments, enclosing the coral
detritus accumulated on their flanks, together with many remains
of the organisms of the external sea.1
Dolomite, Dolomitic Limestone, or Magnesian Limestone.—
This rock is generally due to the alteration of ordinary
limestone, a portion of the carbonate of lime being replaced
by carbonate of magnesia.1 Dolomites resemble ordinary
types, but are liable to contain cavernous hollows and cavities of
retreat, as if the materials had shrunk during the process of
chemical change. The specific gravity is higher than that of
ordinary limestones, being about 2 '8. Commonly the formation
of dolomite in ordinary limestone spreads as a sort of disease in
bands and patches, often resembling igneous veins. The iron
that is often at the same time introduced colours the dolomite
a faint brown, in striking contrast to the dark-grey limestone
about it.
SECT. II.] ROCKS. 119
The rion-effervescence of true dolomite with cold acids may
cause mistakes on hurried examination. The hardness, however,
is only a little above that of calcite limestones.16
Its colour is usually brown or yellow, but white, grey, and
black varieties occur. Dolomite is sometimes earthy and friable,
sometimes splits easily into thin slabs, and sometimes forms large
concretions.1
Rock-salt (see Chapter V., p. 90) occurs in beds and masses
sometimes from 60 to 90 feet thick. It is frequently mixed with
argillaceous, ferruginous, or bituminous earths which give it
various colours, but is sometimes perfectly pure and white. It
is often associated with gypsum.1
Gypsum (see Chapter V., p. 80). — It occurs in the form of regular
beds in irregular concretionary masses and in veins and strings
in other rocks. The crystalline varieties are known as Selenite ;
the cryptocrystalline and fine-grained varieties are called
Gypsum; the compact, very fine-grained and mottled varieties
are known as Alabaster ; while the fibrous varieties with a silky
lustre are called Satin-spar.1
The rock is generally white, with a compact structure, semi-
transparent, and resembling some pure crystalline limestones.
The glancing surfaces of the calcite cleavages in the latter are
represented in some coarser alabasters by the clinopinacoidal
plates of the gypsum crystals ; but, as a rule, the mass is more
compact. The hardness is only 2, and the thumb-nail thus
distinguishes the two types of rock. The white powdery surfaces
of gypsum when struck by the hammer resemble those of
crystalline limestone. The specific gravity is another excellent
test, being only about 2*32. The rock does not effervesce with
acids. In the field the whiteness of the rock, as it appears in
bosses through the soil, or gleams high up among mountain
masses, is a feature that attracts attention at a distance even of
miles. The comparative purity of massive gypsums prevents
their weathered surfaces from being masked by products of
decomposition.15
(ii) Siliceous Rocks.
Flint and Chert. — These terms can be used synonymously for
the concretions and beds of chalcedonic and amorphous silica
found so frequently in limestones and sandy rocks.15 Flint,
however, is brittle and breaks with a very marked conchoidal
fracture, while Chert is tough and breaks with a splintery
fracture.1
The characteristically uniform and often conchoidal surface of
fracture, the semi-transparency of fragments, and the hardness
120 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
( = 7) are useful features in determination. Acids, moreover, have
no effect.15
These concretions have been accumulated in the strata, after
their consolidation, by the solvent action of percolating waters,
which have dissolved the substance of various minute skeletons
of siliceous organisms, and redeposited the material. The chief
accumulations of flint are met with in the Carboniferous limestone
(p. 179), in the Portland and Purbeck beds (p. 174), and in the
Chalk, p. 172 (see Chapter IX.). It is probable, in some cases, that
no small amount of this siliceous material has actually been
derived from the solution of overlying sandstones, which have
happened to contain sufficient lime to render the silica soluble.6
Nodular flints and chert-bands are found to follow the lines of
stratification of the rocks in which they occur. They may also
be looked for in " tabular " forms along planes of jointing or
faulting. In the Chalk the white exterior of the flints is due to
porosity on a microscopic scale, caused by the removal of the
more soluble part of the chalcedonic silica.
With the unaided eye, duller white patches are often seen in
cherts and flints, which are the residue of chalk-mud, or of fossil
forms, mainly sponges, about which the segregation has taken
place. Fossils may be included without change, casts being
formed of them, or their calcareous substance may be partly or
wholly silicified.15
(iii) Phosphatic Rocks.
Phosphatite. — Occasionally beds of small concretions of
phosphate of lime, sometimes called coprolites, rest on clay
surfaces or are scattered in sands or limestones. They are highly
valued for the manufacture of an artificial manure for root-crops
which is named superphosphate of lime. These deposits appear
to have been owing chiefly to the growth and decay of sea-plants
for many generations, on fixed spots near to the shore, since those
plants all contain a quantity of phosphates which are capable of
combining with lime when liberated by the decay of their
organic tissues. These concretions rarely assume a septarian
structure; and the mineral often invests or infiltrates animal
substances.6
Bone-beds (Bone-breccia). — The fragments of bone have usually
become rich dark brown or grey-black, and have a characteristic
lustre. Associated with them is concretionary phosphate of lime,
which forms nodules round them and disguises their outlines.
Some phosphatic deposits consist of black casts of fossils mingled
with irregular concretionary lumps. All cases can easily be
tested chemically.15
SECT. II.] ROCKS. 121
Coprolitic beds and nodules are, properly speaking, formed of
the excrement of animals \ but the name is often given to ordinary
phosphatic deposits (see p. 7 1).1
Guano (see under Apatite^ Chapter V., p. 71) consists mainly of
the droppings of countless sea-fowl, intermingled with their
skeletons and eggs, the decomposed bodies and bones of fishes,
seals, and other massive creatures frequenting the islands on
which it is deposited.11
(iv) Carbonaceous Rocks.
When wood decomposes, the oxygen, hydrogen, and nitrogen
are gradually removed until almost nothing but carbon is left.
Various stages in the process of decomposition under various
conditions produce humus, peat, lignite, brown coal, bituminous
coal, and anthracite, the latter containing the largest percentage
of carbon.
Humus (see Chapter I., Section VI., p. 24) is the vegetable part
of the soil as opposed to the strictly mineral portion.1
Peat is strictly a vegetable accumulation (see Chapter I.,
Section VI.) and occurs in all stages of consolidation from the light
fibrous turf of the surface, in which the several plants are
apparent, to the dark compact peat below.11
Lignite consists of a mass of branches and stems of trees and
plants matted together and retaining their woody fibre.
Coal. — Wherever vegetation has accumulated in swampy
localities necessary for its preservation, coal has been formed, and
hence coal is of every geological age. Its formation in the
Carboniferous period, and generally, was analogous to the growth
of peat. Intercepted drainage killed the forest trees in districts
experiencing a temperate climate, and, as in the English fens or
Irish bogs, the stumps of forest trees are found beneath the
vegetable growth, which was itself a soil for plants of many kinds
now imperfectly preserved. Spores of coniferous trees furnished
bituminous bands. Peat, like coal, alternates with beds of clay.6
The common characters of the coals that serve readily in their
recognition are their very low specific gravity, their hardness of
about 2, and their combustibility.
Brown coal is a lignitic coal, sometimes laminated, of a warm
brown colour. It is sectile and sometimes clayey, and does not
soil the fingers.
Common coal needs no description as to external characters. Its
specific gravity is about 1 '28, and it is also sectile.
Anthracite has a more brilliant lustre, does not soil the
fingers, is more brittle, and has a specific gravity near 1*4. The
flame produced from it is very weak.15
122 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
(v) Ferruginous Rocks.
Very few rocks are free from iron, but it usually occurs in
small quantities, so that its chief importance is as a colouring
agent; see Chapter VI., Section IV., p. 104; and, with regard to its
weathering properties, see Section IV. of this chapter. Magnetite,
ilmenite, specular iron ore, and limonite are, however, found in
many crystalline rocks and occasionally occur in beds or masses.1
Ironstones. — Many concretions consist of brown clay ironstone,
which effervesces with hot hydrochloric acid, the solution
becoming coloured a strong yellow. These nodules consist of
carbonate of iron with brown oxide crusts. The " black-band " of
the Coal Measure rocks is similar. Ironstones very frequently
result from the pseudomorphosis of some ordinary sedimentary
rock, though some arise from deposition as bog iron ore, and
others are merely cemented sandstones.
By the breaking up of concretionary carbonate of iron,
concentric coats of limonite are formed in succession around each
original centre ; where the rock is split up into cuboidal blocks
by jointing, each block on being broken open reveals towards the
centre sections of concentric spheroidal surfaces, marked brown
by the hydrated oxide, which is a stable product insoluble in
water. As these surfaces approach the joint-planes they conform
more to them, and the outermost coat is often box-like and well
consolidated, protecting the interior from further action.
Concretionary layers of limonite, with no apparent connection
with joint-planes, may be found in many sands, and serve to
protect fossils that might otherwise have been entirely dissolved.15
Section III. — Altered and Metamorphic Rocks.
Classification. — However much a rock may have changed in
structure or texture by any of the agents of metamorphism (see
Chapter III., Section III., p. 46), it is tolerably clear that it was,
when first formed, either an original (igneous) or derivative
(aqueous) rock, or has been subsequently made a mixture of both.
The most natural classification of the altered and metamorphic
rocks, therefore, would be to arrange each as a variety of the special
rock out of which it has been formed. Our knowledge does not
admit of this, hence such rocks as can with a fair approximation
to certainty be referred to their natural position are classed as
Altered Rocks ; those in which the original characters are masked
or appear to be wholly obliterated are classed as Metamorphic.
It must always be borne in mind, also, that all the great rock-
SECT. III.] ROCKS. 123
groups shade the one into the other, so that authorities rarely
agree as to the separating lines between them.3
The division adopted in this section is into "Altered Rocks"
and " Distinctly Foliated Rocks."
ALTERED ROCKS.
Quartzite (Quartz-rock, cf. p. 114). — A hard, compact rock, white,
red, or brown in colour, breaking with a peculiar lustrous fracture.
It is distinctly stratified, occurring usually in thick beds. Under
the microscope it is seen to be composed of quartz grains, the
interspaces between which are filled up by a deposit of silica. In
most cases it appears to have been originally an ordinary sand-
stone, the siliceous cement being a subsequent deposit carried in
by percolating waters.3
Quartz-rock, in the greater number of instances, especially
when occurring in veins, seems more recent than mica schist and
gneiss, though by easy changes in composition it becomes nearly
identical with them. The internal evidence of texture seems to
decide the question of the origin of quartz-rock, and to prove
that, however altered by subsequent metamorphic action, it was
originally a mechanical deposit. The degree of compactness
which it exhibits varies extremely, in some cases approaching the
loose granular character of sandstone ; in other cases it has the
density of the quartz of veins.6
Lydian- stone. — A dense black or brownish rock, extremely
fine-grained, the result of the hardening or silicification of a
somewhat carbonaceous shale.3
Spotted Shale. — The shaly mass is full of dark brown or black
spots and patches, with an attempt at regular outlines. These
are mere " pigment spots " or actual embryo-crystals, and show
no true faces or specific characters. Mere contact with a dyke
(see Chapter III., Section I., p. 35) will sometimes produce this
type of alteration in the shales or slates around. At times
recognisable garnets may be developed.15
Porcellanite (Baked Shale). — A pale, close-grained, flinty
rock breaking with a hackly or conchoidal fracture ; due to the
metamorphism of fine clays, shales, or fine tuffs, so called from its
resemblance to porcelain or chinaware.3
Slate. — A hard, compact, and usually more or less aluminous
rock, splitting into thin, parallel layers, more or less oblique to
the original stratification. When the rock still retains evidences
of its primary detrital character, it is known as clay slate, green
slate, roofing slate, etc. When its cleavage planes are so
crowded with micaceous flakes as to present a silvery "sheen " it
124 GEOLOGY FOR ENGINEERS. [FT. II. CH. VII.
is known as a phyllite; when it is distinctly crystalline and
micaceous throughout it is termed a mica slate. The two latter
varieties graduate into the typically metamorphic rock, mica
schist?
Crystalline Limestone (cf. p. 117) is in general stratified ; it fre-
quently alternates with gneiss and mica schist, and sometimes
retains argillaceous partings; it was therefore a water-formed
deposit. Its state of granular or saccharoid crystallisation is due
to changes developed since its deposition, and partly occasioned by
the action of heat on contained water : this change is more
obvious in the deeper-seated than in the newer calcareous
deposits.
The beds of crystalline limestone, whether distinctly stratified
or not, are in general detached and limited, and so entirely
enveloped in gneiss and mica slate as to form but subordinate
members of those widespread rocks.
Though crystalline limestone is a simple rock, its aspect admits
of many variations from unequal admixture with other mineral
substances. Of these the most frequent are mica, talc, and
steatite, the latter of which often communicates a green or
mottled colour to the whole rock.6
Those crystalline limestones which are suitable for ornamental
architecture are termed Marbles, and many marbles are rocks of
this kind, which owe their crystalline character to alteration by
intrusive masses; still, there are also many in which the
crystalline structure is not due to this cause. The term marble
is, however, very loosely employed, and may be generally taken
to signify any rock which takes a good polish and is employed
for decorative or architectural purposes.16
Serpentine. — A massive, compact rock formed of the mineral
serpentine, of a dull-green or brownish colour, often curiously
veined and mottled. It is easily cut with a knife, and frequently
shows scattered crystals of enstatite, chromite, etc. It is a
silicate of magnesia, and appears to be due in most cases to the
alteration of a highly basic rock (peridotite), such &s picrite, etc.3
DISTINCTLY FOLIATED ROCKS.
This group includes the schists and gneisses the origin of which
is still much discussed by geologists. Foliation has been de-
scribed in Chapter VI., Section III., p. 103. The division of foliated
rocks into altered sediments and altered igneous masses is beset
with such enormous difficulties that we must be content merely
to bear in mind the possibility of either origin, and to seek
diligently for elucidation in each case as it comes before us in the
SECT. III.] ROCKS. 125
field. There is, however, a growing feeling that the great
majority of amphibole — and chlorite — schists, a few mica schists,
and many gneisses have their origin in igneous rocks ; while in
many cases original flow, and not metamorphism, is responsible
for their special structures (see Fluidal Gneissic Structure,15
Chapter VI., Section III., Group 1, p. 99).
Classification. — When the foliated rock has a granitoid structure
(apparently differing from granite, syenite, and the other varieties
of holocrystalline igneous rocks mainly in the fact that its con-
stituents are arranged in distinct folia) the rock is known as
a gneiss ; and thus we have granite gneiss, diorite gneiss, syenite
gneiss, and the like. When the foliated rock, however, is fine-
grained in texture, and divides with ease into thin lenticular
sheets (apparently differing from ordinary flagstone, shale, slate,
or tuff mainly in its foliated and crystalline nature), the rock is
termed a schist ; and thus we have mica schist, hornblende schist,
garnet schist, etc., each variety being named usually after its
dominant or characteristic mineral. When the schist becomes
more or less massive, and the foliation is feebly developed, it is
termed Rock, as hornblende rock, garnet rock, etc.3
Gneiss. — -This name was originally applied to a crystalline
aggregate of quartz, felspar, and mica, etc., differing from granite
simply in the fact that its component minerals are arranged in
folia, so that it may with difficulty be split up into subparallel
slabs ; but the name is now generally employed for all foliated
holocrystalline rocks with a granitoid structure. The more finely
schistose varieties shade down into felspathic schists ; the more
granular varieties pass insensibly into granite, syenite, diorite, etc.
The chief varieties are named according to their mineralogical
composition, such as granitic gneiss, biotite gneiss, augite gneiss,
hornblendic gneiss, etc.3
Some writers hold that there is no definite rock known as
gneiss, but that the term is to be applied only to a particular
kind of structure.
The component minerals of gneiss and granite are the same —
quartz, felspar, and mica. They are mixed with the like accidents
and permutations, and occasional admixture of other minerals,
and are subject in both rocks to the same extreme variation of
size. But these rocks differ in the mode of arrangement among
their constituent minerals. The ingredients of granite are so
connected together by contemporaneous or nearly contemporaneous
crystallisation, that one mineral penetrates and is intimately
united with another ; and we are compelled to conclude that they
were not accumulated in distinct crystals ready formed, but that
the minerals never had a separate existence as solids until their
126 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
different geometric forms were slowly developed by crystallisa-
tion. Gneiss almost always suggests, by some degree of imper-
fection of the edges and angles of the quartz and felspar, and
much more decidedly by the laminar arrangement of the mica
and consequent minute foliation of the rock, that its materials,
ready-made and crystallised, were brought together and arranged
by water.
Gneiss is essentially a mass of quartz and felspar, foliated with
thin films of mica which are sometimes exposed by fracture.
As in granite, the felspar is usually orthoclase, but oligoclase is
sometimes associated with the orthoclase, though oligoclase is
more frequent in hornblende gneiss and protogine gneiss j there
are varieties of gneiss in which orthoclase is the only felspar.
Occasionally albite is associated with orthoclase. Orthoclase
varies in colour in gneiss quite as much as in granite, and is
sometimes found in porphyritic crystals. The quartz occurs
either in grains or small lenticular plates made up of many
crystals united together. The mica may be either potash mica
or magnesia mica, and occasionally both micas are found in the
same rock. Sometimes the mica surrounds the crystals of felspar,
giving that mineral a lenticular form. Hornblende is an im-
portant constituent in many gneisses of the West of Scotland,
and chlorite and talc are found in some gneisses of Scotland, so
that gneiss has often been divided into mica gneiss, hornblende
gneiss, and chlorite gneiss.
Structure. — Gneiss varies in structure with the condition of the
mica. In the common type, mica is found in separate laminae,
dividing the felspar and quartz. But when the foliated structure
is indistinct, owing to the imperfect continuity of the mica films,
the rock is termed granitic gneiss, and makes a transition to
granite. On the other hand, the mica may be so abundant as to
isolate the quartz and felspar in lenticular masses ; and in
section this condition gives a delicate, veined aspect to the rock.
Sometimes the mica shows parallelism, giving the folise of the
rock as regular an aspect as exogenous growth in wood ; and this
condition further developed imparts a platey cleavage to the
gneiss.6
Mica Schist. — Mineral constituents. — The kind of mica in mica
schist varies with the locality. In the St Gothard the soda mica
paragonite is found. In some localities the yellowish-white potash
mica is rich in water, and forms the species damourite. The
colours of the mica vary, but dark magnesia mica is most
common. This mineral determines the colour of the schist, which
is grey, or greenish grey, or yellow grey, or may be brownish
black.
SECT. III.] BOCKS. 127
The quartz occurs in grains, scattered between parallel layers
of mica scales. As the quantity of quartz increases, the grains
become large flattened lenticular plates, among which films of
mica are diffused. Occasionally the quartz becomes so abundant
as to be only separated into layers by thin films of finely divided
mica, and such varieties make a transition to quartzite. The
varieties which are poorest in quartz always have small grains
of quartz enveloped in the laminae of mica. The varieties of
structure are similar to those of gneiss ; but the crumpled wavy
structure is one of its most typical modifications.6
Chlorite Schist. — A rare rock compared with mica schist. It
is dark green, with black-green scales on the surfaces of
foliation, and is typically rather fine in grain. The softness is
characteristic, the whole having a soapy feel in the hand. In the
field, the absence of the glancing surfaces of mica and the general
darkness of the rock exposed mark it out from mica schist.15
Talc Schist. — A somewhat rare magnesian schist, light in
colour, generally pale greenish or pure white, with a silvery and
pearly lustre. The rock feels soapy to the hand and its hardness
= lti5
Potstone, the lapis ollaris of the ancients, is a massive variety
of talc schist, composed of a finely felted aggregate of scales of
talc, with chlorite and serpentine. It is also known as indurated
talc or talc slate.3
Hornblende Schist is one of the commonest metamorphic rocks,
usually green-black, with a lustre due to fibrous or somewhat
plate-like hornblende ; quite distinct from that of a dark mica
schist.15
Calc Schist (cf. p. 117) is the schistose representative of the lime-
stones with accessory silicates, forming lustrous specks and rods
upon the planes of foliation.15
Mylonite. — The compact, microscopic shear-breccia typically
formed in the numberless overfaults (thrust-planes) of mountain
regions. It is composed of the flakes and particles of the rocks
which have been sheared, dragged, and ground between the jaws
of the gliding planes. The particles are set in a subcrystalline
paste, which is streaked with inosculating veins and fibres of more
or less opaque matter.3
Granulite. — A foliated rock, differing in structure from mylonite
essentially in the fact that the matrix is holocrystalline, being
composed of microscopic granules of quartz and felspar, forming
a kind of mosaic. The chief varieties are garnet-granulite,
augen-granulite, etc. The granulitic structure is very character-
istic of some of the crystalline schists.3
Flaser gneiss, flaser gabbro, etc. — Igneous or gneissic rocks
128 GEOLOGY FOB ENGINEERS. [PT. II. CH. VII.
which have been deformed by earth- pressure and the like into
lenticular masses separated from each other by folia or wavy
films of finer crystalline material.3
Augen-gneiss, augen-gabbro, augen-schist, etc. — Igneous or
metamorphic rocks showing "eyes " or inclusions of crystals, etc.,
set in a finer crystalline and foliated ground-mass.3
Section IV.— Rock Decomposition.
IGNEOUS ROCKS.
The igneous and metamorphic rocks consist in greater part of
various silicates which are largely subject to external atmospheric
influences. In consequence of this, these rocks, hard and
seemingly indestructible as they generally are in the unaltered
state, are liable to decompose and disintegrate into soft and
yielding masses. As all the sedimentary strata are derived from
the wear and reconstruction of others of older date — all traceable
back to the antecedent igneous rocks — these changes in the
structure of the latter bear upon the composition of the former.
The insoluble essential bases of both are alike — only that in the
sedimentary rocks they exist free, and in the igneous rocks are
usually combined. All the rocks of igneous origin consist of
silica, sometimes free (quartz), but more generally in combination
with the various earths and alkalies, and a few metallic oxides,
forming with them a variety of silicates, amongst which the
felspars very largely predominate.
Felspars are essentially double silicates of alumina, and of the
alkalies and alkaline earths. They contain more or less potash
or soda, and form more or less stable compounds in proportion
to the quantity and nature of the alkalies present. Their com-
position varies in consequence of the bases being liable to be in
part replaced by one another ; the typical composition of the
three geologically more important varieties has been given in
Chapter V., p. 76. These contain silica, alumina, potash, soda,
and lime in variable proportions.
Formation of kaolin. — Exposed to the action of the weather,
the felspars of the hardest granites, and of the analogous
crystalline rocks, are, under certain conditions, decomposed by the
carbonic acid in the rain and surface waters, forming, with the
lime and alkalies present, carbonates which, being readily soluble,
are, with probably some alkaline silicates, removed wholly or in
greater part by the water ; while the silica set free remains mostly
as an impalpable powder (see Chapter I., p. 8). The combination
of silica and alumina on the other hand, being entirely insoluble,
SECT. IV.] ROCKS. 129
remains, combined with a portion of water which is taken up
during the change, and the resultant is a white mealy powder,
unctuous and plastic in water. This is an hydrated silicate of
alumina, or kaolin (china-clay) (see Chapter V., p. 85). This change
shows the loss of a portion of the silica and of all the alkalies ;
while the whole of the alumina, in combination with the other
portion of the silica, remains as an insoluble residue, holding a
definite proportion of combined water. But, as there generally
remain some portions of undecomposed felspar and a variable
quantity of free silica, the actual composition in nature varies
within certain limits.
Origin of clays. — Almost all the china-clays contain, with a
definite hydrated silicate representing the typical kaolin, small
portions of the other elements present in the original rock. This
kaolin is the basis of all clays ; and where the decomposed rock
contains foreign elements, the clays show correspondingly varied
composition. Granite and its ally pegmatite furnish the purest
kaolins. Kaolin is also obtained from decomposed porphyries
and gneisses.4
Decomposition of other Silicates. —The decomposition is not
limited to the felspars. It equally affects the other silicates
which enter so largely into the composition of the more basic
igneous rocks, e.g. hornblende, augite, olivine ; and as in these
rocks free quartz is generally absent, the whole mass disintegrates
and decomposes. The composition of the more important of the
basic rocks is given in Section I. The normal composition of
hornblende, augite, and olivine is given in Chapter V. These
rocks furnish by their decomposition not only kaolin, together
with lime and magnesia, but also a large proportion of the
peroxide of iron resulting from the peroxidation and hydration of
the protoxide ; while a hydrated silicate of the protoxide of iron
is formed as another product of the alteration of the hornblendes
and augites. It is in this way that the widely disseminated iron-
peroxides and glauconite (silicate of iron) have originated.
It is owing to the presence of these complex silicates containing
lime, magnesia, and the metallic oxides that diorite, diabase,
melaphyre, and other basic rocks generally decompose into green
and brown clays. Great bodies of these rocks are also converted
into masses of soft and decayed rock, of grey, green, red, or brown
colours, formerly known under the general name of "wacke."
At Robschutz in Saxony a decomposed diorite is worked as a
fuller's earth, and near Florence a decomposed variety of gabbro is
worked as a fire-clay.
Serpentine — itself an altered rock — is not infrequently more
completely decomposed and changed into magnesian clays, some-
9
130 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
times white and at other times coloured. Some of these clays
contain as much as 33 per cent, of magnesia.
Basaltic rocks. — The alteration in the felspathic bases is very
noticeable, and as these rocks, like the older greenstones, contain
silicates with metallic oxides, they only furnish very impure
clays.
Other basic volcanic rocks, such as dolerite, andesite, etc., are
also liable to decompose ; and so also in a less degree are
the trachytic lavas and scoriae. The vitreous lavas are less
liable to decompose.
Ordinary clays are not generally derived direct from the parent
igneous rock, but are reconstructed, especially in the later deposits,
from older clay beds.4
Origin of Quartzose Sands and Sandstones.— Granites (see
Section I.) consist of a more or less intimate mixture of quartz
and felspar, in proportions varying, on the average, from 40 to 50
per cent, of each, with 5 to 10 per cent, of mica. The quartz
forms a crystalline matrix, which, as the felspar decomposes,
breaks up in fine-grained granites into grains generally of small
size ; or, if it be of coarser grain, then into larger fragments. As
decomposition goes on the whole rock loses its coherence ; and, on
the removal of the decomposed soft parts, crumbles down into a
grit or gravel of quartz, with flakes of the mica. These being
comparatively indestructible, the only further change they under-
go is through wear, by which their angles are gradually rounded
off and the size of the grains reduced. This takes place on shore-
lines, by tide and wave action (see Chapter I., Section V., p. 20).
The result is the production of a fine quartzose, and more or less
micaceous sand, such as may be seen in the many beautiful small
bays on the coast of the Land's End. All the soft and soluble
ingredients of the decomposed silicates have disappeared, and a
simple residue of micaceous quartzose sand, with some amorphous
matter, remains. When, however, as not infrequently happens,
portions of the felspar resist decomposition, the sand becomes
further mixed with a proportion of felspathic debris. It is from
this source that the materials of the various quartzose, micaceous,
and felspathic sandstones of the sedimentary strata have been
chiefly derived. As in the case of the argillaceous strata, such
sandstones are not always derived directly from the crystalline
rocks, but are constantly reconstructed by denudation from the
earlier sedimentary strata of the same class. In these reconstruc-
tions the only change which is effected is a greater amount of
wear of the sand, and the gradual removal of all traces of felspar,
which yields ultimately to the successive changes.4
Extent of Disintegration. — The decomposition of granite is not
SECT. IV.] ROCKS. 131
confined to the surface, but extends to considerable depths. The
process of decay is very variable, depending on the nature of the
felspar, and upon climatic temperature and humidity. Moisture,
or even a damp condition, is the great element in effecting decom-
position, but the influence of cold is important. Thus, while the
granite monuments of Egypt have remained unaltered for ages,
the recent monuments of St Petersburg already show symptoms
of decay. Again, in this country, some of the Cornish and Welsh
granites (Lamorna, Penryn, etc.) furnish solid and enduring
materials for our public monuments, while others (St Austell, etc.)
are so decomposed as to form a mass of quartz grit and white clay
(kaolin) that can be readily removed with pickaxe and spade.
Over large tracts in Cornwall, France, Spain, India. Central
Asia, and elsewhere the granite is thoroughly disintegrated. The
depth to which decomposition extends is, however, very variable ;
sometimes to a few feet, at others to more than 100 feet. The
decay is also irregular, some parts of the same granite resisting
decomposition more than others. Hence the formation of granite
blocks and " tors" (see Chapter I., p. 6).
Graphic granite is very liable to decompose. At Itsasson, near
Bayonne, this rock is decayed to a depth exceeding 150 feet, and
horizontally on the side of the hill for a distance of more than
100 feet. It forms a very fine white kaolin with free quartz.
Some gneisses are also extensively decomposed, forming kaolin
clays more or less pure ; this is of common occurrence in Central
France. Around Rio Janeiro the gneiss has decomposed into a
reddish clay from a few inches to 100 feet deep. In the Pyrenees
the disintegration extends to depths of 40 to 50 feet or more.
The syenites and diorites of Guernsey and Jersey, according
to Professor Ansted, are disintegrated in places to a depth of
50 feet or more ; and he states that a considerable part of the
north of the island of Alderney consists of a thick bed of sand and
fine gravel with boulders, the whole mass being derived from the
decomposition of the greenstone rock in situ.
The ophite (diorite, p. 109) of the Pyrenees is disintegrated gener-
ally into a bright brown argillaceous mass with concentric nodules
or subangular blocks of the unaltered rock remaining in situ, and
to such a depth that the unaltered rock rarely shows in the pits
or railway sections, which are 30 to 40 feet deep. This rock is of
Late Cretaceous and Miocene age.
Serpentine is sometimes decomposed to a considerable depth.
This is frequent in Northern Italy. In addition to the formation
of unctuous clays, the change sets free carbonate of magnesia and
silica, which are deposited in veins traversing the altered rock.
Basaltic rocks are decomposed often to great depths, and
132 GEOLOGY FOB ENGINEERS. [PT. II. CH. VII.
generally give rise to impure ferruginous clay, although at times
the iron has been so far removed as to leave a light-coloured clay.
The grains of titaniferous iron which may be present remain
unaltered.
Laterite and palagonite, which are rocks of considerable local
importance, are merely weathered and altered forms of lava, often
scoriaceous and tufaceous, in which the protoxide of iron has been
changed into the peroxide, and the rock has assumed various
bright colours of red and yellow, and of brown passing to black.
The schistose rocks are also subject to change. A talcose schist
in the neighbourhood of Pau and Bagneres is so altered that the
disintegrated mass is worked as a marl for manure. Other
schistose rocks have been found to pass into an impure fuller's
earth.4
SEDIMENTARY STRATA.
Although productive of infinitely less actual decomposition,
changes in the sedimentary strata, due to the influence of air and
moisture, are nevertheless of importance from the differences
they often produce in the aspect of the strata, the deceptive
appearances to which they give rise, and the extent of the
original decalcification.
Alteration of Colour. — Rocks originally grey, or blue, are
changed to light yellow, red, or brown. Ochreous and even
blackish beds become white, and dark greens pass into browns
and reds. These changes are due to the oxidisation of the
metallic bases by air and moisture, and to deoxidisation by
organic matter (see Chapter L, Section VI., p. 24). Thus some of
the grey argillaceous limestones or marls of the Lias, or of the
Kimmeridge, and similar argillo-calcareous strata, which imbibe
small portions of water, become light yellow or brown for some
distance from the lines of joint and bedding. Sometimes the
whole mass is bleached ; but more frequently central dark cores
are left. This alteration is due to the circumstance that almost
all these argillaceous limestones owe their bluish-grey colour to
the presence of a small quantity of bisulphide of iron (iron-pyrites),
or of some carbonaceous matter. The former, when exposed to
the action of air and moisture, is decomposed and changed by
oxidisation of the sulphur and iron into the sulphate of the
protoxide of iron; and this in its turn is decomposed, the acid
uniting with some of the earthy or alkaline bases present, and the
protoxide passing into a hydrated peroxide. The rock conse-
quently loses the dark colour due to the original pigment, and
retains only the slight tinge due to the presence of the iron-
peroxide.
SECT. IV.] ROCKS. 133
When the colouring is due to organic or carbonaceous matter
alone, the alteration is effected by the slow oxidisation of the
organic matter by the air and moisture. The organic colouring
matter is thus often completely destroyed, while the resulting
carbonic acid is carried off by the permeating waters, either alone
or in combination as a carbonate of some substance.
Freestones. — This alteration, owing generally to the greater
permeability of the oolitic and other freestones, extends in them
to a greater depth than in the more compact rocks. In these it
has generally removed the colour of the whole mass of the strata
above the line of permanent water-saturation (see Chapter XII.),
and it is not until a depth considerably below the surface is
reached that the rock is found to retain the grey colour it
originally had.
Green rocks. — The presence of minerals with a base of iron-
protoxide, as glauconite, gives some rocks a deep bright-green
colour. On exposure, the silicate of iron is decomposed, the
silica being set free, and the iron, taking up a further portion of
oxygen and water, is converted into a hydrated peroxide.
Consequently, the rock loses its green colour, and passes to yellow-
ish brown or ferruginous. This action is very marked on the
surface of the calcareous iron-ore of the marlstone of the Lias ; and
the brown colour of some of the oolitic iron-ores may, owing to
the permeability of the strata and the consequent influence of the
surface waters at depths, be due to a change of this nature.
Some of the fossiliferous iron-sandstones of the Lower Tertiary
strata of Kent are not improbably decomposed green-sandstones,
and possibly some portions of the Red Crag were deposited
originally as green glauconiferous sands.
Argillaceous strata, such as the London Clay, Kimmeridge Clay,
and the Oxford Clay, generally contain concretions and shell-casts
of iron-pyrites which, when exposed to the air, decompose and
form an efflorescence of the sulphate of iron, which ultimately
passes into the brown hydrated oxide. It is to the decomposition
of another small portion of iron-sulphide dispersed through beds
of this class that is due the change which commonly takes place
in the London and other of these clays, from dark bluish grey at
depths to a light burnt-umber-brown near the surface — a change
which often extends to some depth.
Deoxidisation. — On the other hand, the influence of vegetable
matter is effecting deoxidisation is very marked, as shown in the
case of a piece of lignite found in the London Clay around which
the iron was deoxidised and the clay changed from a dark brown
to a light fawn colour.
Bleached gravels. — A change of another kind takes place in
134 GEOLOGY FOR ENGINEERS. [PT. II. CH. VII.
iron-stained superficial gravels, such as are common in the
neighbourhood of London, and in the Hampshire Tertiary area.
These gravels have a bright ochreous colour, caused by the
presence of a small quantity of the peroxide of iron. When they
form, as is often the case, moors and commons covered with heath,
or here and there coated with a thin layer of peat, the organic
matter carried down by the rain-water reduces the iron-salt from
a peroxide to a protoxide, which the free carbonic acid present
converts into a carbonate ; and this salt, being soluble, is removed
by the same surface-waters, leaving the upper part of the gravel
colourless and often quite white. Or it may sometimes be that
the humic acid in the soil removes the iron as a soluble humate.
The yellow staining of the flints is also removed, and they then
present a bleached and white surface. '
PART III.
HISTORICAL GEOLOGY.
THE aims of historical geology are (a) to classify and describe the
rocks of the earth's crust in the order of their formation, and (b)
to ascertain and point out the successive groups of animals and
plants which have made their appearance on the face of the globe
from the dawn of life up to the present time.3
The sciences which deal with these aims are, respectively,
stratigraphy and palaeontology.1
It is usual in geological text-books to describe the various
formations in ascending order, commencing with the lower, but to
the engineer who has to deal practically with the formations as he
finds them, the descending order will be more serviceable and has
been adopted in this part.1
135
PT. III. CH. VIII.
CHAPTER VIII.
PEINCIPLES OF STRATIGRAPHY AND
PALEONTOLOGY.
Section I. — Classification of Stratified Rocks.
Formations. — The stratified rocks of Britain and other countries
appear at the surface, or beneath the soil, in definite geographical
bands or zones, each band showing only one special lithological
type (or special association of types) of stratified rock. Each such
band is formed of the outcropping edges of a succession of more
or less similar strata following one over the other in unbroken
order. The entire succession of strata occurring in each band
forms collectively a thick and continuous rock-sheet, which is
known as a geological formation, and is distinguished by geologists
under a special name, n Each formation is not only identifiable as
a whole by its characteristic lithological features, but also by a
characteristic assemblage of fossils peculiar to that formation
alone. The main guides to the correct classification of the
formations are (1) lithological characters, (2) characteristic fossils,
(3) superposition, (4) conformability.3
(i) Lithological characters. — These are referred to in Chapters
VI. and VII. The lithological character of a bed sometimes
varies, and, therefore, less dependence is to be placed upon it
than upon palseontological evidence. Furthermore, the slight
variation in the lithological characters of sedimentary rocks often
renders it very difficult to assign them to any particular horizon
in the absence of fossils, sandstones, slates, shales, and limestones
of different geological ages often bearing a close resemblance to
one another. Again, rocks differing widely in lithological char-
acter may have been deposited at the same time, as in the case of
the Devonian and Old Red Sandstone rocks, the former having been
thrown down in the sea and the latter in lakes, as proved by the
fossils which they respectively contain. Yet both formations
occupy a position intermediate between the Upper Silurian rocks
and the lowest members of the Carboniferous series.16
(ii) Characteristic fossils. — Each formation possesses as a whole
136
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 137
distinctive organic remains. All the British stratified formations
afford fossils : the special assemblage of fossils (fauna and flora)
found in one formation, however, never occurs in another forma-
tion, but is restricted to the strata of that formation alone.
Some of the British geological formations are strikingly indi-
vidualised in this respect. Thus the Old Red Sandstone is marked
by the presence of its mail-clad fishes, the Coal Measure formation
by its flowerless plants, the Mountain Limestone formation by a
host of special corals, and the like.
(iii) Superposition. — Every geological formation must be newer
than the formation which underlies it, for its strata could not
have been laid down upon the underlying formation until the
deposition of that formation had been completed. And, for the
same reason, every geological formation must be older than the
formation which overlies it. It follows, therefore, of necessity
that the order of sequence or superposition of the geological
formations gives us the order of their deposition in geological
time. Of the entire series of the successive formations, the lowest
must have been the first deposited, and the highest must have
been deposited last. Inverted strata (see p. 42) must be
carefully distinguished.
(iv) Conformability. — Sometimes formations are locally miss-
ing, and there is a break or gap in the ordinary succession both
of the rocks and of the fossils, which is usually marked by an un-
conformability (cf. p. 41); but the relative order of the formations
present remains always the same.3
The grouping of sedimentary rocks into formations is, of course,
more or less arbitrary. Some genera, and frequently species,
which occur in a lower formation are often represented in the
succeeding deposits of a newer formation, and probably, if the
truth were known, it would be found that all the formations
which we now recognise pass from one into another. Because
an unconformity occurs in one limited district it does not
necessarily follow that this break extended over the entire globe.
Allowances must be made for relative distributions of land and
water, which we have often no means of realising, and no doubt
the universal application of limited knowledge often does more
harm than good in this branch of geological inquiry.16
Periods and Systems. — The British formations have been
arranged in five successive major chronological Periods or Cycles,
in descending order : (1) Post-Tertiary or Quaternary ; (2) Tertiary ;
(3) Secondary ; (4) Primary ; (5) Archaean or Pre-Cambrian.
Some of these cycles include only one system, while others include
several. The recognised British systems are, in descending order :
(1) Post-Tertiary or Quaternary; (2) Tertiary; (3) Cre-
138 GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
taceous; (4) Jurassic; (5) Triassic; (6) Permian; (7) Carboni-
ferous; (8) Devonian; (9) Silurian; (10) Ordovician;
(11) Cambrian; (12) Archaean.
The collective life-assemblage represented by the fossils of the
Pre-Carnbrian formations is known as Eozoic, that of the Primary
systems as Palaeozoic, that of the Secondary systems as Mesozoic,
that of the Tertiary formations as Cainozoic. The Palaeozoic
systems may be regarded as divisible into Protozoic (Cambrian,
Ordovician, Silurian) and Deutozoic (Devonian, Carboniferous,
Permian). The Mesozoic and Cainozoic systems may be spoken
of collectively as Neozoic, and the Post-Tertiary formations
referred to as Anthropozoic*
TABLE I. — SEDIMENTARY STRATA IN GREAT BRITAIN.
ANTHROPOZOIC OR QUATERNARY PERIOD.
Recent or Post-Glacial Deposits.
Alluvial deposits, peat bogs, deltas, aerial deposits, loess, brick
earths, raised beaches, valley drifts and gravels, river
and marine terraces, ossiferous caves.
Pleistocene or Glacial Deposits.
Moraine debris.
Clyde beds of Arctic clay and shell.
Kames (in Scotland) and eskers (in Ireland) of sands and gravels.
Upper boulder clay ; un stratified, with fragments of rock
interspersed.
Middle sands and gravels ; fossiliferous.
Contorted drift of East Norfolk ; loams, chalk rubble, and shelly
beds.
Lower boulder clay or till; a stiff clay interstratified with
beds of sand, fireclay, and peat.
Westleton sands and shingle.
Erratic blocks.1
CAINOZOIC OR TERTIARY PERIOD.
Pliocene Formations.
'Forest bed group of clays, lignites, gravels, and sands.
Elephant beds.
Upper
Weybourne Crag.
Chillesford Crag.
Norwich Crag of shelly sands.
^Red Crag, dark red or brown ferruginous sands.
Lower . Coralline or White Crag of calcareous sands.
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. 139
Miocene Formations.
Wanting in Britain.
Oligocene Formations.
Hempstead beds : an upper group of marine beds, marls, and
clays and a lower group of fresh- water and estuarine
marls.
Bembridge beds', sands, coloured marls, and pale, persistent
limestones.
Osborne beds : blue and red clays with calcareous zones.
Headon beds of limestones, etc.
Eocene Formations.
Upper
Middle
Lower
London Basin.
Upper Bagshot : white
and pale yellow sands.
Middle Bagshot : purple
sands, green clays, and
pebble beds.
Upper part of Lower Bag-
shot : light - coloured
sands.
Lower part of Lower Bag-
shot : pebble bed.
London clay : stiff grey
or brown clay with
septaria.
Woolwich and Reading
beds : sands, pebble
beds, plastic clay, and
loam.
Thanet sands : pale sands
with grains of glau-
conite.
Hampshire Basin.
Barton series of clays
and sands.
BracMesham series of
clays, marls, and sands.
Bognor series of clays,
sands, and calcareous
sandstones.
Plastic clays and sands
with occasional bands
of flint pebbles.1
140
GEOLOGY EOR ENGINEERS. [PT. III. CH. VIII.
Upper .
Lower
or Neo- •<
comian.
MESOZOIC OR SECONDAEY PERIOD.
Cretaceous System.
'Upper Chalk (with flints) : soft white chalk, containing
numerous flint nodules more or less arranged in
layers.
Middle Chalk (without flints) : harder and less white
than the Upper.
Loiver Chalk, including Grey Chalk, Chalk Marl, and
Chloritic Marl : a greyish or yellowish marly
chalk.
Upper Greensand : beds of siliceous sand with grains
of glauconite.
Gault : a bluish tenacious clay.
Lower Greensand : Yellow, grey, and green soft sands
with bands of limestone and ironstone and
occasional masses of ragstone and the Atherfield
Clay at its base.
Wealden, subdivided into (a) Weald Clay, and (b)
Hastings beds : clays, shales, sandstones, and
shelly limestones.3
Jurassic System.
fPurbeck beds : marls, fresh- water limestones, and shales.
Upper (or Portland beds : coarse- and fine-grained oolitic lime-
Portland K stones, marls, and sand.
Oolites. Kimmeridge Clay and shale : black bituminous shales
and calcareous clays.
Middle(or
Oxford)
Oolites.
Lower
(or Bath)
Oolites.
Corallian : formed of the Corallian Oolite, Calcareous
Grit, and Coral Rag.
Oxfordian, formed of the Oxford Clay and Kellaways
Rock : fossiliferous calcareous sandstone.
Great Oolite Series, including in its upper portion the
Cornbrash of limestones and marls, the Forest
Marble, and the Bradford Clay • in its lower portion
the Great Oolite proper of thick, cream-coloured
oolitic limestones, the Stonesfield Slate of thin-
bedded limestones, and Fuller's Earth.
Inferior Oolite Series, including the Inferior Oolite
limestones and grits and the Midford sands.
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. 141
Liassic .
I Upper Lias : clays and shells with nodular limestones.
Middle Lias or Marlstone : ferruginous limestone with
micaceous clays and sandy beds.
Lower Lias : blue clays, shales, and bands of limestone.3
Rhsetic
Upper
Trias or
Keuper.
Middle .
Lower
Trias or
Bunter.
Triassic System.
iPenarth beds : green and grey marls, black shales, and
" White Lias " of white and cream-coloured lime-
( stones and marls.
I Keuper Marls : red and green marls with occasional
sandstones and beds of rock-salt and gypsum.
Lower Keuper Sandstone : red and white sandstones
(waterstones), local grits, with occasionally a base
of conglomerate or breccia.
Muschelkalk of Germany : wanting in Britain.
{Upper and Lower Variegated Sandstones : soft, bright-
red, and variegated sandstones and marls with
thick beds of rounded pebbles.1
Upper
Middle
Lower
PALAEOZOIC OR PRIMARY PERIOD.
Permian or Dyas System.
Red sandstones, clays, and gypsum.
/Magnesian limestone.
\Marl slate.
Red and variegated sandstone.
Reddish-brown and purple sandstones and marls, with
calcareous conglomerates and breccias of volcanic
rocks.1
Upper
or Coal <
Measures.
\f-AA\
Middle
Carboniferous System.
'Red and grey sandstones, clays, and sometimes breccias,
with occasional seams and streaks of coal and
spirorbis limestone.
Middle or chief coal-bearing series of yellow sandstones,
clays, and shales, with numerous workable coals.
Gannister beds ; flagstones, scales, and thin coals, with
hard siliceous (gannister) pavements.
(Millstone Grit : grits, flagstones, and shales, with thin
\ seams of coal ; Moor Mock (Scot.).
142
GEOLOGY FOR ENGINEERS. [FT. III. CH. VIII.
Lower or
Carboni-
ferous
Lime-
stone.
Yoredale group of shales and grits, passing down into
dark shales and limestones.
Thick limestone in south and centre of England and
Ireland, passing northwards into sandstones,
shales, and coals.
Lower Limestone shale of south and centre of England,
passing northwards into Calciferous Sandstone
group of Scotland.1
Devonian System.
Devonian Type.
Old Red Type.
Upper.
Middle.
Lower.
Pilton group : grey, red,
and green grits and
slates with calcareous
seams in N. Devon, and
grey, red, and green
slates with volcanic
tuffs and -limestones
(Chudleigh) in S.
Devon.
Hfracombe group : barren
grey slates and grits
in N. Devon; lime-
stones (Torquay), vol-
canic rocks, and cal-
careous slates in S.
Devon.
Lynton group : slates,
grits, and calcareous
beds in both N. and
S. Devon.
Yellow and red sandstones
and conglomerates of
Caithness, Somerset, etc.
Caithness flags of N. Scot-
land : green, grey, and
reddish sandstones, flag-
stones, and conglomer-
ates with volcanic rocks
in S. Scotland, Middle
Cornstones of Hereford,
and parts of Shropshire.
Forfarshire flagstones,
sandstones, and con-
glomerates, with inter-
calated eruptive rocks.
Lower conglomerates and
sandstones of Ross ; lower
part of Cornstones in
S. Wales, viz. : blood-
red shales and marls
with bands of impure
concretionary limestone
termed cornstones.3- 18
SECT. I.]
PRINCIPLES OP STRATIGRAPHY AND PALEONTOLOGY.
143
Upper or
Ludlow.
Middle or
Wenlock.
Lower or
Llandovery.
Upper or
Bala.
Middle or
Llandeilo.
Lower or
Arenig.
Upper.
Middle.
Lower.
Silurian System.
Upper Ludlow Rock : composed of red sandstones
and calcareous grey shales, including (a) the
Downton Sandstone and Passage Beds or Tile-
stones; (b) the Bone bed; and (c) the Upper
Ludlow Shales, soft, incoherent shales and
mudstone.
Aymestry or Ludlow Limestone : concretionary lime-
stone crowded with fossils.
Lower Ludlow Shales : greenish-brown shales and
muds tones.
Wenlock Limestone : flaggy limestone of great thick-
ness, with corals.
Wenlock Shales : a thick mass of greenish-grey shales.
'Tarannon Shales : a thin series of purple shales.
Pentamerus Limestone : a hard calcareous rock with
Pentamerus (Brachiopod).
Mayhill Sandstone : an irregular sheet of coarse grit
and conglomerate.
Lower Llandovery : grits and flagstones.3
Ordovician or Lower Silurian System.
fCaradoc formation of calcareous sandstones and
\ shales ; and the Bala Limestone.
(Llandeilo Flag group : dark argillaceous flagstones,
\ sandstones, and shales.
(Arenig or Stiper Stone group : a thick series of grey
\ flags and dark shales.3
. . Cambrian System.
f Tremadoc Slates : dark-grey earthy slates.
\ Lingula Flags : bluish and black slates and flags
( with bands of grey flags and sandstones.
jMenevian Beds : sandstones and shales with dark-blue
( slates and flags, dark-grey flags, and grey grits.
(Harlech and Longmynd group : purple, red, and grey
( flags, sandstones, and slates with conglomerates.
Archaean and Pre-Cambrian Rocks.3
Torridonian, Lewisian, Pebidian, Uriconian, etc., series.
144
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
TABLE II. — CLASSIFIED LIST OF THE CHIEF GROUPS OF STRATA
IN NORTH AMERICA.17
Periods.
Local Characters, Names,
and Epochs.
Recent
Modern era
Indian shell-mounds.
(
Raised beaches and coast
Terrace
terraces.
Quaternary
1
River terraces.
•
Pleistocene <
Champlain <
Alluvian deposits.
Loess of the Mississippi.
Flood deposits.
Diluvian deposits.
Orange-sand beds.
Gravels and Erie clays.
Glacial |
Erratic blocks.
Boulder-clay and drift.
Pliocene
Sumter
Loams, sand, and phos-
phatic beds of North
and South Carolina.
1
Loup River group.
Shiloh, Yorktown, and Gay
Head beds.
Tertiary ,
Miocene
Yorktown
Richmond and Montmery
diatomaceous earths.
White River beds.
Wind River beds.
The John Day basin of
Oregon.
Vicksburg group.
Green River shales.
Charleston burhstones.
Bridger beds.
Grand Gulf beds.
Eocene
Alabama
Orbitoides limestone.
The Claiborne group.
White limestone of Ala-
bama.
Jackson lignitic clays.
Wahsatch beds.
SECT. I.J PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. 145
TABLE II. — Continued.
Periods.
Local Characters, Names,
and Epochs.
Transition, '
Tertiary, or
Upper
Cretaceous
Laramie
Lignitic series of the Mis-
sissippi, Upper Missouri,
and Green River basins.
(
Fox-hills f
Sandstones of the Fox-hills
and base of Big Horn
Later
group I
Mountains.
Cretaceous j
Pierre J
Plastic clays of Upper
I
group (
Missouri.
Niobrara j
Calcareous marls of the
group 1
bluffs on the Missouri.
Earlier
Cretaceous '
Benton (
group <
Clays and limestones on
the Upper Missouri.
f
Rotten limestone of Ala-
bama and Tennessee.
Dakota
Sandstones and clays with
Mesozoic ,
Age \
group j
tf
lignites of Dakota, Kan-
sas, and New Mexico.
Wanting on the Atlantic
and Gulf borders.
Jurassic J
Oolite and
Liassic
Marls and limestone of
Wahsatch, Laramie
Range, and LTintah.
Auriferous slates of Sierra
„
Nevada.
Red sandstones and con
The Acadian
glomerates of Nova
area
Scotia, Connecticut val-
ley, and Pennsylvania.
Triassic
Coal Measures of Virginia
The Palisade
Q T\C\
and North Carolina.
The Elk and Uintah Moun-
cil H I -<
other areas
tains of Colorado and
Sierra Nevada of Cali-
fornia,
10
146
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
TABLE II. — Continued.
Periods.
Local Characters, Names,
and Epochs.
Permian
Upper
Palseozoic
or Car-
boniferous
Carboniferous
Middle
Palaeozoic,
Devonian "
Sub-
Carboniferous
Catskill
Chemung
Hamilton
Corniferous
Limestones, sandstones,
gypsum, marls, and con-
glomerates of the In-
terior Continental basin,
west of the Mississippi,
Kansas.
The Upper and Lower Coal
Measures of the Alle-
ghany region, Illinois,
Missouri, Michigan,
Rhode Island, New
Brunswick, Nova Scotia,
northern half of Cali-
fornia, and parts of
Wyoming and Utah.
Conglomerates and sand-
stones of Appalachian
region, Virginia, and
Tennessee.
Limestones, sandstones,
and shales of Illinois,
Kentucky, Iowa, Tennes-
see, Michigan, and Ar-
kansas.
Limestones of Utah, Wy-
oming, and Northern
California.
Catskill red sandstone.
Chemung shales and sand-
stones.
Portage sandstones.
Genesee shales.
Hamilton flags and shales.
Marcellus shale.
Corniferous and Onon-
daga limestones.
Schoharie grit — Cauda
Galli grit.
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 147
TABLE II. — Continued.
Periods.
Local Characters, Names,
and Epochs.
Oriskany
Oriskany sandstone.
Upper
Silurian
Lower ]
HelderbergJ
Salina
Water-lime group.
Onondaga salt group.
'
Niagara
Niagara shales and lime-
stone.
Clinton sandstones.
Medina marls and sand-
stones.
Oneida conglomerates.
Trenton
Cincinnati limestone and
Hudson River shales.
Utica shales, Taconic
slates.
Trenton limestones.
Lower
Silurian
Canadian -!
Chazy epoch : limestones.
Quebec epoch : sandstones
and limestones.
Calciferous epoch : sand-
rock.
\
Primordial -
Potsdam epoch : sand-
stones.
Georgia shales and Chil-
howee sandstone.
Acadian epoch : shales and
sandstones of St John,
Ocoee conglomerate.
Archaean
Huronian.
Laurentian.
148
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
•OlOZOUrBQ
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a* -3
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If!
I1!
H
W ® 0} S
Jg g s? «!
O) <D ° fl
§ -3 .§.s|l
IH E P-iSOH
III I
^-1-sl
lll
O
i?.HiR|
,2 d
IIII
-4!
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 149
•0107098^(5
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.|
WtJ
tS
ow
a
If
150
GEOLOGY FOR ENGINEERS. [FT. III. CH. VIII.
TABLE IV. — LIST OF THE SEDIMENTARY AND METAMORPHIC
STRATA OF AUSTRALIA.IT
Age.
Pleistocene .
Pliocene
Miocene
Eocene
Cretaceous
Triassic
Permian
Car-
boniferous
Devonian .
Silurian
Archaean
n
Upper,
Middle,
Lower, or
Oolitic
( Upper
( Lower
( Upper
{ Lower
( Upper
I Lower
Local Formations.
Black-soil plains.
Ossiferous caves containing extinct
gigantic kangaroos and emus.
" Deep leads," mostly capped by basalt.
Portland beds of Victoria and "deep
leads " ; Murray River beds of South
Australia.
Fresh-water "deep leads" with plant
beds.
Marine beds of Victoria.
Desert sandstone of Queensland.
Fresh- water beds of N.S.W. and Queens-
land.
Marine clays.
Wainamatta, Hawkesbury, and Clarence
series of N.S.W.
Carbonaceous series and Bacchus Marsh
Sandstone of Victoria. Ipswich, Bur-
rum, and Clifton coal-beds of Queens-
land.
Newcastle and Bowenfels Upper Coal
Measures, N.S.W. ; Coal Measures,
Queensland ; Middle Coal Measures of
East Maitland, N.S.W.
Upper Marine beds, N.S.W.
Lower Coal Measures, N.S.W.
Lower Marine beds, N.S.W.
Port Stephen's beds, N.S.W.
Brown sandstones and quartzites.
Murrumbidgee beds.
Yars beds, etc., N.S.W.
Mudstones of Yarralumla.
Slates, grits, limestones, Gordon River
beds.
Gneiss and schists of Silverton, N.S.W.
Gneiss of Bathurst and S.W.A.
SECT. I.] PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. 151
TABLE V. — LIST OF THE SEDIMENTARY STRATA OF
NEW ZEALAND.17
Age.
Local Names.
Recent
Pleistocene .
Pliocene
Upper Miocene .
Lower Miocene .
Upper Eocene
Cretaceo-Tertiary
Neocomian .
Jurassic
Liassic „
Triassic
Permian .
Upper Carboniferous .
Lower Carboniferous . )
Upper Devonian . . )
Lower Devonian .
Upper Silurian .
Lower Silurian
Archaean and Plutonic (
rocks . . . (
Moa beds, alluvia, shingle plains.
Cave deposits — shore deposits.
Terrace plains, alluvial gold drifts,
pumice and lignite beds, Kereru beds.
Wanganui series.
Waitotara and Awatere beds.
Taipo, Awamoa, Margapakeka, and
Pareora beds.
Mount Brown beds, Oamara beds, Num-
mulitic beds.
Ototara stone, Fucoidal greensands,
Amuri limestone, Coal formation, pro-
pylite breccias.
Conglomerates with coal, porphyries,
greensands.
Mataura series, coal seams.
Pututaka beds, Flag-hill beds.
Cattin River and Bastion series.
Otapiri series, Wairoa series, Oreti
series.
Kaihiku series, Mount Potts and Glos-
sopteris beds.
Wanting 1
Maitau series, Te-anan series.
Reefton beds.
Baton River slates and limestones.
Mount Arthur series and graptolite
slates.
Gneiss, mica schists, syenites, and
granites.
152
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII,
TABLE VI. — LIST OF THE SEDIMENTARY STRATA OF
SOUTH AFRICA.17
Age.
Recent
Pleistocene 1
Pliocene 1 .
Miocene
Eocene
Cretaceous .
Jurassic J ™«>hage
1 formation
Triassio f Stormberg
(Permio-
Triassic)
Carboniferous
Devonian
Silurian 1
Cambrian 1
Archaean ?
series
Karoo
series
Local Names.
Tufa, shell-breccias, and blown sands.
Clays, sandstones, and lignites of the
Cape flats.
Shell beds and raised beaches on the
seaboard of the East Province.
Wanting.
Umtafuna and Impengati beds.
Trigonia beds, \
I Wood bed 1 400 feet.
< Saliferous beds, C
( Zwartkop sandstone, )
Enon conglomerate, 300 feet.
Cave sandstones, 150 feet.
Red beds, 600 feet.
Stormberg beds with coal, 2000 feet.
Sandstone and shales, 5000 feet.
Kimberley or Olive shales and con-
glomerates, 2300 feet.
[ Upper Ecca beds, 2700 feet.
< Ecca or Dwyka conglomerate, 500 feet.
( Lower Ecca beds, 800 feet.
Witteberg and Zumberg quartzites, 1000
feet?
Table Mountain sandstone, 4000 feet.
Bokkeveld beds.
Malmesbury beds ; mica schists and
slates of the Cape; Namaqualand
schists and gneiss.
Section II. — Palaeontology.
Palaeontology is the study of fossil beings ; and it treats of the
living beings — animal and vegetable — which have inhabited the
earth at past periods in its history.
A fossil or "petrifaction" is any body, or the traces of the
SECT. II.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 153
existence of any body, whether animal or vegetable, which has
been buried in the earth by natural causes.
Fauna and flora mean the entire assemblage of the animals and
of the plants respectively belonging to a particular region or a
particular period.1
Fossilisation or petrification refers to the particular state of
preservation of fossils, (a) In the majority of cases the original
substance of the fossils is preserved unaltered. This is usually the
case with all bodies originally stony — such as shells, corals, and
the like — and in a less degree with teeth, bones, and scales of
animals, and even with trunks, branches, and leaves, (b) Some-
times the original substance has been replaced by fresh mineral
matter leaving an exact model not only as to external form but
often as to minute structure. The replacing material may be
silica, iron pyrites, iron oxide, iron carbonate, and even sulphur,
malachite, vivianite, etc. Thus wood, shells, corals, and the like
may become silicifled. Sometimes (c) the original substance
may be wholly removed, leaving in the rock a hollow mould.
Sometimes (d) this mould may become afterwards filled up by
fresh mineral matter, giving a solid cast of the original fossil.3
Classification of Animals. — Animals are arranged by zoologists
into two grand divisions — the Invertebrata, which are divided into
eight sub-kingdoms ; and the Vertebrata, which forms a single
sub-kingdom.1 In the following arrangement the order is an
ascending one.
INVERTEBRATA.
Protozoa. — Animals of the lowest type, of a jelly-like substance,
in many cases secreting beautiful shells.
Rhizopoda include (1) Foraminifera having mostly calcareous
shells. These date from Silurian and possibly Archaean times and
are very abundant, often form-
ing thick beds of limestone ; e.g.
Chalk of Cretaceous age, Num-
mulitic Limestone of Eocene age,
Atlantic ooze still forming.
Genera : Globigerina, Miliola,
Nummulites (fig. 36). (2) Radio-
laria having siliceous shells of
great beauty. The remains are
rarely found fossil.2
Spongida. — Sponges are soft FIG. 36.— Nummulitic Limestone,
animals with an internal skeleton
of horny fibres, or of spicules of lime or of silica. The latter
kinds occur abundantly as fossils from the Cambrian period.2
154
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
Cselenterata (Corals and Zoophytes). — Simple or compound
animals with a distinct body-cavity, and often a firm, stony
skeleton of radial plates or tubes, or of both.
(1) Hydrozoa, include jelly-fishes, millepores, corals, and
graptolites ; the last named had pen-like, horny skeletons, the
glistening impressions of which, in pyrites, are common in Silurian
rocks. Genera : Monograptus, Diplograptus, Didymograptus,
Dictyonema, Rastrites (figs. 37-40).
FIG. 38.— Diplo-
graptus.
FIG. 39.— Didymo-
graptus.
FIG. 37. — Monograptus.
a, spirilis ; b, cyphus.
FIG. 40. — Rastrites.
FIG . 41. — Lithostrotion.
FIG. 42.— Calceola
sandalina.
(2) Actinozoa include sea-anemones, sea-fans, and corals. Corals
may be single as in the mushroom corals (Fungia), cup corals
(Caryophyllia, etc.); or compound, as in the reef-building corals
(Astrcea, etc.).2 The space inhabited by the zoophyte is more or
less circular in section, with divisions called septa reaching from
the circumference towards the centre.5 They are divided into : —
Tetracoralla or Rugose (wrinkled) corals, with septa in fours
of Palaeozoic times: Lithostrotion^ Calceola (figs. 41, 42).
SECT. II.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 155
Hexacoralla or Zoantharia, with septa in sixes, e.g. Madrepora,
Favosites (figs. 43, 44).
Octocoralla or Alcyonaria, with septa in eights, e.g. red coral,
Heliolites, Syringopora (figs. 45, 46).2
Echinodennata (echinus, a spine ; derma, skin). — Usually
five-rayed animals with a rough skin strengthened by calcareous
fie 43.— Madre-
pora.
®£g3
FIG. 44.— Favosites.
(Young specimen.)
FIG. 45.— Heliolites.
FIG. 46. — Syringopora.
FIG. 47. — Pentacrinus.
FIG. 48. — Encrinus
liliiformis.
particles or by plates so fitted as to form a shell covered with
movable spines. They include : —
(a) Fixed forms, mostly with jointed calcareous stalks: —
(1) Crinoids or encrinites, "sea-lilies," ranging from Cambrian,
abundant in Carboniferous and Jurassic, e.g. Pentacrinus, Encrinus
liliiformis (figs. 47, 48) ; (2) Cystideans or bladder-like forms ; and
(3) Blastoids or bud-like forms, both found in Palaeozoic only.
156
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
(b) Free forms, viz. (4) Echinoidea or "sea-urchins,"2 whose
hard external crusts with knobs or tubercles and perforations
arranged geometrically are very noticeable and are abundant in
the Chalk ; 5 (5) Asteroidea or star-fishes; (6) Ophiuroidea or brittle-
stars ; and (7) Holothurea or sea-cucumbers.8
Annulosa (ringed) or Vermes (Worms). — The chief fossil
forms belong to Annelida, which encase themselves in a calcareous
tube, e.g. Serpula (tubes of), Arenicolites (tracks and burrows).5
Arthropoda or Articulata (jointed). — This sub-kingdom com-
prises all creatures without a backbone, which have jointed or
FIG. 49.— Cypris.
FIG. 50.— Estheria.
FIG. 51.— Eurypterus.
articulated limbs. They have segmented bodies6 and a hard
skin of chitin, often calcified.
(1) Crustacea, the jointed shell-fish, with many paired legs,
gills, and a firm crust. The chief groups are : — The lobsters and
crabs (Mesozoic) ; Barnacles ; Ostracods, Cypris (fig. 49) ;
Phyllopods, Estheria (fig. 50), and in the older strata Eurypterids —
Eurypterus (fig. 51), Pterygotus, and especially Trilobites. These
varied in size from that of a pin's head to over 2 feet in length.2
They had symmetrical jointed bodies, large compound eyes, and
two lines or indentations running down them giving them the three-
lobed appearance from which they derive their name.5 Chief
genera: Paradoxides (fig. 53), Olenus and Olenellus (fig. 52)
SECT. II.] PRINCIPLES OF STRATIGRAPHY AND PALEONTOLOGY. 157
(Cambrian), Asaphus, Ogygia (Ordovician), Phacops, Calymene
(Silurian).
(2) Arachnida include spiders and scorpions, both dating from
Palaeozoic.
(3) Myriapoda include millepedes and centipedes, with very
many paired limbs, dating from the Carboniferous.
FIG. 52.— Olenellua.
FIG. 53. — Paradoxides.
FIG. 54.— Fenestella.
FIG. 55.— Spirifer.
(4) Insecta include beetles, flies, moths, bees, etc., mostly
having distinct head, thorax, abdomen, six legs and four wings.
They date from Silurian in France, Devonian in America, and
Carboniferous in Britain.2
Molluscoida (Mollusc-like). — A provisional group allied both
to Mollusca and to Yermes.
(1) Polyzoa, small compound animals common in rocks of all
ages, encrusting shells, or free, and aiding largely in the formation
158
GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
of some limestones. The miscalled Coralline Crag is rich in
polyzoans. Ex., Fenestella (fig. 54), the "lace-coral."
(2) Brachiopoda have shells with two valves placed front and
back, each symmetrical, but usually unequal, the front one with a
beak. Chief genera : Lingula, Rhynchonella (fig. 56), Terebratula
(fig. 58), Spirifera (fig. 55), Productus (fig. 57), Atrypa, Orthis,
Strophomena?
FIG. 58. — Tere-
bratula.
FIG. 56.— Rhyn-
chonella.
FIG. 57. — Productus.
FIG. 59.— Gryphaea.
FIG. 60. — Cyrena.
FIG. 61. — Hippurites.
Mollusca. — Soft-bodied animals enclosed in a tough muscular
skin (mantle) and usually covered with a calcareous shell often
very thick and of great size, either in one piece or in two
" valves."
(1) Lamellibranchiata differ from Brachiopoda in breathing by
leaf-like gills, the two valves of the shell being usually equal, often
unsymmetrical, and placed right and left. The forms with but
one shell-muscle occur only in the sea, as Ostrea (oyster), Gryphcea
(fig. 59). Those with two may occur in either salt water, as
Cardium (cockle), Mytilus, Cyprina; or in fresh water, as Unio
(mussel), Anodonta, Cyrena (fig. 60), Hippurites (fig. 61).
SECT. II.] PRINCIPLES OF STRATIGRAPHY AND PALAEONTOLOGY. 159
(a.)
FIG. 62. — Gasteropods. a, Bellerophon ; b, Limnsea ; c, Planorbis ; d, Paludina.
FIG. 63.— Nautilus. FIG. 64.— Gomatites. FIG. 65.— Ceratites.
FIG. 66.— Ammonites. FIG. 67.— Turrilites. FIG. 68.— Scaphites.
160
GEOLOGY FOR ENGINEERS. [FT. III. CH. VIII.
(2) Gasteropoda : snails, whelks, cowries, etc., having univalve
shells, usually spiral, breathing either by gills or in the land and
fresh- water forms by a lung-sac. Ex., Bellerophon, Limncea, Plan-
orbis, Paludina (fig. 62).
(3) Cephalopoda : octopus, cuttle-fish having long arms bearing
suckers around the mouth, which contains powerful jaws. Chief
genera: Nautilus (fig. 63), Orthoceras (fig. 69), Clymenia,
Goniatites (fig. 64), Ceratites (fig. 65), Ammonites (fig. 66),
Turrilites (fig. 67), Hamites (fig. 71), Scaphites (fig. 68),
Belemnites (fig. 70).2
FIG. 69.— Orthoceras. FIG. 70.— Belemnites.
FIG. 71. — Hamites.
VERTEBRATA.
Fishes. — The internal skeleton varies from osseous to carti-
laginous. The external covering may be armour scales or no
protection. Scales are (1) ganoid (shining), or formed of bone
covered with enamel ; 3 (2) placoid (plate-like), when the body is
covered with horny plates or bristled with small eminences like
the shagreen of the shark ; 18 (3) cycloid, when they are bony or
horny, destitute of enamel, with a smooth surface often bearing a
central spine, and having rounded margins. (4) Ctenoid are of
similar composition, but are jagged at the edges like the teeth
of a comb. Tails of fishes may be diphycercal* (double-tail )>
heterocercal (with unequal lobes), or homocercal (with symmetrical
lobes).3
* The vertebral column is straight throughout, and its terminal portion is
symmetrically surrounded by the caudal fin. In the others this terminal
portion is bent obliquely upwards, and the lower part of the caudal fin is,
developed into a distinct lobe, so that the tail becomes bilobed in form.3-
SECT. II.] PRINCIPLES OP STRATIGRAPHY AND PALAEONTOLOGY. 161
Groups of Fishes.— (1) Teleostei (perfect-boned), with the whole
of the internal skeleton osseous and the tail homocercal ; e.g. most
modern fishes.
(2) Ganoidei ; ganoid scales, osseous or cartilaginous skeleton,
and heterocercal tails, e.g. Palgeozoic fishes.
(3) Elasmobranchii (plate-gilled) include sharks and rays, skin
unprotected or with scattered, isolated scales, cartilaginous
skeleton ; e.g. Hybodus, Acrodus.
(4) Dipnoi (double breathers) include mud-fishes ; e.g. Ceratodus.
(5) Marsipobranchii (pouch-gilled) include the Ostracodermi or
shell-skinned, Cephalaspis and Pteraspis.
Amphibia. — These have the character of fishes in the young or
tadpole stage, and of reptiles in their adult state. Besides the
Batrachians, frog, toad, etc., and the recent tailed forms, newt,
etc., there were Labyrinthodonts, bone-clad forms with peculiar
teeth, and often of colossal size, Mastodonsaurus, found only in
Carboniferous, Permian, and Triassic rocks.2
Reptilia can be distinguished from Amphibians by the articula-
tion of the skull to the vertebrse, the complexity of the lower jaw,
and by the teeth having one fang.5 Of the ten orders known only
four still live : —
(1) Tortoises and turtles, dating from Jurassic times.
(2) Snakes and serpents, unknown before Tertiary times.
(3) Lizards, including the Permian Proterosaurus (the earliest
known reptile), the Triassic Telerpeton, and the gigantic Cretaceous
Mosasaurus.
(4) Crocodiles, including the Stagonolepis of the Trias, Teleosaurus
of the Oolite, and Goniopholis of the Cretaceous.
The chief extinct orders, all Mesozoic only, are : —
(5) Ichthyosauria, from the Trias to Chalk.
(6) Plesiosauria, Trias to Chalk.
(7) Ornithosauria or Pterosauria, flying reptiles, Pterodactylus,
etc., from the Lias to Chalk.
(8) Dinosauria, huge reptiles ; Iguanodon, Megalosaurus,
Atlantosaurus (100 feet long).2
Birds. — The earliest known form is Archceopteryx of the Upper
Oolites, with a long bony tail, toothed and very like a reptile. Bird
fossils are never abundant, but are fairly common in Tertiary strata.2
Mammals. — (1) Marsupials; having pouches for carrying the
young.
(2) Cetaceans ; whales, porpoises, dolphins.
(3) Sirenians ; manatee, etc.
(4) Edentates (toothless) ; armadillo, Glyptodon, Megatherium.
(5) Insectivores ; mole, Stereognathus.
(6) Bats.
u
162 GEOLOGY FOR ENGINEERS. [PT. III. CH. VIII.
(7) Rodents ; beaver, hare, mouse, etc.
(8) Ungulates, including horse, pig, rhinoceros, tapir, Palceo-
therium ; also the Ruminants, ox, elk, deer, etc. ; and Probosci-
deans, elephant, Mastodon, Dinotherium.
(9) Carnivores ; lion, cat, dog, bear, hyaena.
(10) Primates ; monkeys, apes, and man.2
Classification of Plants. — Plants have been arranged by
botanists into two main divisions or sub-kingdoms — namely, the
Phanerogams or flowering plants; and the Cryptogams or non-
flowering plants.3 The following arrangement is in descending
order.
PHANEROGAMS.
Angiosperms. — True flowering plants, often bearing bright
bells or brilliant clusters of bloom, and always having their seeds
enclosed in some more or less conspicuous form of enveloping fruit.
Dicotylce or Dicotyledons. — Plants with two cotyledons or seed-
lobes — also called Exogens from their mode of growth outwards
forming annual rings of growth, as in all our shrubs and forest trees.
They date from Upper Cretaceous and abound in Tertiary strata.
Monocotyledons or Endogens. — Plants with one seed-lobe, in-
creasing by additions in the interior. Ex., grasses, palms, lilies,
arums. They date from Carboniferous age.
Gymnosperms. — Plants with naked seeds, i.e. not contained in
a seed-vessel; mode of growth exogenous, like that of the
dicotyledons. Cycads, palm-like plants with hard leaves, easily pre-
served. They date from Permian times.
Conifers. — Firs, pines, yews, etc., with resinous wood (the fibres
of which have peculiar markings) and usually cone-shaped fruits.
These date from Devonian.
CRYPTOGAMS.
Pteridophyta or Fern-plants. — Lycopods or Club-mosses, the
most abundant of Carboniferous plants. Equisetites or Horse-tails,
with pointed stems. Ex., Calamites, Equisetum. Ferns, often
abundant and of tree-like size, from Devonian or Silurian times.
Ex., Cyclopteris, Neuropteris, etc.1
Bryophyta or Moss-plants. — Mosses form beds of peat ;
Sphagnum, the bog-moss ; Liverworts.1
Thallophyta. — Fungi — Mushrooms, lichens, etc. — rare as fossils.
Algce — Sea-weeds, etc., occur fossil from Cambrian period. Some,
which secrete mineral matter, form rock-deposits, e.g. Corallines
and Nullipores, with calcareous incrustation ; Diatoms, with beauti-
ful siliceous cases, and Chara, with encrusted stems and fruits,1
SECT. I.]
CHAPTER IX.
THE GEOLOGICAL SYSTEMS.
Classification of Strata. — If classed as to their place and mode
of formation strata are terrestrial, lacustrine, fluviatile, or marine.
Terrestrial deposits on old land surfaces are marked by tree-
stumps in position as they grew, with tree-stems, leaves, fruits,
land-shells, insects, etc., in the old soil round their roots.
Lacustrine deposits in old lake beds and Fluviatile deposits in old
river beds are marked by : —
(a) Beds of shell-marl or limestone, full of fresh-water shells.
(b) Clay containing these shells or insects, leaves, fruits, etc.
(c) Iron-stained sand, gravel, and conglomerate with fresh-
water and land plants, fishes, etc.
Marine deposits on old sea bottoms are marked by sea-weeds,
sea-shells, echinoderms, corals, foraminifera, etc.2
Section I.— Anthropozoic or Quaternary Period.
This period includes all that time which elapsed between the
close of the Tertiary period and the present day. There is no
break either stratigraphical or palseontological between the
Tertiary and the Quaternary, but the latter is signalised by the
commencement of the Glacial epoch. It is not possible to
arrange the Quaternary accumulations in strict chronological
order, because their relative antiquity is so often indeterminable.
In the glaciated regions of the northern hemisphere the various
Glacial deposits are grouped as the older division under the name
of Pleistocene, and the younger accumulations which lie above
them are named Recent or Post-Glacial.8
RECENT OR POST-GLACIAL FORMATIONS.
Human Relics. — In most of the non-glacial deposits we find
evidence of the existence of man, in the form of implements of
stone, bone, etc., and the study of this special branch of geology
163
164 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
shades insensibly into the science of archaeology.1 The following
is the chronological classification of the deposits yielding traces of
human workmanship which is generally employed by archaeo-
logists : —
TT- . ( The period covered by the records of human
' ( history down to the present time.
( (c) Bronze and Earlier Iron Age.
Prehistoric < (b) Neolithic or Newer Stone Age.
( (a) Palaeolithic or Older Stone Age.3
The rude implements of Palaeolithic man occur in association
with the bones of wild animals, many of which are now extinct.
The smooth and polished implements of Neolithic man occur in
association with the bones of wild animals similar to those now
existing, together with those of the ordinary domestic or tamed
animals, and with various objects of human manufacture.3
Non-glacial Deposits. — The most characteristic phenomena of
recent times and the preceding non-glaciated areas of Pleistocene
times are as follows : —
Cavern deposits occur usually in limestone districts in the caves
and rock shelters inhabited by early man and are largely
Palaeolithic,3 e.g. the Brixham Cave.
Alluvial deposits (cf. p. 15). — Under this head are comprehended
all accumulations and deposits resulting from the operations of
rivers. These accumulations are often of great thickness, and
consist for the most part of alluvial silt, masses of gravel and
shingle, with occasional beds of fine, dark-blue unctuous clay and
layers of shell-marl, e.g. in the Thames and Severn valleys. The
river-terraces (cf. p. 16) which belt the slopes of most inland river
valleys are composed of sand, shingle, and silt, and give evidence
of the former flood-levels of the river. It is usual to distinguish
these river gravels as low-level and high-level, the former being
the more modern and containing relics of Neolithic man, while the
older and higher terraces contain traces of Palaeolithic man.
Among alluvial formations must be grouped those wide-
spreading foreign sheets of gravel, sand, and mud, such as those
of the river-plains of Eastern North America, South America,
Siberia, the valley gravels of California, Australia, New Zealand,
etc.3
Lacustrine deposits. — Silted-up lakes are numerous in almost
every country, and many parts of alluvial valleys are but the
sites of former lakes and marshes filled up and obliterated. The
organic remains found in lake-deposits are strictly fresh-water and
terrestrial — fresh-water shells, as Limncea, Planorbis, and
Paludina (fig. 52), in the marls; marsh plants, as the reed,
SECT. I.] THE GEOLOGICAL SYSTEMS. 165
bulrush, and equisetum, in the peat-moss; terrestrial plants, as
the birch, alder, hazel, oak, pine, etc.3 With these occur bones
and skeletons all pointing to Neolithic times.
Fluvio-marine formations. — In the "deltas" or large expanses
of low alluvial land which have accumulated at the mouths or in
the estuaries of rivers are found marine shells, etc., in the lowest
beds which are of Palaeolithic age, and in the higher beds forms of
more recent times.
Aerial deposits of sand-drift, which form "links" and "dunes"
on the coast, contain a few Neolithic relics.1
Raised beaches and submerged forests (cf. p. 29) afford evidences
of the great variations in the level of the shore-line which have
taken place in Quaternary time.3 Such raised beaches are found
fringing the south coast of England ; 17 the higher ones " probably "
belong to the final stages of the Glacial epoch, since some of them
are partly covered by Glacial clay.3
Submerged forests and peat-lands occur in the Bristol Channel,
at the entrance to the Mersey, and on the Lancashire coast, as
well as on the Lincoln coast.17 They consist of a bed of peat
or semi-lignite from 2 to 6 feet in thickness, abounding in
roots and trunks of trees in the lower portion, and in mosses
and aquatic plants in the upper and lighter-coloured portion.
The trees are chiefly oaks (often of great dimensions), Scotch firs,
alders, birches, hazels, and willows; and throughout are em-
bedded hazel-nuts, seeds of various plants, and the wing-cases
of insects.3
PLEISTOCENE OR GLACIAL FORMATIONS.
At the period of maximum cold the ice-sheet extended over
only a part of the northern hemisphere, and possibly reached out
also from the Antarctic regions into South America and
Australasia. In Britain the southern limit of the ice was the
hill-range bounding the Thames valley on the north. In the
areas which were not reached by the ice the non-glacial deposits
described above must have commenced in Pleistocene times.1
Glacial Deposits. — These are collectively known as the Drift
or Glacial Drift which forms a more or less continuous mantle
overspreading the rocky floor of the British Islands, and all
Europe north of a parallel through the valley of the Thames.
It consists of a sheet of clay, sand, and gravel, lying below the
soil, and resting unconformably upon the rocks which form the
solid floor of the country. It is usually thickest and most
compact in the open plains and deep valleys, and thins away
upon the ridges and swells of the higher grounds. Sometimes it
is scores or even hundreds of feet in thickness ; sometimes it is
166 GEOLOGY FOB ENGINEERS. [PT. III. CH. IX.
reduced to a mere film. It is composed of more or less
tumultuous masses of clay, sand, and gravel (sometimes rudely
stratified, sometimes destitute of stratification), usually containing
pebbles and rounded and angular blocks or boulders (from a
pound to many tons in weight) of rocks many of which are
foreign to the district where they are met with, while the
surface of the clay itself is often scattered over with similar
erratic blocks (cf. p. 19), many miles from their original home. The
more loosely compacted types of Drift are known as Boulder Clay
(a title which is sometimes applied to all these clayey drifts in
general), and the most closely compacted varieties are known in
Scotland as Till. Associated with the boulder-bearing clays are
masses and sheets of gravel and sands, which sometimes occur
grouped together and form a middle member of the Drift series,
having till below them and ordinary boulder-clay above. Some-
times they occur as sheets overlying boulder-clay, and sometimes
they occur alone, overspreading wide tracts of country (Fluvio-
glacial Deposits).
The rocky floor on which the Drift rests is often polished and
grooved in different directions (striated), and the boulder-clays
are locally accumulated in longer mounds (moraines) or shorter
mounds (drumlins), and the gravels in wide expanses or narrow
sinuous ridges (Kames, Eskers).z
Great Britain. — The Glacial series is well developed in the
eastern counties, notably on the coast of East Norfolk, where the
" contorted drift " consists of loams, chalk, rubble, and shelly beds
which have been powerfully contorted. In Lincolnshire occurs
the ordinary triple series of Upper Boulder Clay, Middle Sands and
Gravels, and Lower Boulder Clay. There is little drift on the high
lands of Central England, but on the western side there is a thick
covering of Glacial deposits, the triple series recurring in
Shropshire, Cheshire, and Lancashire.17
In Scotland the Till or Lower Boulder Clay overlies rock-
surfaces sometimes highly glaciated, and is succeeded by clays
interstratified with beds of sands and gravels.
Up the Clyde valley come the later brick earths with their
Arctic molluscan fauna.17 The Kames or peculiar elongated
ridges (or rarely mounds) of sand and gravel are comparatively
frequent in Scotland ; but in Ireland, where they are known as
Eskers, they are most abundant. These Esker systems extend
sometimes for over a hundred miles, but are modified by local
circumstances. On low ground they are well-defined ridges
which break into irregular mounds and short ridges crossing high
ground, but again becoming well defined when the high ground
is passed. If a hill occurs, the esker will be either deflected
SECT. I.] THE GEOLOGICAL SYSTEMS. 167
round it or there will be a break in the system, as it ends on or
near one side of the hill but sets in again on the other side. The
eskers of the central plain of Ireland are often associated with
the bogs, either running in lines between two large bogs, or
partially or entirely surrounding flat places, which seem to have
been converted into bogs in consequence of the eakers damming
the drainage of the country, the superfluous water simply soaking
through the porous base of the esker.18
Boulder clay. — In passing from the south to the north, the
white Chalk, the dark-grey Kimmeridge and Oxford Clays, the
light yellow Oolitic strata, the grey Lias, the red Sandstones and
Marls, and black beds of the Carboniferous rocks, come succes-
sively to the surface ; and it is found that the Boulder Clay not
only partakes in each area of the nature and colour of the under-
lying rocks, which the ice has ploughed up, but also maintains
the colour and is composed largely of the debris (clay and gravel)
of the rocks which it successively overlaps for a considerable
distance to the south of their outcrop, whereas on the north side
there is an entire absence of such debris.17
Erratic blocks. — Some of the Scandinavian rocks were carried
to Eastern England; abundant boulders of the metamorphic
Highland rocks are spread out over the low grounds of Central
Scotland ; and those of the igneous rocks of Galloway, the Lake
District, and the Arenig Mountains of North Wales are scattered
over much of the Midland area of England.3
Continental Europe. — The ice-sheet which covered the greater
part of Britain formed part of a vast continental ice-sheet which
attained its greatest thickness in the mountains of Scandinavia.
From this centre one portion flowed westward into the North
Atlantic, while other portions passed northward into the Polar
Sea, eastward over Finland, and southward over Sweden and the
islands of the Baltic. Throughout this area the rocks are
glaciated and covered in places with a tenacious boulder-clay full
of striated stones and boulders. This clay extends as far as the
coast of Germany, where it is replaced by a thick accumulation
of sand and gravel, with large boulders, which lie scattered from
Tcheskaia Bay to a point a few miles south of Moscow and Warsaw,
and thence by Leipsic and Hanover to the coast of Holland.18
Huge blocks of Finnish granite are scattered over the plains of
St Petersburg, and extend to the neighbourhood of Moscow. In
Poland and North-Eastern Germany boulders from the rocks of
Finland are mingled with others from North Sweden ; while
throughout North-Western Germany, Hanover, and as far as
Holland, the boulders consist of gneiss, granite, diorite, and
Silurian rocks from the southern parts of Sweden.18
168 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
Phenomena corresponding to those characteristic of the Glacial
deposits of Northern Europe and Britain occur in many other
parts of the northern hemisphere. We have evidence that
during the Great Ice Age the Alpine glaciers coalesced into a vast
sheet of ice which poured out over the lower grounds northward
to the foot of the Black Forest and into Bavaria and Austria,
westward down the valley of the Rhone nearly to Lyons, and
southward far into Piedmont, Lombardy, and Venetia — filling up
the valleys and lowlands, transporting gravels and erratics, and
piling up vast moraines.3
North America. — A northern continental ice-sheet seems to
have extended southward in the earlier stages of the Ice Age to
New York and St Louis. The great drift-mounds and moraine
hills of the Glacial period have been traced far and wide across
the entire breadth of the continent from Long Island to the
Rocky Mountains.3
Asia. — After passing the Urals there is no trace of land-
glaciation, although there is evidence of extreme cold, and
mammoth remains have been found in the frozen ground in Siberia.
The glaciers of the high range of the Caucasus had formerly a far
greater extension, and in the range of the Lebanon the remains of
old moraines are still conspicuous. In Northern India traces of
glaciation have been found in the valleys of Sikkim and Eastern
Nepal down to 5000 feet. In the Western Himalayas perched
blocks (cf. p. 19) are found at still lower heights.18
Africa. — The denudation of great part of the Katberg,
Stormberg, Krome and other ranges lying between the latitude
of 30° and 33° in South- Western Africa, with peaks rising to the
height of from 5000 to 1000 feet, has been effected by the agency
of ice.18
Australasia. — In New Zealand, Tasmania, Patagonia, and
other countries lying now within the temperate regions of the
southern hemisphere, the evidences of extended glaciation in the
form of boulder-clays, gravels, etc., are as distinct as those of the
most typical areas in Europe or America.3
K Section II. — Cainozoic or Tertiary Period.
yJ The Tertiary period embraces all the sedimentary accumula-
tions which were formed between the close of the Cretaceous
period and the commencement of the Glacial epoch. Its strata
^ are of great lithological variety, and take part in the structure of
all the continents and their great mountain chains. During the
period of its deposition most of the species of animals and
plants which inhabit the lands and seas of the present day came
SECT. II.] THE GEOLOGICAL SYSTEMS. 169
into being, taking the place of older forms which became extinct.3
The percentage of existing species gradually increases upwards
and gives name to the successive systems or groups, e.g. Eocene
— dawn of recent species ; Oligocene — few recent ; Miocene —
minority of recent ; Pliocene — majority of recent.1
Fossils. — The era of Birds, Mammals, and Dicotyledons.
Vertebrata. — The Fishes are dominantly Teleostean, but teeth of
Elasmobranchs are locally common. The Birds include many
forms which have now disappeared from north temperate regions,
together with the last of the toothed birds (Odontopteryx).
Among Mammals there are a few Marsupiala and Cetacea ; the
Ungulata are largely represented. Among the odd- toed forms
we have Dinotherium, Mastodon (Miocene), and the true elephant
(Pliocene) ; rhinoceros (Miocene) and the allied Chalicotherium ;
tapir (Miocene) and the Brontotherium and Pal&otherium ; the
horse (Pliocene) with its ancestors Orohippus, Miohippus, and
Hipparion. Among the even-toed forms are camels, deer,
antelopes, and gazelles and their relatives, e.g. Sivatherium of India.
The Carnivora are represented by most of the recent families,
and the Primates by anthropoid apes, apparently arboreal in habit,
and equalling men in stature.
Invertebrata. — Foraminifera (Nummulites) (fig. 36) occur
occasionally in Britain, but are marvellously abundant in the
Mediterranean regions. Polyzoa are locally common, with but few
Brachiopoda. The Gasteropods include marine types, e.g. Fusus con-
trarius, now more characteristic of tropical and subtropical regions,
and terrestrial and fresh-water forms, e.g. Limncea, Paludina,
Planorbis (fig. 62), like those of temperate climates of the
present day. Lamellibranchs are locally abundant, the genera
being of recent types.
Flora. — The Eocene rocks of Britain are rich in angiospermous
plants, both Monocotyledons and Dicotyledons, and there is
evidence of warm conditions. The genera include many now
more characteristic of African, Australian, American, and Asian
regions. In the Pliocene deposits of Europe we find the extreme
southern forms gradually disappearing as we ascend the succes-
sion, and their place taken first by North American (evergreen
oaks, planes, Sequoia, etc.) and East Asian types (bamboo,
cinnamon), and finally wholly by the ancestors of the present
European flora.3
Great Britain. — The typical development of British Tertiary
rocks is found in the London and Hampshire basins and the
Eastern counties, parts of Norfolk, Suffolk, and Essex.3
The Pliocene strata are best developed in the Eastern counties.
The remains of the Forest Bed may still be seen on the Norfolk
170 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
coast.1 This group rests on clays with which are associated sands
and gravels known as the Elephant £ed, with remains of elephant,
rhinoceros, hippopotamus, horse, bear, beaver, and deer.6 The
Norwich Crag is mammaliferous ; the Red Crag extends along the
coast between Aldborough and Walton-on-the-Naze ; phosphatic
nodules, used for manure, are found near its base. It rests
generally on the White or Coralline Crag, but sometimes on the
London Clay.5
Miocene strata appear to be wholly absent from Great
Britain.
Oligocene strata occur only in the Hampshire basin, and include
the formations of the (1) Hempstead, (2) Bembridge, (3) Osborne,
and (4) Headon beds, which are made up of deposits of marine,
estuarine, and fluviatile origin (Fluvio-marine series), with a large
array of fossils many of which occur also in the Oligocene strata
of the Continent.
The Eocene of the London basin is made up of (1) the Thanet
sands, (2) Woolwich and Reading beds, (3) London Clay, and (4)
Bagshot sands ; and these strata include beds of marine, fluviatile,
and estuarine origin, with animal and vegetable remains charac-
teristic of a warm or sub-tropical climate. The Eocene formations
of the Hampshire basin include (1) Plastic clays, (2) Bognor series,
(3) Bracklesham series, and (4) Barton series, the rocks and fossils
of which admit of general parallelism with those of the London
basin.3
Continental Europe. — In the Paris basin of Northern France
the Eocene and Oligocene strata contain many limestones and are
rich in fossils. In Belgium they show older strata than those of
Britain, but are mainly fresh-water. In Germany the Oligocene
strata are conspicuous for their abundant beds of brown coal. In
the Mediterranean region, extending from Spain to the Himalayas
and from the Northern Alps to the Sahara, the Tertiary rocks are
remarkable for their masses of Nummulitic limestone laid down in
the clear waters of the broad sea which overspread this region in
Eocene times. At the close of the Eocene period the floor of this
region became subjected to great earth-movements accompanied
by volcanic action. It was ridged up into chains of islands which
afterwards became transformed into our present mountain-ranges
— the Alps, Atlas, Carpathians, Himalayas, etc. These ridges
were separated by areas of depression, regional 'and local ; such
are the areas now occupied by the Mediterranean, Black, and
Caspian seas. The history of later Oligocene, Miocene, and
Pliocene times is the history of the gradual transformation of
these irregularities into the present geographical conditions of
this vast region. The shallow seas in. the more Alpine districts
SECT. III.] THE GEOLOGICAL SYSTEMS. 171
became changed into gulfs, lakes, and river plains — in time partly
filled up by deposits like the Nagelflue and Molasse of Switzerland,
the sands and clays of the Vienna and Hungary basins, and the
fresh-water marls of Auvergne. Where the conditions remained
longer, submarine strata were formed like the sub-Apennine series
of Pliocene times, which covers a large portion of the outer
Apennines and of the island of Sicily. It consists of clays,
marls, and limestones thousands of feet thick.3
North America. — Marine Tertiary beds floor all the middle
parts of the Mississippi basin, from New Orleans up to St Louis.
The Rocky Mountain ranges, like those of the Alps, underwent
their last upheaval in Tertiary time. They show enormous thick-
nesses (13,000 feet) of fresh- water strata of Eocene, Miocene, and
Pliocene ages, with abundant plant and mammalian remains
(Miohippus, Lophiodon, Rhinoceros, etc.).3
Asia. — In India there is a complete series of Tertiary formations
(12,000 to 15,000 feet), the latest of marine origin being of
Miocene date. The Indian Pliocene rocks are sandstones, con-
glomerates, and clays (Siwalik beds) of fluviatile origin, laid down
along the outer skirts of the Himalayas, and remarkable for the
abundance of their extinct Mammalia (Sivatherium, Elephas, Hippo-
potamus, etc.). The Nummulitic limestone is continued onwards
from the Mediterranean through Palestine, Persia, Afghanistan,
and along the Himalayas to the farthest confines of India.3
Australasia. — Older Tertiary deposits cover much of Victoria.
The lowest rocks are clays with giant forms of cowries and volutes ;
the upper beds are clays and lignites with great sheets of basalt.
In New South Wales the region appears to have remained a land-
surface for the greater part of Tertiary times, and was eroded by
the streams into deep river beds, which now afford the auriferous
gravels of the country. These fluviatile deposits, which were
buried from sight by outflows of volcanic material, have yielded a
large number of extinct marsupial forms. Tertiary rocks occur
also in Tasmania, and cover large areas in New Zealand, where
they are associated with contemporaneous igneous rocks, are rich
in marine fossils, and are valuable because of their locally auri-
ferous character.3
rn*-*^^
Section III. — Mesozoic or Secondary Period.
A great palseontological break occurs in Europe between the
shallow-water (often estuarine and fresh-water) Tertiary deposits
and the ocean-formed chalk of the Cretaceous period, though
unconformity of the strata is rarely apparent. In North America,
Syria, and Egypt passage beds bridge this gap.2 Here all the
172 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
great Saurian reptiles, the Ammonites and Belemnites, die out and
are replaced by the distinctive genera of Tertiary times.
CRETACEOUS SYSTEM.
•^\fA^3^y^
Fossils. — The age of Iguanodon, Mosasaurus, Hippurites.
Vertebrata.'2 — The Fishes include Elasmobranchs : Acrodus, Ptycho-
dus, Ganoids, and Teleosteans. Amphibia were rare, but Reptilia
abundant : Ichthyosaurus, Plesiosaurus, and Pterodactylus, the
remarkable Dinosaurian Iguanodon. The marine, serpent-like
Mosasaurus was confined to the Cretaceous rocks. Birds are rare
and mammals unknown in Britain, but some remarkable birds
and remains of mammals have been found in North America.
Invertebrata. — Of Cephalopoda, Belemnites (fig. 70) and
Ammonites (fig. 66) are very abundant, but here die out. The
Lamellibranch Hippurites (fig. 61), which are absent in Britain,
are characteristic of the thick Mediterranean limestones. Echino-
derms are frequent, but Corals, Polyzoa, Brachiopoda, and Arthro-
poda less abundant. Foraminifera abound.3
Great Britain. — The western boundary of the Cretaceous area
ranges from Flamborough Head to Swanage ; east of that line the
Cretaceous strata are warped upwards into the Wealden anticlinal
(around which the strata outcrop in concentric bands), and
warped downwards into the two broad hollows of the London and
the Hampshire basins. In Ireland Upper Cretaceous strata occur
below the basalts of Antrim, and in Scotland below those of
Morven and Mull.3
Continental Europe.— The Cretaceous rocks of Europe belong
apparently to two distinct geological provinces — the Brito- Russian,
ranging from Ireland to the Urals, marked by the presence of the
White Chalk ; and the Mediterranean, ranging from Spain to the
Balkan Peninsula, etc., marked by the presence of the Hippurite
limestones, etc. The divisions of the Cretaceous generally ac-
cepted on the Continent are those of the French scheme, accord-
ing to which the Lower Cretaceous is composed of the Neocomien,
Urgonien, Aptien, and Albien ; and the Upper Cretaceous of the
Cenomanien, Turonien, Senonien, and Danien. The term Neo-
comien, which is, strictly speaking, the name of the marine
equivalents of the Wealden, is sometimes applied to the whole of
the Lower Cretaceous. The fluviatile Wealden itself occurs in
North- West France, Belgium, and Hanover ; the White Chalk in
North France, Denmark, Sweden, Prussia, and Russia. In Saxony
the Upper Chalk formations are represented by massive sandstones
(Quader), and along the line of the North-East Alps by barren
sandstones or grits ( Vienna Flysch). In the Spanish Peninsula,
SECT. III.] THE GEOLOGICAL SYSTEMS. 173
Pyrenees, the Alps, and the Atlas, the Hippurite or Mediterranean
Cretaceous is developed, and extends through Southern Asia to
the Himalayas.3
North America. — Cretaceous rocks cover a large proportion of
the continent. In the eastern districts they are shown as a
broad band of fresh-water strata (Potomac formation, etc.) rang-
ing from New Jersey round the southern extremity of the
Alleghanies into the centre of the Mississippi basin. In Texas
they consist of marine limestones, and even of white chalk with
fossils of the Mediterranean type. From Texas the Cretaceous
strata continue northward to the shores of the Arctic Ocean,
occupying a more or less connected area some 2000 miles in
length by 600 to 800 miles in width. Its greatest development
is in the Western States, where the strata have a collective thick-
ness of some 16,000 feet, and the Laramie series, which is at the
top, extends over an area of some 18,000 square miles and contains
coal-beds varying from 5 to 20 feet in thickness. Cretaceous
strata extend to North Greenland, Vancouver Island, and the
Queen Charlotte group.3
South America. — Cretaceous strata occur in mass at many
points along the chain of the Andes from Venezuela to Cape Horn ;
the marine fossiliferous deposits have taken part in the great
earth-movements which have affected the region, and rise to
heights of from 10,000 to 20,000 feet above the present sea-
level.3
Asia. — Besides the two types found in Continental Europe, a
third type is met with in the lands surrounding the Indian and
Pacific Oceans (Indo-Pacific Province) and occurs in South India,
Japan, and Aleutian Islands as well as in California and
Vancouver. The rocks are often of shallow-water origin, and
occasionally contain workable coal-seams. In the central part of
Southern India their highest strata are fresh-water beds, and they
are associated with the famous Deccan traps — sheets of basalt of
a collective thickness of more than 5000 feet, and covering an
area of 200,000 square miles.3 A small outcrop of Lower Cre-
taceous occurs in Cutch. Marine strata are well developed around
Trichinopoly and Pondicherry and again slightly in the Narbada
valley. Cretaceous beds also occur in Sind and the Salt Range
of the Punjab with Hippurite limestones, which are also found in
Syria, Arabia, and Persia.17
Africa. — The Libyan desert of North Africa is floored by
Cretaceous rocks which are of the type of the White Chalk. In
South Africa occur beds related to the Indian Cretaceous.3
Australasia. — In Queensland Cretaceous strata cover large
tracts,3 and in New Zealand Upper Cretaceous strata are met with
174 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
of great thickness, including the Coal formation, which contains
only brown coals ; but on the west coast seams of good bituminous
coal are found in sandstones and conglomerates, possibly the
equivalent in time of the Lower Greensand.17
/\
JURASSIC SYSTEM.
Fossils. — The age of Cycads, Ammonites, and Reptiles. Verte-
brata. — The Fishes included Elasmobranchs, Ganoids, and some
Teleostei, and the Amphibia were represented by a few Stegocephala.
But Reptiles are characteristic : Ichthyosaurus and Plesiosaurus ;
of Crocodilia, Teleosaurus ; of Pterosauria, Pterodactylus ; and of
Dinosauria the colossal Megalosaurus and Cetiosaurus. The first-
known Birds were found in the Upper Oolite beds of Solenhofen
and Marsupials in the Stonesfield slate and Purbeck beds.
Invertebrata. — Abundant six-rayed Corals, Crinoids, Star-fishes,
and Echinoids occurred. The most striking feature is the abund-
ance of the Cephalopoda, Ammonites and Belemnites, Lamelli-
branchs are numerous : Trigonia ; also Gasteropods and Brachio-
pods, Terebratula (fig. 58), Rhynchonella (fig. 56). Flora — Ferns,
Pecopteris, Glossopteris, Cycads and Conifers. 2> 3
Great Britain. — There are two sections — the Lias and the
Oolite — the former consisting typically of dark shales alternating
with thin beds of blue or grey limestone, and the latter of
alternations of massive calcareous rocks with thick sheets of soft
grey clays and marls.3
The Liassic strata stretch across England in a narrow strip,
varying from 1 to 20 miles wide, from Lyme Regis to Whitby
and the mouth of the Tees. The Oolitic strata form a wider
tract directly to the east of the Lias.5
The Lias is marine throughout, but affords beds of iron-ore in
Yorkshire (Cleveland) ; the Lower Oolites are marine and estuarine
towards the north ; while the Middle Oolites are invariably
marine. The Upper Oolite is best developed at Swanage and
Portland, and shows fresh-water beds in its upper zones (Purbeck).
Outliers of the Lias occur in England at Needwood, Whitchurch,
and Carlisle ; in Ireland in the county of Antrim, and also in Scot-
land. In Skye and Ramsay the Lias is 1200 feet thick, and is
followed by the Inferior Oolite, Great Oolite, and Oxfordian. At
Brora estuarine beds of Jurassic age have been worked for coal.3
Continental Europe. — The formations of the English Jurassic
are continued into France and Germany, and are well displayed
round the Paris basin, in the Franco-Swabian area, and N.W.
Germany. In these areas the same divisions and fossil zones are
recognisable as those in England, but the nomenclature is that of
SECT. III.] THE GEOLOGICAL SYSTEMS. 175
D'Orbigny. In the Jura and in Provence the strata are rich in
limestones, but are greatly folded. The Jurassic formations
extend along the whole course of the Alps, but vary in thickness
and lithological members, the Upper Oolite being the most widely
extended. The Russian type of the Jurassic covers much of Russia
and Siberia, and forms a broken zone round the (north) polar
regions.
Products. — In Swabia petroleum occurs in the Upper Lias marls ;
seams of coal of the Yorkshire Jurassic type occur in Bohemia
and in Hungary. Lithographic slabs are procured from the lime-
stones of Solenhofen in Germany.3
North America. — In the United States Jurassic rocks cover
broad areas in Nevada, Dakota, Utah, and Colorado. In the
western districts, especially in Colorado, they have afforded a
remarkable series of reptilian forms.3
South America. — Jurassic beds occur in the Andes of Chili
and Peru.17
Asia. — In India a thick development of Oolite rocks more than
6000 feet thick occurs in Cutch. The upper divisions (Rajmahal,
etc.) of the great fresh- water Gondwana series of Central India are
also of Jurassic age.3 Upper Jurassic strata are also represented
in Punjab and the Himalayas.17
Africa. — The Uitenhage formation of South Africa, which
consists of saliferous strata, sandstones, limestones, shales, and
conglomerates, represents the whole of the Jurassic series.17
Australasia. — The Burrum Coal Measure formations of Queens-
land,3 the Wainamatta series of argillaceous shales and thick
sandstones of New South Wales, and the Carbonaceous formation
of Victoria, which is 5000 feet thick, are of Jurassic age.17 A
massive series of marine and fresh-water beds with coals occurs in
New Zealand.3
TRIASSIC SYSTEM.
Types. — There are three fairly distinct types or paleeontological
facies : (1) the Marine type of the Alps ; (2) the Mixed or semi-
marine, semi-continental type of the German Trias; (3) the
inland or continental type of Great Britain, South Africa, and
Eastern North America.3
Fossils. — The advent of Mammals and Ammonites. Vertebrata.
— Of Fishes — Elasmobranchs, Hybodus, Acrodus ; Ganoids,
Palceoniscus ; andDipnoids, Ceratodus. Of Amphibia — Labyrintho-
donts, Mastodonsaurus, Trematosaurus. Of Reptiles — Lizards,
Ichthyosaurus, Notosaurus, Telerpeton, Rhyncosaurus ; creatures
with crushing teeth, Placodus ; Crocodiles, Stagonolepis. Of
Anomodonts (in S. Africa) the mammalian-toothed Galeosaurust
176 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
the equal-toothed Pariasaurus, and the double dog-toothed
Dicynodon (rordonia, etc. Of Mammals — the teeth and jaws of
Microlestes.
Invertebrata. — Cephalopoda are abundant, Ceratites (fig. 65),
Nautilus (fig. 63), Orthoceras (fig. 69), and a few Ammonites.
Crinoids are also abundant, Encrinus liliiformis (fig. 48). Lamelli-
branchiata are numerous, Myophoria, Gervillia, and Avicula
contorta. Gasteropoda occur. Of Crustacea, Estheria (fig. 50),
and Brachiopods, Terebratula, are representative.
Flora. — Pecopteris, Equisetum, Conifers and Cycads.
Great Britain. — The Triassic system is most fully developed
in the central parts of South Britain, and varies from 750 to 5000
feet in thickness. The Lower Trias or Bunter is formed of the Upper
and Lower Variegated Sandstones, with the intermediate Pebble-
beds ; and the Upper Trias or Keuper is made up of the Keuper
marls and waterstones. The Bunter series is almost barren of
fossils, except in its intermediate zones, where the derived pebbles
contain a few fossils of pre-Triassic age. The Keuper is equally
barren in organic remains. Triassic strata occur also in the
basins of the Sol way, the Moray Firth, upon the western coasts
and islands of Scotland, and in some of the counties of North-East
Ireland. The Rhsetic or " passage beds " occur as a thin band
from north of Yorkshire to the Dorset coast.2
Continental Europe. — Triassic rocks occupy the greater part of
Germany ; the Bunter locally affords beds of dolomite, and sand-
stones with plant remains. The distinct central member, the
Muschelkalk, which is absent in Britain, is generally rich in
fossils, while the lower half of the Keuper is marked by impure
coal and the upper half by its abundant gypsum beds.
The marine development of the Trias is typified by the thick-
bedded limestones, dolomites, and calcareous shales of the Eastern
Alps, rich in invertebrata of all types. The individual beds
cannot be paralleled with the German formations, but on the
whole are their equivalents in time.3
World-wide Distribution. — The marine strata of the pelagic
or Alpine facies of the Trias are of almost world-wide distribution :
they occur in the Maritime Alps, Apennines, Spain, Balkan
Peninsula, Turkestan, Himalayas, and practically surround the
Pacific Ocean, being met with in Peru, Colombia, Nevada,
California, British Columbia, Alaska, Japan, New Caledonia, and
New Zealand ; they also occur in Siberia and Spitzbergen. In all
these regions their limestones yield the Ammonoid forms Ceratites,
Trachyceras, and the Lamellibranchs Daonella, etc.
Triassic beds of the continental type occur in South Africa
(Karoo series), where they yield the rich reptilian fauna already
SECT. IV.] THE GEOLOGICAL SYSTEMS. 177
described. They occur also in South India (Panchet beds), where
they yield a few of the same forms. In the coastal regions of
North-East America the Trias is represented by red sandstones
and shales with abundant footprints of Dinosaurs (Connecticut),
with local sheets of melaphyre (Palisades, etc.) and with plant-
remains (Virginia). Similar Triassic strata occur also in South
America (Argentina).3
Section IV.— Palaeozoic Period.
The Primary strata are marked by Palaeozoic or ancient forms
of life, all the species and all but sixteen genera then existing
being now extinct, as also certain whole families, viz. Graptolites,
Rugose Corals, Cystideans, Trilobites and Eurypterids, Ortho-
ceratites and Labyrinthodont (or frog-like) reptiles. Crinoids or
Encrinites and Brachiopod shells were then very abundant in
number, and very varied in form. It was the age of Brachiopods.
The fishes had heterocercal (unequally-lobed) tails, but no true
bony skeleton, though often covered with bony plates. The
land plants were chiefly Ferns, giant Lycopods, and Conifers.
Note. — Entire absence of flowering plants, birds, and mammals.2
DYAS OR PERMIAN SYSTEM.
The Carboniferous system in Britain is overlain by a great
thickness of red sandstones, shales, limestones, and marls which
was formerly considered to be one system and was named the New
Red Sandstone. Subsequently, it was found to be composed of two
distinct systems, the older of which is known as Permian and is
of Palseozoic age ; the younger, as Triassic, of Mesozic age.3
Fossils. — The advent of reptiles. Fossils are nearly confined to
the marl, slate, and limestone bands. There are but few distinctive
forms, most being mere survivals from the Carboniferous, which are
of ordinary size in the lower beds but higher up become stunted
and finally disappear. 2- 3
Vertebrata. — Fishes : Palceoniscus, Platysomus, in the marl
slate, and copper shales. Labyrinthodonts : JSranchiosaurus,
Lepidotosaurus, and a true reptile (lizard), Proterosaurus.
Invertebrata. — The Polyzoan Fenestella retiformis (fig. 54) ;
the Brachiopods, Productus (fig. 57) ; Spirifer (fig. 55) ; the
Lamellibranchs, Bakevellia, Avicula, etc. Gasteropods are rare.
The Cephalopod Nautilus (fig. 63).
Flora. — In Great Britain and Europe Callipteris and Walchia
are characteristic ; in Central India and southern hemisphere
Glossopteris and other Mesozoic types.3
12
178 GEOLOGY FOE ENGINEERS. [PT. III. CH. IX.
Great Britain. — On the east side of England, from the coast
of Northumberland to the plains of the Trent, the Permian rocks
consist of a great central mass of limestone ; but on the west
side of the Pennines, and extending southwards into the central
counties, the calcareous zone disappears and we have a great
accumulation of red, arenaceous, and gravelly rocks. The Lower
division is typically developed in the Vale of Eden, where it
consists of brick-red sandstones and breccias; the red rocks
extend into the valleys of the Nith and Annan in Scotland, and
the breccias, further south in Staffordshire, attain a thickness of
400 feet. The Upper division is best seen at St Bees, near
Whitehaven. The Permian rocks of North Ireland consist of
fossiliferous magnesian limestone with red marls at its base, and
at the city of Armagh, of boulder beds and limestone breccias.
Products. — Some of the finest building-stones of the country,
such as the Mansfield sandstones and the magnesian limestones
of Durham, York, and Nottingham ; marls for brickmaking and
a thick bed of rock-salt.3
Continental Europe. — In Northern Germany the Permian is
made up of the Zechstein and Rothliegende, which constitute the
so-called Dyassic type of the system. The Rothliegende occurs
also in Bohemia, Saxony, the Saar district in S.W. Germany, and
at many localities in Central France. In the typical Russian
district of Perm the Permian strata cover vast areas, and consist
of sandstones, limestones, and marls and shales in repeated
alternation. In the Alps the Permian is represented by the
Verrucano, and in the Tyrol by the sandstone and quartz-
porphyry series of Botzen, and by the richly fossiliferous marine
Bellerophon limestone. In Carinthia and Sicily the entire series
is marine.
Products. — Copper-bearing deposits of Germany and Persia and
many workable coal-seams in France.
North America. — Permian rocks are rare. The highest
division (the so-called Barren Measures) of the Coal Formation
of the Alleghany region have been referred to the Permian, and
contain an admixture of Coal Measure and Permain forms,
together with genera characteristic of the Jurassic period. In
Texas certain mottled clays, sandstones, and limestones, overlying
the local Coal Measures, are referred to the Permian. Marine
rocks of this age also occur in Spitzbergen?
South America. — Strata of the Indian Gondwana type with the
GlossOpteris flora have been met with on the east of the Andes in
Brazil and Argentina?
Asia. — In Southern India the Permian beds consist of great
thicknesses of sandstones and shales, with fresh-water and
SECT. IV.] THE GEOLOGICAL SYSTEMS. 179
terrestrial fossils ; the plants include Glossopteris and other
Mesozoic types, which are sometimes so abundant as to constitute
valuable seams of coal. They form the lower half of the
Gondwana series and comprise the Talcher, which is distinguished
by its remarkable conglomerates or boulder beds (which are met
with not only south of the Nerbudda River but in the Salt Range
of the Punjab) and the Damuda with workable coal-seams.3
Africa. — The beds in South Africa are similar to those of
South India. A group of sandstones and shales (Lower Karoo or
Ecca beds), with local coal-seams, affords examples of Glossopteris
and shows at its base a remarkable boulder bed (Dwyka) which has
been compared with the Indian Talcher.3
Australasia. — Similar beds to those of Southern India and
South Africa occur in Australia, hence it has been suggested that
these three widely separated regions formed part of a single
Permian continent. At Bacchus Marsh and elsewhere in Victoria
sandstones and shales with the Glossopteris flora, associated with
boulder conglomerates with striated pebbles, like those of the
Talcher, are met with. The Upper or Newcastle division of the
workable Coal Measures of New South Wales is also, probably,
of Permian age. In Queensland the local Permian is made up of
a lower marine series with abundant Brachiopods and Cephalopods,
and an upper fresh-water series with Glossopteris, etc.3
CARBONIFEROUS SYSTEM.
Fossils. — The age of Cryptogamic plants, Corals, and Crinoids.
The fossils of the coal-bearing strata are marine, estuarine, and
terrestrial. The marine forms are generically related to those
of the Carboniferous Limestone, viz. abundant Corals, Encrinites,
Polyzoa, Brachiopods, Cephalopods with rare Eurypterids,
Phyllopods and Trilobites and occasionally Sponges, Annelids, and
Protozoa. Scales and teeth of large Elasmobranch fishes are not
uncommon. The estuarine forms include mussel-like forms, such
as Anthrocosia. The terrestrial forms embrace rare forms of
Articulata, such as scorpions, beetles, crickets, may-flies, etc., but
the most abundant are plants including many Cryptogams and a
few Gymnosperms. Among the Cryptogams are tree-like Lycopods,
Ferns, and Equisetums.3
Great Britain. — This great system attains a maximum thick-
ness of 20,000 feet and is made up of limestones, grits,
sandstones, shales, coals, and ironstone. Calcareous strata with
marine fossils prevail generally in the lower half of the system, and
sandy and shaly strata with abundant land-plants in the upper half.
The Upper Carboniferous or series of Coal Measures is djvide4
180 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
from the Lower by the arenaceous formation of the Millstone grit,
which, however, is absent in the Midlands. The Middle or Grey
Coal Measures afford the richest and most valuable seams of coal
mined in the British coal-fields. Three of the chief British coal-
fields occur in the neighbourhood of the Bristol Channel ; six in
the Midlands and the Welsh border ; five in the Pennine region ;
three in Central Scotland ; and three in Ireland. In Southern
Britain coal-measures have been proved to exist below some of
the more recent formations.
The Lower or Carboniferous Limestone series is typically
developed in the Southern Pennines, where the Yoredale group of
shales, limestones, and sandstones is from 1500 to 4500 feet thick
and the limestone from 2000 to 4000. The same series recur in
North Wales. In South Wales, Mendip Hills, and the Forest of
Dean the Yoredale rocks are wanting. Passing northwards the
limestone is gradually replaced by a series of sandstones, grits, and
shales with occasional bands of limestone, which develops into
the Calciferous group of sandstones, shales, and cement stones
with a series of workable coals overlain by the Carboniferous
Limestone, in the north of Scotland and north of Ireland.
In Central Ireland the whole of the Lower Carboniferous is
practically represented by limestones ; but in the extreme south
of the island it becomes replaced by cleaved grits, sandstones, and
shales. In the central parts of Devonshire the whole of the
Lower and perhaps some of the Upper Carboniferous is
represented by a mixed group of greywackes, flags, shales, and
thin bands of limestone of the "Culm" type.3
Continental Europe. — Carboniferous strata of the Pennine
type occur in Belgium and North France ; of the Culm type in
West and North Germany, Central Europe, and the Alps ; and of
the Limestone (S. Wales) type in Russia and the Urals. The
chief European coal-fields are those of France and Belgium,
Westphalia, Saarbriick, Silesia, Bohemia, and Russia, Central
France, and Northern Spain.3
North America. — Carboniferous rocks are grandly developed
in Nova Scotia and the United States. Their lower divisions
(sub-Carboniferous) are typically limestones, with abundant
marine fossils ; and the beds of their highest division, which
form the rich coal-fields of Nova Scotia, Pennsylvania, and the
basins of the Ohio and Mississippi, are prolific in terrestrial plants
and afford many remarkable terrestrial Articulata.3
Asia. — Carboniferous rocks, covering large areas, occur in
Northern China; they are made up of limestones below, with
Fusulina, etc., rich coal-measures in the middle, and marine
sandstones above.3 Coal and Carboniferous limestone occur in
SECT. IV.] THE GEOLOGICAL SYSTEMS. 181
Japan. Coal of this period is also found is some islands of the
Indian Archipelago and in Borneo.1^
Africa. — Rocks with Carboniferous fossils occur in the Sahara
and in Egypt? Coal has been noticed on the banks of the
Zambesi, and occurs in the Transvaal and Orange River Colony.
Natal coal is of Triassic or Permian age.17
Australia. — The Carboniferous rocks of New South Wales are
formed of two distinct divisions, both containing workable coals.3
DEVONIAN SYSTEM.
Types. — There are two distinct types of strata: — (1) the
Fresh-water or Old Red Sandstone, and (2) the Marine or
Devonian.
Fossils. — The age of Fishes and Eurypterids.
Old Red Sandstone.
The Fauna is remarkable for the predominance of tishes.
Their skeletons were cartilaginous, and their tails either diphy-
or hetero-cercal.2 Their bodies were sometimes naked, but more
generally protected by an armour of plates, or a covering of
granules, ganoid, or placoid scales, and in some cases were even
provided with various protective spines (Ichthydorulites). The
Old Red fishes included lamprey - like forms (Palceospondylus),
shell - skinned fishes (Ostracodermi), joint -necked Ganoids
(Coccosteus), fringe-finned Ganoids (Holoptychius, etc.), Dipnoids
or mud-fishes (Dipterus, etc.), and shark -like Elasmobranchs
(Acanthodes, etc.).3
Associated with these fishes occur the giant Crustacea or
Arachnoids — Eurypterus (fig. 51), Pterygotus, and Stylonurus.
The Flora are few in the Lower series and include Psilophyton
and other marsh plants ; those of the Upper Old Red Sandstone
are more common and embrace Lycopods and Ferns.
Devonian.
Graptolites die out; a peculiar sponge, (?) Stromatopora,
abounds, as do Corals (Favosites, fig. 44 ; Heliolites, fig. 45 ;
Calceola, fig. 42), which form the " Madrepore marble " of Torbay
and Plymouth. Trilobites are few, but large allied Crustaceans
(Eurypterus, Pterygotus) continue. Brachiopod and Lamelli-
branch bivalve shells, and the Cephalopods, Orthoceras, Clymcnia,
Goniatites, Nautilus, are the most abundant fossils.2
Great Britain.— The Old Red type occurs from the Bristol
182 GEOLOGY FOE ENGINEERS. [PT. III. CH. IX.
Channel through Wales, Central and North-East Scotland to the
Shetlands j and also largely in Ireland. Everywhere a great
break occurs between the Upper and Lower groups. It consists
of lake deposits (in Lakes Orcadie, Lome, Caledonia, Cheviot, and
Welsh lake), and in Scotland often abounds in fish remains and
land-plants. There, too, interbedded volcanic rocks occur, more
than 6000 feet thick, and form the Pentland, Ochil, and Sidlaw
hills, which are " the basal wrecks of extinct volcanoes."
The Devonian type occurs only to the south of the Bristol
Channel, and consists of a great thickness of grey and blue slates,
schists, sandstones, and limestones rich in corals.2
Continental Europe. — Old Red Sandstone strata occur in
Northern Russia with typical fishes, in Norway with remarkable
igneous rocks, and in Spitzbergen and Bear Island with abundant
plant remains. Devonian strata sweep almost uninterruptedly
through Europe from Calais to the Urals. In Belgium and the
Rhine provinces all three divisions are present in great thickness
and rich in characteristic fossils. In Russia they are almost
horizontal, and interbedded with red sandy strata with Old Red
fishes. In Bohemia they consist of limestones rich in Orthoceratites
and Goniatites. Devonian rocks are also met with in many other
parts of Europe.3
North America. — Old Red Sandstone beds are met with in
Gaspe and New Brunswick, containing Lycopods, Calamites,
Ferns, and even a few Conifers, often in such abundance as to
form seams of coal. In the United States Devonian rocks include
a Lower division of sandstones and limestones (Oriskany and
Corniferous), with Brachiopods and plants ; a Middle division of
black shales and limestones (Hamilton) with Goniatites and
Productus; and an Upper division (Chemung) which is formed,
in some districts, of limestones rich in Clymenia, and in others
of red sandstone (Catskill\ with Upper Old Red Sandstone fishes.3
Products. — The Devonian strata are the source of immense
supplies of petroleum which, it is thought, has distilled from
decomposing animal matter in the limestones, such as fishes,
crustaceans, and mollusca, whose hard parts are embedded in it
in great abundance. This is also the source of the deposits of
bitumen.5
(UPPER) SILURIAN SYSTEM.
Fossils. — The age of Brachiopods ; abundance of Corals,
Encrinites, and Trilobites ; advent of Vertebrates : Fishes,
Scorpions, and Insects.
The Fishes are Elasmobrancliii or Ostracodermi (Cephalaspis^
Pteraspis, etc.). The Arachnida or forms of scorpions appear.
SECT. IV*.] THE GEOLOGICAL SYSTEMS. 183
Among Crustacea, Trilobites are abundant, Phacops, Calymene, and
Homalonotus being commonest. Cephalopoda and Gasteropoda
are represented by Orthoceras and Better ophon (fig. 62).
Brachiopoda are the most prevalent and characteristic of the
system, especially Pentamerus and Spirifer. The Echinodermata
are represented by star-fishes and crinoids, and of the
Ccelenterata both Actinozoa and Hydrozoa are abundant. Of the
former the Corals are chiefly rugose, while in the latter the great
group of Graptolites becomes extinct within the limits of the
system.
Of Flora but little is known ; but both Lycopods and Ferns
appear to have been in existence.3
Great Britain. — The typical area is that of Shropshire, and the
same type is prolonged south-westward along the Welsh border
(where an older formation — the Lower Llandovery — makes its
appearance at the base of the system), and south-eastward and
eastwards into the areas of Woolhope, the Malvern Hills, and
South Staffordshire, etc.
Silurian strata of the greywacke type sweep through the
central parts of Wales from Cardigan to Denbigh, the Llan-
dovery being represented to the south by great thicknesses of
grits, and to the north by a few bands of graptolitic shales,
while the equivalents of the Wenlock and Ludlow are more or less
barren grits and shales, which thicken northwards (Denbigh
Grits and Flags}. The same is the case in Westmoreland, where
the Skelgill and Browgill Shales answer to the Llandovery, the
Coniston Grits and Flags to the Wenlock, and the Bannister Slates
and Kirkby Moor Flags to the Ludlow formations. Rocks of the
greywacke type floor almost the whole of the Scottish uplands.
The most widespread formation is the Gala Group (Tarannon),
which is underlain by the thin but richly graptolitic formation of
the Birkhill Shales (Llandovery), and overlain by another
greywack^ formation, the Riccarton Beds (Wenlock). In
Ireland, Silurian rocks of the Shropshire type occur in Galway
and Kerry, and of the types of those of Birkhill and Gala of the
southern uplands, in Londonderry, Cavan, and Down.3
Continental Europe. — In Northern Europe the Silurian rocks
are usually limestones, as in Norway, Central Sweden, Gothland,
and Esthonia, but are locally intermixed with or replaced by
carbonaceous shales with graptolites, as in Scania and Dalecarlia.
In Bohemia two lithological members occur, an Upper or Cal-
careous series and a Lower or Graptolitic division. In France and
Belgium the Ludlow is partly represented by marls, and all the
formations below by grey and black shales with graptolites.3
North America. — The majority of the rocks remind us both of
184 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
those of Shropshire and Scandinavia. The Llandovery is repre-
sented by sandstones and shales (Oneida and Medina], the Wen-
lock by shales and calcareous beds (C 'Union and Niagara], and the
Ludlow by three formations — the Salt Group of Onondaga, the
Water-lime with Eurypterus, and Lower Helderberg limestones with
abundant forms of Pentamerus.3
Asia. — Lower Silurian rocks occur in the Salt Range and Simla
area, while in the great chain of the Himalayas, Silurian rocks,
flanked by Secondary formations, form part of the central axis of
the range. In China, Silurian Graptolites and Orthoceratites
have been met with. Similar formations spread over large tracts
in Southern Siberia, in the Altai Mountains, and in Asia Minor.17
Australia. — Silurian fossils occur in Southern and Western
Australia, and it is in the highly metamorphosed Silurian rocks
that auriferous quartz-veins so frequently occur.17
ORDOVICIAN OR LOWER SILURIAN SYSTEM.
Silurian strata cover a wider extent of the earth's surface and
are more uniform in nature of deposits and fossils than any other
group.
Fossils. — The age of Graptolites, Trilobites, and Cystideans.2
Graptolites are the characteristic fossils ; they are found in more
or less abundance in strata of all lithological types, but are most
prevalent in the thin-bedded black shales. Of Trilobites the
primordial genera Olenus, etc., die out, and Asaphus, Calymene,
and Ogygia Buchii first appear. Corals were few, but Brachiopods
and Cystidean Echinoderms swarmed. Gasteropods first appear,
ex. Bellerophon ; also Cephalopod-chambered shells.3
Great Britain. — The typical area is North Wales. The Arenig
beds, containing enormous sheets of volcanic rocks, sweep round
the Merionethshire anticlinal, forming the ranges of Cader Idris,
the Arans, and the Arenigs, etc. They are succeeded by a great
thickness of barren, dark shales, representative of the Llandeilo.
The final division is made up of grey flagstones and shales with
the typical Bala limestone near the base, and the Hirnant lime-
stone near the summit. The upper of the two volcanic groups of
the Ordovician of North Wales is comparatively thin near Bala,
but expands to an enormous thickness, and forms the mountain-
ranges of Snowdon and Penmaenmaur, etc. In the Lake District
the two volcanic series are connected by intermediate masses of
lava and ashes and form collectively the Borrowdale series. This
is underlain by the Skiddaw slates and overlain by the Coniston
limestone. In the Girvan district of South Scotland the lowest
Ordovician rocks are the Ballantrae volcanic series. The Llandeilo
SECT. IV.] THE GEOLOGICAL SYSTEMS. 185
seems to be represented by the Stinchar group of conglomerates
and limestones, and the Bala by the Ardmillan series of flagstones
and shales. At Moffat, etc., the Upper Llandeilo and Bala beds
are represented by the Glenkiln and Hartfell shales of the thin-
bedded Moffat series.
In Ireland Ordovician rocks occur in many areas. In Tyrone
and Mayo they are of the Girvan type ; in Down and Cavan they
have a Moffat facies ; at Kildare and elsewhere in Central Ireland
they call to mind the Bala beds of the Lake District ; in
Wicklow and Wexford they are of the type of the sedimentary
and volcanic rocks of North and South Wales.3
Continental Europe. — The Ordovician rocks attain their widest
extension in Scandinavia and Esthonia, where they are thin,
horizontal, and rich in fossils ; their calcareous members have a
far more varied fauna than those of Britain, but their graptolitic
members agree with ours almost specifically. The same is also
the case in Belgium, but the rocks are more disturbed. Impor-
tant beds occur in Western France and in Bohemia*
North America. — The Ordovician strata are of two main types —
the calcareous type of the central regions, including the Calciferous
sandstone, the Chazy and Trenton limestones, and the Utica and
Hudson River (Cincinnati group) shales ; and the greywacke and
black-shale type of the Hudson River and the lower reaches of the
St Lawrence, including the graptolite-bearing Quebec group
(Point Levis beds) and the Marsouin and Norman's Kill shales,
etc.8
Asia. — Lower Silurian rocks occur in the Salt Range and the
Simla area of India.11
Australasia. — Ordovician strata with abundant Arenig and
Llandeilo graptolites are found in Australia (Victoria) and also
in New Zealand.3
CAMBRIAN SYSTEM.
This system consists of a vast succession of reddish grits, con-
glomerates, shales, slate, and quartzite ; but there is no gneiss,
and there are few schists and fewer limestones. It is divided into
Upper, Middle, and Lower zones.5
Fossils. — The characteristic fossils are Trilobites, of which the
genus Olenellus (fig. 52) characterises the Lower, Paradoxides
(fig. 53) the Middle, and Olenus the Upper Cambrian. The chief
invertebrate groups which occur, in addition, are Cephalopoda,
Gasteropoda, Lamellibranchiata, Pteropoda, Brachiopoda, Asteroidea,
Crinoidea, Hydroida, and Sponges. Vertebrata are very doubt-
fully represented ; and of plants only sea-weeds are known to
186 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
Great Britain. — The Cambrian rocks of Merionethshire have
been estimated at 15,000 feet in thickness. They consist of the
Tremadoc slates, Lingula flags, Menevian beds, and Harlech series.
At St David's (9000 feet thick) the Upper Cambrian is repre-
sented by the Tremadoc beds and Lingula flags, the Middle by
the Menevian beds and Solva group, and the Lower by the Caerfai
group, with evidences of Olenellus.
In Shropshire the Cambrian consists of three members — the
Wrekin quartzite at the base, the Cornley sandstone, and the
Shineton shales. In the Malvern Hills the Cornley or Hollybush
sandstone lies at the base of the Cambrian, and is followed by
black shales with Dolgelly trilobites and green shales with
Dictyonema. In the Nuneaton district there are two members —
the Hartshill quartzite and Stockingford shales. In the North-
west Highlands of Scotland a band of Cambrian strata ranges from
Eriboll almost to Skye, comprising (1) Eriboll quartzite; (2)
Fucoid beds; (3) Salterella grit; (4) Durness limestone. Near
Bray Head in Ireland occur certain coloured slates and grits
usually referred to the Cambrian.3
Continental Europe. — Cambrian rocks are well displayed in
Scandinavia and North- West Russia ; they are usually horizontal
and of no great thickness, but yield a rich fauna, the character-
istic genera of the Lower, Middle, and Upper Cambrian being all
present. Cambrian rocks occur in Bohemia, Bavaria, France,
Belgium, Spain, and Sardinia.3
Asia. — In India, Lower Cambrian fossils occur in the Salt
Range of the Punjab, and Higher Cambrian fossils are met with in
Northern China.3
North America. — Well developed in North America. The
Lower Cambrian or Olenellus zone is shown in the Rocky
Mountains, the Alleghanies, the Taconic Ranges, and in New-
foundland. The typical Middle Cambrian fossils are most
abundant in New Brunswick (Acadia), Massachusetts, and
Newfoundland. The Upper Cambrian (Potsdam sandstone, etc.)
formations occur in Canada and in the basin of the Mississippi ;
and the highest Cambrian or Dictyonema beds in the valley of
the St Lawrence.3
Australasia. — Olenellus has been found in Western Australia.
Section V. — Eozoic Period.
ARCH^AN AND PRE-CAMBRIAN ROCKS.
All rocks of greater antiquity than the oldest fossil-bearing
strata of the Cambrian are grouped together as Pre-Cambrian or
SECT. V.] THE GEOLOGICAL SYSTEMS. 187
Archaean. These rocks, unlike those of the subsequent fossiliferous
systems, have not yet been satisfactorily divided into formations
and systems. They present themselves under three types : (a)
coarsely crystalline gneisses and schists associated with plutonic
igneous rocks ; (6) finely crystalline schists and gneisses associated
with more or less metamorphosed sedimentaries and volcanics ;
and (c) unaltered sediments and contemporaneous lavas, ashes,
and tuffs.3
Fossils. — The only undisputed fossils yet obtained from the
Pre-Cambrian rocks of Britain are worm-burrows and worm-tracks.
The crystalline limestone of the original Laurentian of North
America have yielded the problematic fossil Eozoon Canadense, some
of the more or less metamorphosed groups of the Huronian type,
a few traces of Annelids, sponges, and plants \ and the unaltered
formations, forms of Protozoa, Mollusca, and Molluscoidea.3
Great Britain. — In the North-West Highlands occur the
Leivisian, which consists of coarsely crystalline gneisses and schists
more or less inclined ; above which and unconformably rests the
Torridonian, composed of masses of chocolate-coloured sandstones
and conglomerates approximately horizontal.
In Ireland there are gneissic rocks with micaceous and chloritic
schists associated with granite in Wicklow, and gneisses and
serpentinous limestones in Galway and Donegal, in all probability
of Archaean age.
In Wales occur the Dimetian, composed of granites and gneiss,
covered by the Arvonian strata of volcanic rocks, and on the
latter rest unconformably the Pebidian strata of slates and shales,
etc.
Pre-Cambrian rocks occur in Anglesea, Shropshire (the Long-
myndian and Uriconian series), in the Malvern Hills, at the Lizard,
and at Charnwood Forest in Leicestershire.3
Continental Europe. — Pre-Cambrian rocks of all the British
types cover large areas in Scandinavia and Finland, occur also in
France and Spain, and in the cores of the great European
mountain-ranges. 3
Asia. — In India there are two great " massifs " of gneissic and
crystalline rocks — the one forms the extensive upland and
plateau tracts that extend from Ceylon through the Madras,
Bengal, and Bundelkhand districts to Assam ; the other constitutes
the colossal framework of the Himalayas. These Archaean gneisses
are succeeded unconformably by a series 2000 feet thick of
quartzites, conglomerates, schists, slates, breecias, and limestones,
associated with contemporaneous bedded igneous rocks.17
North America. — These rocks occupy a connected area of two
million square miles, having Hudson Bay as its centre. They
188 GEOLOGY FOR ENGINEERS. [PT. III. CH. IX.
comprise a lower or Basement complex consisting of coarsely
gneissose types, which was formerly called Laurentian, and an
upper division known as AlgonJcian or Proterozoic which is divided
into a Keewenawan, resting on an Upper and a Lower Huronian —
all of finer crystalline schists and sediments, etc.3
South America. — These rocks range through the greater part
of Brazil, Guiana, and Venezuela and occur again in the Andes of
Chili."
Africa. — Pre-Cambrian rocks form considerable rocks tracts in
Algeria and on the eastern borders of Egypt.17
Australasia. — Gneissic and crystalline rocks occupy large tracts
in South -Western Australia. They are also well developed in
New Zealand.17
PART IV.
GEOLOGICAL OBSERVATION.
WHILE a theoretical knowledge of geology, as set forth very
briefly in the three preceding parts of this book, is of great value
to the engineer as a necessary groundwork to any study of the
subject, the methods of geological observation, which are dealt
with in this part, are of fundamental importance. In the practical
application of geology to engineering, which is the subject of the
concluding part of this book, it is essential that the geological
factors should be based on the most reliable data. Hence all
geological observation must be carried on in the most accurate
and careful manner possible. Every fact which throws light on
the area observed must be carefully noted and the record must
be both full and accurate. The observer should train himself not
to jump ^ to conclusions, but to view every bit of evidence with
regard to the nature and structure of the rocks which are con-
cealed from view, and must learn to interpret rightly such facts
as are patent, without minimising their value or too greatly
exaggerating it.1
189
PT. IV. CH. X.
CHAPTER X.
OUTDOOR WORK.
EQUIPMENT.
IT is essential that the equipment or outfit for the field should be
as light as possible. No one can do good work of any kind with
a regular "Christmas tree" slung round him. The following
instruments are, however, indispensable : hammer, knife, lens,
compass and clinometer, note-book, and tape or other measure.1
Hammer. — The most important implement of the geologist.
It may be light or heavy, short- or long-handled, according to the
nature of the work to be done and the fancy of the individual,
but it must be heavy enough to break up any ordinary rock
which is met with. Most geologists recommend a square face
with a chisel-shaped tail end, the cutting edge of the chisel being
at right angles to the axis of the handle shaft. Sir A. Geikie
considers a hammer weighing one pound, or a few ounces more,
quite sufficient for the ordinary purposes of a field geologist ; but,
when it is intended to collect specimens, a hammer weighing two
or three pounds or even more will be required, and a small chip-
ping or trimming hammer as well. If, therefore, the engineer has
time and opportunity to devote to geological observation he will
do well to collect rock specimens for future reference, and should
provide himself with both hammers.1
Chisel. — Though many geologists prefer to dispense with a
chisel, there is no doubt of its convenience where blocks of rock
have to be worked out from a cliff-face, or in any place where the
hammer fails to get an easy hold. A good "cold chisel" some
4J to 5 inches in length is suitable. If it is too short, it may
become driven in down joint-cracks before its work is done and
before the block is wedged away from the parent-mass.15
Bag and Belt. — Elaborate hammer-belts seem quite unnecessary.
The specimen-bag is commonly slung by a strap passing .over the
right shoulder, so that it can be steadied and partly supported by
the left hand when it becomes full and heavy. An additional
190
SECT. I.] OUTDOOR WORK. 191
strap for the hammer cumbers the chest, and even in a belt the
head has to be prevented from touching and wearing through the
clothes. It is simple enough to slip the hammer into the side
bag itself, the handle projecting from the forward end under the
flap. The left hand, by resting on the handle, can then easily,
during long walking, keep the bag from rubbing unpleasantly on
the hip.
A walking-stick is indispensable on steep or roughish ground,
and where long slopes and taluses are in question its use will
make observations possible that might otherwise involve genuine
risk. A steep hillside should be traversed with the stick in the
inside, not in the outside, hand.15
A compass is a necessity for the pedestrian. It may be com-
bined with the clinometer, as in the convenient box instruments
often made. Many of these, however, do not allow sufficient
length in the edge which is to be held coincident with the line of
dip observed. Anyone can construct a clinometer from an
ordinary protractor — a swinging index, or even a weighted thread,
being hung from the centre of the straight edge so as to reach the
graduated arc. Of course the 90° marked on the protractor reads
as 0° when a dip is to be taken ; thus, if the index points to 84°
the dip is 6°, and so on (see Section II., Geological Sections).15
Tape-measure.— To find the relation of the point where
observations are being made to features marked upon the map,
and thus in one's notes to localise the observation, is often difficult
in a wide and open country. Even the map on the scale of 6
inches to a mile cannot represent every rock and projecting boss,
arid measurements must be made extending from some recognisable
point to the place of observation. The tape-measure, so important
in determining the thicknesses of beds on faces of a quarry, is often
of use in direct measurement on the surface of the ground, for
which purpose it should be at least 40 feet in length.15
An Abney's level is useful for contouring and measuring angles,
combining as it does the properties of a level and of a clinometer.15
A common triplet pocket-lens, or any useful form which will
bear rough usage, must always be carried in the field, as indeed
it should be carried by the geological observer every day of his
life, whether in town or country.15
A note-book is indispensable, and should have some blank pages
for outline sketches.15
Section I. — Geological Surveying.
All geological observations should be recorded on geological
maps or plans and geological sections.
192 GEOLOGY FOR ENGINEERS. [PT. IV. CH. X.
A geological plan consists of a plan or plans of all the geological
deposits with the boundary lines between formations clearly
shown, and containing as much information as possible with regard
to the structural characters of the district. It may be necessary
to prepare two or more plans of the same area in order to show
the various formations.
A geological section not only gives the outline of the surface
features but also the geological formations and structural char-
acters as far as they can be traced. It will often be desirable to
prepare several sections in different directions across a given area.
The first requisite is an accurate plan of the topography of the
area, with the nature of the surface depicted by contour lines or
hachuring, etc.
Some sort of map is usually available, and if one on a suitable
scale of, say, 6 inches to the mile is not obtainable, an enlargement
of the local map should be prepared.
Taking this into the field as a basis for his traverses and section
lines, the observer should record as much information as possible
on the plan and the remainder in his note-book. The actual
sections and geological plans can then be prepared at home.
The geological plans and sections taken together should contain
full information as to the geological structure, viz., dips,
curvatures, dislocations, etc., and all possible information of
economic value.1
MAPS.
A geological map of a given area consists of a map or plan of
the surface features in which are shown the boundary lines of
each bed or stratum exposed in the area. These boundary lines
are the lines where the lower margins of the strata cut the
surface of the earth, and the boundary line of any given stratum
coincides with the outcrop of the stratum below it.1
Contours. — It is presumed that the observer is familiar with
the system of representing on a map, by means of contour lines,
such undulations of the ground as are of any prominence : each
contour line passes through all the points at which a horizontal
plane would cut the surface of the ground.
To anyone accustomed to the use of contours it will be obvious
that the boundary lines of strata must bear some relation to the
contours. This relation is as follows : —
(1) When the strata are horizontal, the boundary lines
coincide with the contours. This is obvious.
(2) When the strata dip towards a hill the boundary lines are
less winding than the contours.
SECT. I.] OUTDOOR WORK. 193
The truth of this can be seen if we imagine the dip increased
till the strata are vertical, for the boundary lines would then
become parallel straight lines.
(3) When the strata dip away from a hill the boundary lines
are more winding than the contours.
This is true so long as the dip is less than the slope of the hill,
but if it is greater the boundary lines wind in a reverse way to
the contours.1
Tracing Boundary Lines. — The object of the observer is to
trace the lower margin of the stratum on which he is standing.
He will, therefore, first look for any natural sections or artificial
exposures such as cliffs, quarries, road and railway cuttings, etc.,
and, selecting these as his principal points, he will locate them on
his map and then make a traverse of the intervening country,
noting all the geological features.
Preliminary traverse. — In making such a traverse it is
desirable to select such roads, paths, streams, or other lines which
conveniently divide the area to be traversed. If there is a coast-
line it should be carefully examined. The observer should then
work along these lines, going over the ground on either side. In
this way the whole area will be traversed and nothing important
omitted.
The points to be noted are those which tend to throw light on
the geological characteristics of the area, especially (1) all
indications of the nature of the rocks, as described below; and
(2) the chief structural features as described in Section II.
Considerable experience is needed to enable the observer to
place a proper value on the different indications he may meet with,
and it may often happen that many miles may be walked before
a boundary line can be accurately determined ; but in such cases
points should be provisionally fixed, and further indications must
be sought for.
It should be noted that when there is very little change of
feature the boundary line will be found to run higher up than
would at first appear, owing to the movement of rocks and soil
from the higher to a lower level.
When obscure areas are met with, such as grass-lands, marsh,
moors, indications should be sought outside these areas, and when
sufficient data are available the boundary lines may be traced by
means of the surface features.
The same rule applies in unravelling the details of geological
structure — by going further afield some clues may be obtained
which will throw fresh light on the situation.1
Indications of nature of rocks. — While making this traverse the
engineer should look out for every indication of the nature of the
13
194 GEOLOGY FOR ENGINEERS. [PT. IV. CH. X.
rocks, whether exposed or underlying the surface. The natural
and artificial exposures, which form the principal points of the
survey, will afford specimens for rough classification in the field
as described in Section III., as well as for more careful indoor
examination (vide Chapter XL).
In places where no such exposures can be found, it will be
necessary to dig through the surface soil and subsoil, if any exist,
in order to obtain an indication of the nature of the underlying
rocks. While doing so a look-out should be maintained for any
weathered portions of rock which have worked up from below.
The soil and subsoil will also afford valuable indications. The soil
is derived from the subsoil which, in its turn, is derived from the
underlying rock, and the nature of the subsoil may be detected
by examining the heaps thrown up by burrowing animals. Light
soils are derived from sands and gravels, and heavy soils are
generally due to the presence of clay.
Vegetation is also an indication of the nature of the rocks from
which the soil and subsoil have been derived.
Oak flourishes on clay, while fir-trees grow freely on light
sandy soils.
As regards animal life on limestone soils, common snails are
very abundant, and partridges, rabbits, and snakes are common on
light soils.1
GEOLOGICAL SECTIONS.
While the geological map shows the various outcrops in a
given area, the structural features, e.g. dips, faults, thickness of
strata, unconformable strata, curvature, etc., can be best described
by means of a section.
To a certain extent a geological section must be considered an
ideal one, inasmuch as some of the details of what is below the
surface of the ground must remain uncertain ; but the indications
obtained in the process of geological surveying will afford a
sufficiently accurate basis for filling in the details of the section.
In running a section, a line should be selected which traverses
those parts of the area which are geologically most important,
and which is, as nearly as possible, at right angles to the strike of
the beds ; if necessary, the bearing must be changed from time to
time to fulfil these purposes.
If an accurately contoured map is not available, the inequalities
of the surface of the ground must be recorded in the usual way
by means of a theodolite and level and chain, or, if great accuracy
is unnecessary, by pacing and Abney's level or clinometer. All
outcrops, artificial or natural exposures, wells or borings, dips,
fault, etc., should be noted on the section.1
SECT. II.] OUTDOOR WORK. 195
Section II. — Structural Characters of Rocks.
It has already been pointed out in Section I. that, while making
the geological survey of an area, all possible indications of the
nature of the rocks should be looked for and noted down. The
structural characters of rocks are dealt with separately in this
section for the sake of convenience, but it is not meant that a
separate examination of the district must be made on this account.
The structural characters of the rocks should be noted while the
geological survey is being made.
Referring to Chapter III., we note that the first question for
consideration is whether the rocks met with are igneous, aqueous,
or altered, and in forming our conclusion we must bear in mind
that igneous rocks are usually crystalline and aqueous rocks are
very generally fossiliferous. We must remember, however, that
some altered rocks are crystalline and that some igneous rocks,
composed of fragmentary volcanic materials, are stratified or
bedded. Again, the jointing of igneous rocks and the lines of
foliation and cleavage in altered rocks must not be confused with
lines of stratification in aqueous rocks (see Chapter III., Section
II., p. 38).i
The structural characters of igneous and metamorphic rocks
need no further reference beyond that given in Chapter III.,
Sections I. and III.
As mentioned in Chapter III., Section II., p. 37, the changes
which occur in aqueous rocks are (i) stratification ; (ii) inclination ;
(iii) curvature ; (iv) joints ; (v) dislocation. For convenience we
will take (i), (ii), and (iii) together; as regards (iv), joints, see
p. 43.
STRATA AND THEIR INCLINATION.
Principle of Stratification. — The law of continuity of strata
(see Chapter VIII., p. 137) must be firmly impressed on the
observer, who should not be misled by the temporary absence of
a particular bed or beds in any of the sections he has observed.
He must look out for alterations of strata, overlap, unconformable
strata, etc. (see Chapter III.), and by comparison of the various
sections observed he will be able to deduce the regular order of
stratification in the district which he is surveying.
JDip andJStrike (see Chapter III., Section II., p. 40). — Strata are
said to dip when they are inclined ; the direction of the dip is the
point of the compass towards which the strata slope, and the
amount of the dip is estimated by the size of the angle which the
196 GEOLOGY FOR ENGINEERS. [FT. IV. CH. X.
layers make with the plane of the horizon. For example, the dip
may be 40° to the south, or 60° to the north-east, and so on, the
limits of variation of dip being the horizontal and the perpendicular.
The direction of the dip is ascertained by means of a pocket-
compass, and the amount of dip with a clinometer. The dip may
be stated by the incline of 1 in a given number of units of
length; thus a fall of 1 in 100 corresponds to an angle of 6°.
The opposite term to dip is rise ; if the beds dip to the west, they
rise to the east.
The strike of a set of beds is denned to be the plane at right
angles to the direction of dip, on the course of a horizontal line
on the surface of inclined beds ; it coincides, therefore, with the
line of outcrop when the surface is horizontal. Consequently, the
edges of inclined strata, viewed in the line of their strike, will be
level, whilst a section at right angles will exhibit the true direc-
tion and maximum amount of slope of the strata. If, then, a bed
dips due east, its strike is due north and south. Through
knowing the strike, we do not necessarily learn either the direction
'of the dip — because it may be to either side of the line — or of its
amount ; yet to ascertain the true dip it is requisite that the line
of strike be determined, inasmuch as the direction and amount
of dip will vary with the section obtained. Thus, if the strike be
due N. and S., then all the sections, except the one at right
angles, will give a false dip ; if the dip be 45° E., then the varia-
tions in dip will be from W. and E. to N. and S., and from 45°
to 0°.9
Measurement of dip. — In observing a dip, the plane of the
graduated arc of the clinometer must be held parallel to a vertical
rock-face on which the beds appear exposed, and the distance
between the eye and the rocks should be reasonable, in order that
the straight-edge may appear coincident with a considerable length
of the dipping strata. The instrument is tilted until this edge
appears to lie along some well-marked line of stratification ; the
plummet or index then points to an angle equal to the angle of
dip observed. Several observations are desirable as checks to one
another ; any evidences of lenticular or current-bedding (cf. p. 38)
must be noted, and the compass-bearing of the face of rock utilised
must also be observed.
The dip thus found is very probably only an apparent dip, and
is less than the true dip, which runs in some other direction. Two
or more observations taken near to one another will settle this
point. Thus, where there are two dips seen on different walls of
the same quarry, or in closely adjoining quarries, and where these
are evidently not due to mere local slippings or to the very
common creep of the higher beds dowu the slope of a. hillside,
SECT. II.] OUTDOOR WORK. 197
then the direction and amount of the true dip can be found by
the simple geometrical method of Mr W. H. Dalton.
The directions of the walls, or rock-faces, on which the dips are
seen are determined with the compass, and two lines are drawn to
represent them on paper, giving
the angle rab. Should one dip
in the actual quarry-sections incline
towards a and the other away
from a, one of the lines drawn «/
must be produced, so that the dips
represented in direction by the
lines a r and a b both either incline
towards or away from a.
Draw ac perpendicular to a b,
and of any convenient length,
say, for greater accuracy, about 3
inches ; and draw a s perpendicular
to ar and equal to ac. From c r
and s draw lines making with ac FIG. 72. — Measurement of dip.
and as respectively angles equal
to the complements of the observed angles of dip and cutting
a b and ar in d and t. Then the angles ad c and at s represent
the angles of observed dip along the directions a b and ar
respectively.
Join d t ; this line represents the strike of the beds, a e, drawn
from a perpendicularly to it, gives us the direction of true dip.
Draw af perpendicular to a e and equal to ac or as; join fe.
The angle aef, when measured with a protractor, gives the
amount of true dip.
The matter is clear if the three triangles ast, acd, and afe
are imagined as bent up so as to stand perpendicularly to the
plane atd, which remains horizontal. The points s, c, and /
coincide, and a plane laid upon the dipping lines s t> fe, and c d
will represent truly a surface of one of the strata observed in the
field, when both the apparent dips were inclined away from a.
d t is a horizontal line in this surface, and is therefore the strike ;
the line/e now perpendicular to it, and also in the same surface,
represents the true dip both in compass-bearing and in inclination
to the horizon.15
Calculating the Thickness of Strata.— By knowing the upper
and lower boundaries of a stratum and its average dip, one can
readily determine approximately the depth at which it will be
found under any given spot, and its thickness. In fig. 73, suppose
AB to represent the level surface of the outcrop of a bed, the
thickness of which, and the depth of its lower surface below the
198
GEOLOGY FOR ENGINEERS.
[PT. IV. CH. X.
B
point B, it is desired to ascertain; the dip having previously
been observed to be 30°,
and the distance AB
to measure 300 yards.
It is clear that B C
at right angles to the
horizon will be the
depth, and B D at right
angles to the dip will
be the thickness of the
bed.
Now, in the right-
FIG. 73.— Calculating thickness of strata,
angled triangle A D B,
«?, .'. BD = sin AxAB,
or the thickness of the beds = sin 30° x 300 = J x 300 =150 yards.
Again, in the right-angled triangle ABC the angle at A and
the length of the line A B are known, so that
= tanAxAB;
that is, B C, or the depth of C below B = tan 30° x 300 =174 yards
nearly.
Any two terms being given in either of the equations, the third
can be obtained for each.9
Outcrop and Strike. — As the strike is always at right angles
to the direction of the dip, it must continually change with the
latter. It must not, however, be confused with the outcrop, which
is the line where any particular formation cuts the surface.
As explained in Chapter III., p. 41, the strike must coincide
with the outcrop when the surface of the ground is quite level,
and also when the beds are vertical. At all other times they do
not coincide, but the outcrop wanders to and fro across the strike
according to the changes in the angle of inclination and in the
form of the ground.1
Curvature. — If any of the upper beds which have come to the
surface, in any district, are found to be setting in again and dip
in the opposite direction away from their line of strike, an anticlinal
is indicated ; and similarly, when the beds dip inwards in opposite
directions a synclinal may be expected. The above is true whether
the beds are faulted or not.1
Overlap. — This may be detected, even when there is no section
which displays it, by the boundary lines of the two beds gradually
SECT. II.] OUTDOOR WORK. 199
drawing nearer to one another and the outer or lower one dis-
appearing beneath the inner or higher bed.1
Unconformity (cf. p. 41). — There will usually be a considerable
difference in inclination, and the boundary lines will generally
draw near to one another at a considerable angle.1
DISLOCATION.
See under Dislocation, in Chapter III., Section II., p. 44.
The presence of a fault may be anticipated from the follow-
ing :—
(1) The abrupt ending of an outcrop, or the want of continuity
of definite bands or beds.
When a bed passes under another unconformable one the out-
crop of the first bed will terminate abruptly, but in this case the
line of junction will be a wavy line following the dip and surface
features of the newer unconformable bed, whereas the line of a
fault will be a straighter line.
(2) An abrupt change in the strike due to an abrupt change in
the direction of the dip.
Changes in direction of dip and strike often occur in beds which
are not fractured, and at times the change is very sudden, but in
such cases the changing dip forms a curve where the direction
changes, whereas if the beds are fractured by a fault there will be
a sharp angle at the point where the direction of dip changes.
(3) A considerable change in amount and direction of dips of
the same bed in adjacent sections.
Change in direction of dip may indicate flexure (cf. p. 42), but
when there is change in amount of dip as well a fault is indicated.
(4) The presence, between outcrops of any two formations, of a
formation not in its normal position ; or the absence, between
outcrops, of a formation which is usually present.
This may be an indication of either a fracture or an uncon-
formity ; other indications must be looked for.
(5) When a bed fails to appear at the place where, from its dip
as previously observed in section, it was expected; or, the
appearance of a bed at a place where, from its dip, it was not
expected.
This is an indication of either a fault or a flexure.1
Tracing Faults. — The faults which are seen on cliff faces or
other exposed sections are very often comparatively small ones.
The larger faults can seldom be actually seen, although their
presence can be detected by surface indications. One reason for
this is that along the fault-lines of larger faults the walls of the
fracture are subjected to great crushing force which causes them
200 GEOLOGY FOR ENGINEERS. [PT. IV. CH. X.
to crumble away, and thus the opening becomes filled with debris
and the fault is concealed. Again, later deposits frequently cover
the older rocks, and thus the dislocations among the latter are
hidden from view.1
Section III. — Determination of Rocks.
SELECTION OF SPECIMENS.
Position. — As a general rule specimens are of little utility or
interest to the geologist unless gathered actually in situ. A talus-
heap (cf. p. 9), still worse a road-heap, the materials of which may
have come from anywhere, affords very tempting but very mis-
leading material. Some " specimens " seen in their true position
are, however, far too large to be carried away. In such cases a
sketch giving dimensions, or a photograph, must suffice, and chips
from various parts may serve subsequently as illustrations of the
whole.
Soils are best collected in artificial cuts or on the banks of
streams, some 2 feet or so below the ordinary cultivated and
altered surface.
Well-developed crystals of minerals are to be hoped for only in
cavities and on the walls of open joints.1
Bock-specimens should be broken out from larger masses, so
as to secure fresh unweathered surfaces. It is often useful, how-
ever, to show the amount of resistance of the rock to atmospheric
action by collecting the surface-crust also. The difference in
colour between such crusts and the interior is often striking, as
may be seen in brown clay-blocks with blue cores, or in the blue-
grey "felstones" (cf. p. 109) of Wales, which weather to a porcel-
lanous white.
The rock-specimen should be broken, with as little chipping as
possible, into a square fragment with the larger surfaces repre-
senting the lines of bedding if possible. Pieces about 2 inches
long and 1J inches wide, and about the same thickness,
are of a convenient size. When first detached each specimen
should be wrapped in paper and the locality, formation, and
bed should be written on the wrapper. The specimens can then
be easily labelled and numbered and particulars entered in a note-
book in due course.15
EASILY DISTINGUISHABLE CHARACTERS.
It must be clearly understood that the characters described
below and referred to in the accompanying table (p. 203) are
only such as can be easily detected in the field. In all important
SECT. III.] OUTDOOR WORK. 201
or doubtful cases the specimens should be examined at home and
the minerals separated as described in Chapter XL, p. 212, when,
with the aid of the fuller description of structure and other
physical characters given in Chapter VI. and the descriptions of
the rocks given in Chapter VII., it is hoped that the reader will
be able to identify any ordinary rock.1
Structure. — The various kinds of structure referred to in the
table are : —
Crystalline.
Compact or Homogeneous.
Foliated or Schistose.
Fragmental.
Granular.
Vitreous.
Cleaved.
Earthy.
Concretionary.
Crystalline includes all types in which crystalline texture can
be detected by the eye, but the minuter forms, such as crypto-
crystalline, etc., are included under Compact.
Compact or Homogeneous includes all close-grained and
lithoidal rocks.
Foliated or Schistose rocks are those of a distinctly foliated
character; see Chapter VII., Section III., p. 124.
Fragmental (see Chapter VI., Section III, Group 5, p. 101) in-
cludes breccia, conglomerate and volcanic agglomerates, tuffs, etc.
Granular. — This term refers rather to texture than structure ;
see under Texture, Chapter VI, Section III., p. 98.
The remaining terms, Vitreous, Cleaved, Earthy, Concretionary,
are described in Chapter VI.1
Hardness. — The pocket-knife must be used freely, as in the
case of minerals, in estimating the hardness of a rock. The angle
of a steel hammer, drawn across the face, often gives similar
information. All rocks tend, however, to have a hardness a little
below that of their principal constituents (see Chapter VI.,
Section IV., p. 104), owing to looseness of texture or development
of decomposition-films between the grains, but granular limestones
can at once be distinguished by the knife from the unscratchable
quartzites. Basalt, which is scratched with some difficulty when
fresh, can in this state never be confused with black limestone or
compact dark shale — mistakes that have often been made during
the hurried examination of hand specimens.15 See also Chapter
XL, Section I.
Streak. — While the specimen is being scratched to ascertain
202 GEOLOGY FOR ENGINEERS. [PT. IV. CH. X.
its hardness, the streak or colour of the powder produced by
scratching should also be observed.1
Feeling. — May be rough, as trachyte ; smooth, as mica ;
unctuous, as talc, steatite, and serpentine (slightly) ; or meagre,
when the surface seems to rub off in powder under the finger as
chalk.20
Smell. — This is apparent in some limestones containing
hydrogen as well as carbonic acid, which, when rubbed, smell
strongly of carburetted hydrogen ; also in some varieties of quartz.
Some clays have an earthy smell when breathed upon.20
Effervescence. — If a drop of dilute nitric, sulphuric, or hydro-
chloric acid in the proportion of 1 part acid to 5 parts water
be applied to the fresh fractured surface of a rock, it will cause
rapid effervescence if the rock is a pure carbonate of lime, slow
effervescence if the rock is partly composed of carbonate of lime,
but none at all if the rock is a sulphate or silicate.1
Colour and Lustre (see Chapter VI., Section IV., p. 104).— The
colours of weathered fragments and fresh-fractured surfaces should
be carefully noted and the lustre, if any, of the latter should, be
observed.
The various kinds of lustre recognised by experts in the case of
minerals are given in Chapter IV., Section III., pp. 67, 68, and
the same terms are applicable to rocks, but it will generally
suffice, for the purposes of the rough outdoor examination under
consideration, to note whether the freshly fractured surface is or
is not lustrous.
Fracture. — The usual forms are given in Chapter VI., Section
IV., p. 104.1
[TABLE.
OUTDOOR WOfcK.
203
1«!
II
P 5*a
3? 5
1° I
I 1
S -2
S
>1 131
S>» en >> t*>
^5 S^: t< a
II I|I II
te or
ted
!53 2g^
. «
H ^
.a S?o c 5
i -a £|l|-
No. of
Specimen.
204
GEOLOGY FOR ENGINEERS.
TABLE Nil.— Continued.
i
cs 'S
1 *g
I ll
,2 .S ft
The Rock is probably
«S s I* "
Ij j I Jj
Silica in the form of
Jasper, hornstone,
flint, or chalcedony.
to
I1!
03 ^ f^ ^ g> g
2
o
05
N
« 9
: : : I : 1 111
w * £ a
o
'S
s
: : : : : : if : :
§
1
1
S ^ ^^ ^ •£,
— (_
4) ?
S & a §
= " 3 ~ 1 S f
^ 00 F^ c^
u>
|
1
00
£
a* >*%
if j = « I! }.•'•> Sjljl
S
E
32
.11 la a|
^g- - =31 11 -
"^8 tg 88
^3 5s* °S
tit
4> c8
No. of
Specimen.
.-1 O»C<5-*iO COt^OO
S
OUTDOOR WORK,
205
S3 II
111 1!
S** js°
PI
,— i u
nolite, fel
rphyrite.
ill
I s
1
^ I
e« 5
iO (O
<M CJ
206
GEOLOGY FOR ENGINEERS.
ine-graine
tains scatl
of iron py
ii
Hi
OJ
rs c8
* I fe-
ll II'
£
(U C «
SI 5
|s I
Greenish to
white
No. of
Specimen.
CHAPTER XI.
INDOOR WORK.
Section I. — Further Examination of Rocks.
THE rough examination in the field will frequently prove
insufficient and it will often be necessary to isolate the mineral
constituents and examine them separately as described in
Sections II. and III. of this chapter ; but before doing so the
following further tests should be applied and may suffice to
determine the nature of the rock.1
PHYSICAL CHARACTERS.
Hardness. — The rough test made by scratching with a knife
(see Chapter X., Section III., p. 201) may be supplemented by
following the instructions for testing the hardness of minerals
given in Section III. of this chapter. The precautions to be
observed in the case of minerals apply also to rocks.
Specific Gravity. — This is often a good guide to chemical
constitution. The general methods of determining specific gravity
are detailed in the next section (Determination of Minerals}. The
specimen must be selected with the following precautions : —
1. It must be representative of the mass under examination,
and sufficiently large to include all the constituents in their
correct average proportions.
2. It must be free from flaws and cavities.
3. It must be unweathered, except in certain special investiga-
tions.
To observe the first precaution, it is often necessary, and indeed
safer, to use Walker's rather than the refined chemical balance,
which will not weigh a specimen of more than 100 grammes.
The method devised by Mohr for measuring the displaced
water is highly satisfactory in dealing with crystalline rocks of
coarse grain and any specimen which it is inadvisable to reduce
in size. The displacement-apparatus consists in simple form of
207
208 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
an inverted glass bell-jar furnished below with an indiarubber
tube and clip, and supported on a stand. The water placed in
the vessel can be thus run off from below, accuracy being ensured
by using the clip rather than a tap, and by letting the tube
terminate in a jet formed of glass tubing. A horizontal wooden
bar bearing a needle is laid across the top of the vessel, the
needle projecting about 3 or 4 cm. downwards. To ensure
constancy of position, the points where the bar habitually rests
on the glass rim should be marked with a file or by gummed slips
of paper.
The vessel is filled with water ; the end of the needle is lightly
greased, and allowed to project into the liquid. Looking up
from below at the bright, totally reflecting surface of the water,
the clip is released, and the water is allowed to run off until the
needle-point just disappears from view. It now exactly touches
the upper surface of the water and gives us a standard to which
to refer. The specimen, which has been weighed upon a strong
but accurate balance, is then lowered by a fine thread or wire
into the vessel, the water rising higher by the addition of its
bulk. When all bubbles have disappeared, a graduated
measuring-glass is taken, the divisions of which correspond to the
units of weight used in the determination of the weight in air-
Thus, if grammes were used, the glass will be graduated in cubic
centimetres. Into this glass the water is run off until the needle-
point, observed from below as before, again exactly touches the
surface of the water. The amount run off gives the bulk of
water (d) displaced.
G = weight in air
To observe the second precaution, some rocks, such as porous
sediments or pumiceous lavas, must be reduced to powder and
determined with the specific gravity bottle, the finest dust being
sifted or blown off to avoid choking of the small tube in the
stopper.
To observe the third precaution, it is often well to pick up clean
chips from specimens trimmed in the field, which, selected from
a large number, will serve both for the determination of specific
gravity and the making of microscopic sections, if required.
Since the range of specific gravity in rocks, the coals being
omitted, rarely exceeds the limits 2 '2 to 3 '4, many very diverse
rocks have the same specific gravity, and the results are not of
value in absolute determination. But in the case of igneous
rocks, provided that specimens are selected and examined from
different parts of an exposure, an excellent idea can be formed,
SECT. I.] INDOOR WORK. 209
from the specific gravity alone, of the silica percentage of the
mass.
THE CHEMICAL EXAMINATION OF ROCKS.
A number of ordinary qualitative tests may be applied to
rocks, and the examination with acids, hot or cold, is naturally of
great value in the detection of carbonates.
Pure dolomites such as at times occur among crystalline
masses will effervesce only when the acid is heated, but
magnesia occurs in many limestones in which the acid test is
unavailing. The ordinary dolomitic limestones thus effervesce
very freely in cold acid, and the magnesia can only be safely
determined by precipitation from solution by hydric disodic
phosphate in the ordinary way. On the other hand, we must
here repeat the warning that a rock which gives no effervescence
when touched with strong cold acid may yet belong to the group
commonly styled limestones, being in fact a dolomite ; and the
resemblance, except in hardness, of some of these rocks to
compact grey gypsums or even quartzites makes it necessary to
emphasise this caution.
Preparation of Material. — The treatment of a rock with acid
is frequently important as revealing an insoluble residue, which
should always be examined further. The division, however, of
every rock into a soluble and insoluble portion, prior to analysis,
is now regarded as of little value, and the ordinary plan pursued
is to make a thorough fusion of a weighed quantity of the powder
with carbonate of potash and carbonate of soda. The powder
must be obtained by breaking up little fragments of the rocks
still further upon an anvil. The fragments may be wrapped in
stout brown paper so as to avoid the introduction of particles of
steel from the hammer or anvil used. Finally, freed from any
whisps of paper, the material is ground and reground, a portion
at a time, in a fair-sized agate mortar until the powder is
practically impalpable between the fingers. Too much care cannot
be given to this simple preparation of the material used in the
analysis, since imperfect fusion may result if the particles are not
sufficiently fine, and the silica ultimately separated will contain
gritty, undecomposed matter. Although the precautions and
details of the methods employed must be left to chemical works
and to personal practice, it may be of service to remind the reader
of the successive operations performed during a simple rock
analysis, such as would suffice for ordinary determinative purposes.
Naturally, the list of substances that might be looked for and
separately estimated in an elaborate analysis of material from the
14
210 GEOLOGY FOE ENGINEERS. [PT. IV. CH. XI.
earth's crust is as long as that of the known chemical elements,
but the proportions in which the below-mentioned oxides occur
are often of fundamental geological importance.
Summary of Determinative Chemical Analysis of a Rock.
1. Loss on ignition. — Dry the powdered rock in a water-bath
at 100° C., transfer about 1 gramme to a platinum crucible, and
determine the weight of the quantity thus used. Then ignite
strongly over a gas blowpipe and weigh again. Ignite a second
time and weigh, repeating this until the weight is constant.
The difference thus found is due to loss on ignition, which
generally represents water. Where it is necessary to determine
carbon dioxide, a sample of the powder must be decomposed by
acid in an apparatus in which either the gas evolved is allowed
to escape and is determined by loss, or in which it is collected in
an absorption tube by soda-lime and weighed.
2. Silica. — Prepare a fusion mixture by minutely mixing
13 parts by weight of potassium carbonate with 10 parts sodium
carbonate. Add to the ignited powder in the crucible, or to a
fresh sample if the heating has caused it to fuse or frit together,
about 4 times its weight of fusion mixture, mixing carefully and
very thoroughly with a rod or platinum spatula. Fuse at first
over a Bunsen burner, the lid of the crucible being kept on and
avoiding too great heat at the outset. Then apply the blowpipe
until the whole mass runs freely together and ebullition ceases.
The crucible lid should be easily lifted off with the platinum
forceps so that inspection of the mass can be made from time to
time.
Remove and stand the crucible on a cool surface such as an
iron plate, so that the fused mass may crack away from the wall
of the crucible. Place in a porcelain or platinum dish with
hydrochloric acid and water, covering quickly with a clock-glass
to avoid loss by effervescence of the carbonates. Warm, and
allow to stand until decomposition is complete. Evaporate to
dryness, breaking up any lumps with the spatula, and heat
finally to about 120° C. in an air-bath. Moisten again with
strong hydrochloric acid, add water, and warm. The silica should
now float about lightly in the liquid when stirred, while all the
bases are in solution. Filter off the silica, ignite, and weigh. If
gritty matter occurs amid the silica, the fusion has not been
satisfactory, and the process must be begun again.
3. Alumina and ferric oxide. — Add to the filtrate a few drops
of nitric acid, in order to ensure the conversion of ferrous to
ferric salts. Then add ammonia in very slight excess and boil.
SECT. I.] INDOOR WORK. 211
Filter off the precipitate of alumina and ferric oxide, obtaining
the nitrate a. When thoroughly washed, redissolve the pre-
cipitate into another vessel, and divide the subsidiary nitrate
thus obtained into two measured quantities. Thus it may be
made up to | a litre by dilution in a marked flask, and 250 c.c.
may be drawn off with a pipette. In this portion precipitate
alumina and ferric oxide as before, filter, ignite, and weigh.
Draw off 100 c.c. from the portion remaining in the flask, and
determine the iron in this volu metrically by means of bichromate
or permanganate of potash. Make a check-determination by
drawing off another 50 or 100 c.c. Divide the weight of iron
found by '7, which will give the weight of ferric-oxide. Deduct
this from the joint oxides, the alumina being thus found by
difference.
4. Lime. — To the original filtrate a, which must contain
ammonia in excess, add excess of ammonia oxalate. Allow to
stand for twelve hours. Filter, and ignite strongly ; weigh, and
repeat till the weight is constant. The precipitate is thus
converted into lime.
5. Magnesia. — Ammonia being in excess, add hydric disodic
phosphate to the filtrate, stirring very carefully with a rod, since
the precipitate clings to any parts of the beaker that may have
been in the least degree abraded by touching. Stand for twelve
hours and filter cold. Wash the precipitate with a mixture of
1 part ammonia and 3 water, and ignite, the filter being burnt
separately in the lid of the crucible. Where a large quantity of
magnesia is expected a porcelain crucible should be used, to
avoid injury to the platinum. The ignited precipitate is the
pyrophosphate (Mg2P207). To estimate as magnesia, multiply by
•36036.
6. Potash and soda. — These alkalies are best determined by
the Lawrence-Smith method. Mix intimately 1 part of the
powdered rock (about J a gramme) with 1 part of ammonium
chloride and 8 parts of pure calcium carbonate. Heat for an
hour in a deep platinum crucible, which is best supported almost
horizontally over a flat-sided Bunsen flame, and under a conical
iron shield. The flame must be applied very gradually at first to
avoid rapid volatilisation of the ammonium chloride, and the
temperature should at no time rise above dull redness. The
decomposition is effected without complete fusion. Dissolve out
the fritted mass in water in a dish, and filter. The filtrate
contains the metals of the alkalies in the form of chlorides, with
some portion of the materials used in decomposition.
Precipitate the lime from the filtrate by ammonium carbonate ;
filter and evaporate down, testing the filtrate as it becomes more
212 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
concentrated with a drop or two of ammonium carbonate
solution. If lime is still present, precipitate it and filter again.
Evaporate to dryness in a small dish, and gently drive off by
further heating the ammonium chloride and ammonium carbonate.
A dark stain may appear, which is due to impurities in the
ammonium carbonate, and may be neglected. Excessive heat
must be avoided, lest a portion of the chlorides of the alkali
metals should be lost. Weigh the joint chlorides in the dish while
the latter is slightly warm.
Dissolve in water, add platinic chloride, and evaporate almost
to dryness on a water-bath. Add alcohol, and allow to stand
for some hours, the precipitate of potassic platinic chloride
being insoluble in alcohol. Filter on to a weighed filter, wash
with alcohol, and dry at 100°. Weigh with the filter without
ignition.
To calculate this precipitate as potash multiply by '19272.
Divide this result by '63173, which gives the weight of the
potassium chloride in the joint chlorides. Deduct this from the
joint weight and multiply the remainder by '53022. This gives
the weight of soda.15
Fusibility. — Though it is seldom desirable, on account of
their complexity, to treat rocks before the blowpipe as if they
were simple minerals, yet in a few cases the determination of the
fusibility proves of service. The older writers relied, indeed,
more upon this character than has since been thought desirable,
and the nature of the glasses produced was closely studied. It is
obvious that the application of the flame, in the absence of an
acid, will decide between a soft rock composed of silicates and
a limestone, the former in all probability fusing to a glass while
the latter becomes luminous and crumbling. The natural
glasses also have various degrees of fusibility, the more highly
silicated fusing with greater difficulty than the basic. Thus
obsidian fuses at about 5 of von Kobell's scale (see Section IV.,
Observation of Fusibility, p. 230) and tachylyte as easily as 2 '5.
Care must be exercised, however, in dealing with these glasses
that the splinters used do not present unusually thin edges.
In the case of an igneous rock that has undergone alteration,
the fusibility can be of little service, since a very small admixture
of hydrous minerals such as zeolites may suffice to considerably
increase the fusibility of the mass.15
Section II. — Isolation of Constituents.
In the case of a coarse-grained rock, clearly composed of hetero-
geneous materials, it is not difficult to break out with the hammer
SECT. II.] INDOOR WORK. 213
or pliers fragments or crystals of individual constituents, which
can then be submitted to special tests.
Many sedimentary rocks, such as sandstones, can be broken up
with the pliers or even with the fingers, and the grains spread out
on paper for identification. Other rocks, such as clays, may be
broken up after prolonged treatment in water, the materials of
varying fineness being successively washed off into separate
vessels, and an often valuable residue of larger grains, small
fossils, etc., being finally left behind.
When a rock is, however, compact and coherent, its constituents
can be isolated only with difficulty ; and at the beginning of the
nineteenth century a large number of masses were classed as
homogeneous, or even as mineral species, which were in reality
fine-grained rocks in which it seemed impossible to determine the
constituents.15
MECHANICAL ANALYSIS.
The crushing of crystalline rocks, with a view to the isolation
of their constituents, is best performed between folds of smooth
cloth or even paper, to avoid the introduction of extraneous
metallic or mineral material. Any fibres from the paper used
will generally wash off on soaking.
The powder of the rock, which must be fairly coarse, is passed
through sieves of various mesh, until a sample is procured, as
coarse as possible, in which each grain consists of only one mineral
species. For this purpose the sieves used in chemical laboratories
are convenient, several fitting one above the other. The crushed
mineral is placed in the topmost, which has the widest mesh, and
the whole being shaken, each sieve selects a sample increasing in
fineness till we reach the lowest pan.
The objection to the use of sieves lies in the fact that some of
the constituents may be much more friable than others, and hence
for quantitative purposes no one sample may be satisfactory.
The contents of each sieve must be examined in order to determine
if any mineral has become eliminated from this cause. The
sample, when selected after examination with the lens, may be
picked over by the aid of that instrument, or upon the stage of a
microscope with a low power. A fine brush should be moistened
with water (Dr Sorby recommends glycerine) and brought in
contact with the grain to be picked out. It is then dipped just
below the surface of a little vessel of distilled water, and the
grain is detached at once and sinks.
In this way, by care and patience, a quantity of any one con-
stituent can be accumulated sufficient even for a chemical
analysis. But for merely qualitative tests a very few grains will
214
GEOLOGY FOR ENGINEERS. [pT. IV. CH. XL
a
be sufficient, and excellent material can be quickly obtained, to
which microchemical reagents may be applied.15
Washing. — The removal of light material, such as clay, fine
dust, etc., from heavier or coarser constituents may be performed
by ivashiny, as in an apparatus described by M.
Thoulet (fig. 74). A large tube a, terminating in
a tap below, is fitted with a rubber cork through
which a finer tube, 6, passes. A tube c opens
through the side of a. The powdered material is
placed in a, and water is introduced through b.
This rises in a and flows over at c, carrying with it,
if the operation is sufficiently prolonged, all the
light substances thus washed out of the material.
In separating minerals of different specific
gravities water is introduced at c, and flows out
up b when a has become full. This current keeps
the powder well disturbed, and by regulating it
none of the material escapes up b. Check the
flow gradually, and the grains of different characters
will descend successively, forming distinct layers
at the bottom. These can be drawn off by the
tap and a fairly pure amount of any particular
constituent collected. Plate-like minerals, such as
mica, will probably appear among the upper layers.
It is clear that simple forms of such an apparatus
can be constructed with glass tubes, corks, rubber
tubing, and a clip to act as a stop-cock.15
Magnetic Separation. — In using the simple
Tet's 'washing magnet, grains of composite character, containing
apparatus. only minute particles of magnetite, may become
taken up ; but such can be subsequently removed by
picking, if the iron oxide itself is required to be pure. It is use-
ful to make a little sliding of tissue-paper as a cover for the end
of the magnet used. This is kept in contact with the end while
passing over the powdered rock, and the magnetic particles adhere
to it. On withdrawing the magnet to the collecting vessel the
cap is thrust forward and the material falls off into the vessel.
M. Fouque has extended this simple method with considerable
effect. He uses an electro-magnet, connected, if necessary, with
six Bunsen cells. By successive increases in the strength of the
current the constituents of a rock can be fairly sorted one from
another — first the magnetite, then the pyroxene, the olivine
and the felspars and allied minerals which contain traces of
magnetic substances. A residue of felspars and " f elspathoids "
finally alone remains. The frequently occurring glassy matrix of
SECT. II.] INDOOR WORK. 215
igneous rocks affords the greatest cause of error, as indeed in
most isolations by other means. If it is pyroxenic it may, by
inclusion in the felspar, cause the removal of a large quantity of
the latter, leaving only the purer quality ; but in many cases it is
highly silicated and scarcely ferriferous, and cannot be separated
from the felspars that are to be tested by Szabo's or other reaction.
As we have already stated, microscopic examination must decide
on the suitability of such selected material for refined determina-
tive tests.
In practice with Fouque's method, the ends of the electro-
magnet may be covered with thin paper, to prevent the adhesion
of non-magnetic particles to any moisture on the surface of the
iron. The powder is placed on a large card and jerked close
under the poles. When a certain amount of material has been
attracted, the card is withdrawn and a clean card or paper substi-
tuted ; the current is then interrupted, and the particles fall off
and are collected.15
Dense Liquids. — If a solution of known density is to hand, and
a specimen, though it has been completely freed from bubbles,
floats upon the surface, while others sink, with more or less
rapidity, some idea of their relative specific gravities may be
obtained.
Further, if the liquid is diluted until a particular specimen
swims about in it and remains sluggishly wherever it is placed,
the liquid and the mineral will be of the same specific gravity.
That of the liquid may be determined by throwing in a series
of specimens already determined until one is found that will
neither float nor sink to the bottom, or by suspending a weight
from a chemical or Jolly's balance, and comparing the readings
given when it is immersed in water and in the liquids respectively.
The most suitable liquids are (1) solution of borotungstate of
cadmium, first prepared by D. Klein, and now very widely used.
This is a pale yellow liquid, with a density of 3*28; it can be
diluted with water and again concentrated by heating over a
water-bath until a hornblende crystal just floats upon the surface.
Any overheating will cause the salt to crystallise out on cooling
down, when a fresh dilution will be necessary. Though poisonous,
the borotungstate is not irritant like the mercury solutions; it
can be carried about in a stoppered bottle in the solid state and
dissolved in distilled water when required. A few ready-made
solutions of known density, kept carefully stoppered, will be very
useful in the discrimination of gems. The only objections to this
liquid are that it decomposes carbonates, so that specimens before
use should be treated with a mild acid; and that it tends to
crystallise readily upon the stoppers of bottles or the glass rods
216 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
used in stirring. The rods and vessels used should always be
washed with distilled water, the resulting very dilute solutions
being kept together in a bottle, to be concentrated by evaporation
when time allows.
Another liquid (2) that promises very well has been brought
forward by R. Brauns. He uses methylene iodide, which must be
diluted with benzole, and not with either water or alcohol, and
which, to preserve its pale straw-colour and transparency, must be
kept as much as possible from the light. When it has become
darkened, as must eventually happen, the colour can be restored
by putting a few globules of mercury into the bottle and shaking
the whole together for a few minutes. This liquid, from its not
crystallising when concentrated by evaporation, is very clean and
agreeable to use, but it does not seem so adapted for researches
made beyond the reach of laboratories as does the borotungstate
of cadmium solution.
The use of these dense liquids is a most valuable method for
the isolation of constituents of rocks. It is clear that if we pre-
pare a solution of density intermediate between the densities of
any two constituents, one of these will float up to the surface
while the other will sink. If the lighter mineral is the only one
to be collected and examined, the operation may be performed in
an ordinary beaker and the surface-material skimmed off with a
spatula. For economy of the liquid the beaker should be fairly
narrow, since some depth of liquid must be used to allow of
perfect separation. If Klein's convenient borotungstate of
cadmium solution is used, the powdered rock must be treated
beforehand with dilute acid to ensure the removal of the
carbonates.
The material must be well stirred on immersion, and both top
and bottom layers stirred later to prevent entangling of inappro-
priate constituents in either. The particles when removed must
be well washed with distilled water, or with benzole if methylene
iodide is used in the separation. The washings are collected in a
dish and evaporated down until a concentrated liquid is again
obtained for future use.
The material separated, when washed and dried, should be
carefully searched over with a lens of low microscopic power,
since some composite grains are sure to be included. Any doubt-
ful object must be rejected if a quantitative analysis is contem-
plated ; or, for ordinary qualitative tests, only the purest grains
must be selected.15
Use of Acids. — A more dangerous method of isolating particular
minerals from the powdered rock. Strong acids are likely to pro-
duce surface-decomposition of the minerals that are to be ulti-
SECT. II.] INDOOR WORK. 217
mately examined. It is obvious that the nature and strength of
the solvent used in each instance must be left to the judgment of
the observer.
M. Fouque employed hydrofluoric acid in the isolation of the
minerals of the lavas of Santorin. He placed about 30 grammes
of the rock-powder, from which the finest and the coarsest particles
had been sifted off, in a platinum dish into which concentrated
hydrofluoric acid has been poured. The materials were inserted
cautiously and stirred together ; the process of decomposition was
arrested at any required stage by pouring in water and washing
off the fluosilicates, fluorides, and gelatinous products that had
been formed. The materials, when washed, should be rubbed
with the finger under water to free them from the last traces of
the jelly.
In this way the amorphous glassy matrix may be removed
from around many minerals, though it may be difficult to free
felspars completely from it without seriously attacking the
crystals. The ferromagnesian minerals are attacked only after
long immersion, hence they can be isolated from quartz and
felspars with comparative ease. The acid is thus found to attack,
first, the glassy matrix, then the felspars, then quartz, and lastly
the ferromagnesian group (pyroxene, amphibole, olivine) and
magnetite.15
The determination of the proportions in which particular
minerals are present in a rock can, of course, be effected by weigh-
ing the original powdered material and the successive groups of
isolated constituents. Delesse long ago employed a rougher
method, which is simple and very reasonably accurate. It is thus
of especial value to observers far removed from refined apparatus.
Delesse chose a plane or even polished surface of the rock, or, in
special investigations, the six surfaces of a parallelepipedon cut
from it. He covered each such surface with a sheet of gold-
beater's skin or fine paper, increasing the transparency if necessary
by soaking the covering and the face of the rock in oil. The
covering was affixed with gum.
The outlines of the minerals were then traced through with a
pencil or fine pen, and the various minerals were coloured with
different tints. The tracing was removed from the rock and
gummed to a sheet of lead or tin-foil. The outlines were cut
round with a pair of scissors and the pieces of the same tint were
sorted together. To avoid errors due to irregular thickness of the
gum and paper, each sorted group was treated in water and the
fragments of the foil alone finally used.
These groups of fragments were then weighed and compared
with the total weight of foil that corresponded to the area or areas
218 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
of the rock selected, the proportions of each mineral being thus
ascertained. Delesse found it convenient to estimate fine lamellar
minerals, such as mica, by difference.
When a good balance is at hand, the paper may probably be
cut out and estimated directly, without transference to the foil.15
Section III. — Determination of Minerals.
Mode of Occurrence. — The relation of the mineral specimen to
its surroundings should in all cases be observed prior to its
extraction. Its occurrence in veins or diffused through a rock-
mass, in concretionary forms or in well-developed crystals, its
deposition upon earlier-formed constituents or its inclusion in
other substances that have aggregated round it — these are a few
of the many points that may help in its final determination.15
Extraction. — The rock-constituents, having been isolated, must
be either simple minerals or mineral aggregates, and some one or
all of the following methods of examination may be applied with
a view to their determination. The test of hardness, and some
observations on form and cleavage, may often be employed without
the removal of the mineral particle from its surroundings.15
EXTERNAL FORM (cf. p. 58).
Preliminary Examination. — The pocket-lens will aid consider-
ably in examining the crystalline form of minerals that have con-
solidated under favourable conditions ; but the undue development
of certain faces, or the almost complete suppression of others,
renders the interpretation of natural forms far more difficult than
would appear from the symmetrical drawings and models with
which the elementary student becomes at first familiar. Not even
the measurement of the angles will distinguish between an elongated
cube and a prism of the tetragonal system ; but, in such a proble-
matical case, some other test is certain to be available which will
virtually decide the question of the species to which the mineral
belongs. In the preliminary examination with the eye or with the
lens twin-structures may occasionally be detected. Thus the char-
acteristic Carlsbad twinning of orthoclase, whether in granitic or
trachytic rocks, is very generally observable upon broken surfaces ;
the basal cleavage is inclined in reverse directions in the two
halves of which the crystal is built up ; hence the one half will
show, as the specimen is turned about in the hand, a series of
brightly reflecting surfaces, while the other remains dull or even
earthy-looking. Repeated twinning, as in plagioclase felspars,
often reveals itself by the appearance of fine alternating duller or
more lustrous bands.15
SECT. III.] INDOOR WORK. 219
Measuring Crystal Angles. — It is often useful, and in some
cases is absolutely necessary, to determine the angles made by
certain planes of the crystal. Even where works of reference are
not to hand, the determinations can be forwarded to a friend more
fortunately situated ; and the angles thus measured and compared
will, from their constancy in the same species, serve to explain
faces and forms of the most anomalous development. With
sufficient practice upon familiar specimens the well-known contact-
goniometer of Carangeot is capable of giving excellent results. In
its simplest and perhaps handiest form it consists of two small
flat bars of steel or brass, in each of which a slot is cut extending
from near one end to the centre, the other half remaining solid.
A little bolt is passed through the slots, and the bars are clamped
together by a nut above. By releasing the nut and drawing
back or thrusting forward either of the bars, their cleanly-cut
inner edges may be applied to any two planes of the crystal that
are not parallel to one another, the measurement being taken
when the edges of the bars are perpendicular to that formed by
the intersection of the two planes of the crystal. When exact
contact has been made, which may best be secured by holding up
the crystal and the instrument and observing that no light
passes between the planes and the edges of the bars, the bars are
carefully clamped together and again applied to the planes in
question. If no shifting has taken place during clamping it only
remains to determine the angle between the inner edges of the
bars.
Determination of angle. — This is best done by applying the
instrument to a semicircular or circular protractor, which indeed
forms an integral part of the contact goniometer. The point of
intersection of two corresponding edges of the bars, or else of
their middle lines, is made to coincide with the centre from which
the angles have been marked off on the protractor. The bolt of
the clamp is usually bored on the under side, and is dropped
over a pin so fixed that either the edge of one bar, when it is
brought against a stop rising from the circle, or else its middle
line, coincides with the line joining 0° and 180°. The angle is
read off between the prolongations of the bars and not between
the edges that were actually applied to the crystal. When the
middle line is used, the prolongation of one bar is cut back to half
its width.15
PHYSICAL CHARACTERS.
Determining Cleavage (cf. p. 62). — Proper direction. — The
readiest way of determining the cleavage of a crystal is to place the
edge of a knife or small chisel upon a face parallel to that of some
220 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
principal form and strike a light blow with a hammer, when, if the
direction is near that of a principal cleavage, a more or less flat-
faced fragment will be removed. If, on the other hand, no cleavage
is obtainable in the direction of the blow, the fractured surface
will be uneven or irregular, or will show traces of step-shaped
structure in the direction of the true cleavage plane. Thus
fluorspar, whose crystals are chiefly cubes, cannot be cleaved
parallel to that form, but yields with the greatest ease in the
direction of an octahedral face.
Easy cleavage. — Some minerals, such as mica and gypsum, are
very easily cleavable, and many may, with slight effort, be
divided by the finger-nail, or the point of a knife, or needle, into
laminae of extreme thinness. In the case of mica there seems to
be no limit to the capacity for cleavage, as laminee may be
obtained thinner than the edge of any cutting tool that can be
brought to bear upon them.
Developing cleavage. — In some instances cleavages may be
developed in imperfectly cleavable crystals by strongly heating
and suddenly cooling them in water. Quartz crystals, when so
treated, occasionally develop faces parallel to those of the unit
rhombohedron ; and under ordinary circumstances they break
with a fracture like that of glass. Easily cleavable minerals,
such as salt, galena, fluorspar, and calcite, usually decrepitate or
fly into pieces when suddenly heated, the fragments obtained
being regular cleavage forms.12
Hardness. — Anyone seeking to determine minerals should be
thoroughly well acquainted with Moh's scale (see Chapter IV.,
Section III., p. 66). The relative resistance of each member to
the point of a good pocket-knife should be carefully observed in
succession, until No. 7 is reached, which is not scratched by
steel. If a specimen of each member is passed lightly over the
surface of a file, or the file is drawn across an edge of the member,
different amounts of material will be removed from each, and the
sound produced, at first sight, will become more grating as the
higher members are used.
Rough scale. — The observer may, however, rely largely upon
certain simple instruments alone. Thus : —
(a) Minerals unscratched by a good knife have a hardness (H)
of 6 or upwards.
(b) Minerals scratched with a knife have H = 5'5 or less.
(c) Minerals scratched by a bronze coin have H = 3'0 or less.
(d) Minerals scratched by the thumb-nail have H = 2 '5 or less.
Few minerals are harder than 7, and the relative degree of
resistance to the knife afforded by the softer substances will
commonly assign them their places, even when an actual scale of
SECT. III.] INDOOR WORK. 221
hardness is not to hand. Few persons will find serious difficulty
in thus distinguishing between degrees 3, 4, 5, and 6, while the
thumb-nail decides the lowest degrees of all in an equally efficient
manner. A thin soft mineral, such as talc or mica, wrapped
about a harder core, as may occur in schists, presents occasionally
a difficulty ; and it must be remembered that decomposition
renders many substances softer than the values given in text-
books, which are those of typical specimens.
The hardness of small fragments of minerals can be best
ascertained by drawing them across a substance already determined.
The grains may be cemented on to a slip of wood with " electric "
cement, made of 5 parts resin, 1 part bees-wax, and 1 part red
ochre.15
Precautions. — In a fibrous specimen a scratch directed across
the fibres will always indicate a lower degree of hardness, than the
true one ; the scratch should therefore be parallel to the fibres,
or, still better, on the surface of a transverse fracture.
A sound, undecomposed specimen should always be selected,
since the hardness of minerals is greatly affected by decomposition.
Many minerals are softer when first obtained than after they
have been kept some time in a dry cabinet. In crystals, the
edges and angles are often considerably harder than the faces,
and those of the primitive form than of the modifications.39
Determination of Specific Gravity (cf. p. 66). — This is in
principle very simple, the substance being first weighed in air
arid then in water ; the difference between the two weights gives
the weight of an equivalent volume of water, and the quotient of
the original weight by the difference will be the specific gravity.
An exact determination is, however, a matter of considerable
nicety. When the substance contains cavities, it is necessary to
powder it before taking the specific gravity.12
Chemical balance. — The most familiar method of determining
the specific gravity of a body is that involving the use of an accurate
balance and a set of chemical weights. The specimen is suspended
by a light silk thread from the hook on the under side of a small
pan, which replaces the ordinary pan of the balance. It is
weighed in air (w) and then immersed in a glass of distilled water ;
all bubbles are carefully removed,1 the water being boiled if
necessary, or the vessel being placed for some time under an
air pump ; the weight of the specimen when suspended in water
is then determined (w')t and the specific gravity G = -,. In
w — w
1 To remove bubbles with a brush, withdraw the specimen and paint it
over, as it were, with water, which should be worked well into the hollows.
On again immersing, the bubbles will have broken and disappeared.
222 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
accurate determination the water used should be at a standard
temperature ; English observers have chosen 60° F.
Specific gravity bottle. — The use of this method involves appli-
ances of some delicacy. The bottle should be small, to suit the
probable amount of material to be used ; a 25-gramme flask is
large enough. Fill it with distilled water, insert the perforated
stopper, and wipe off any water that has flowed over. Place the
powdered or fragmentary specimen on the pan of the balance on
a scrap of smooth paper, a counterpoise to the paper being laid on
the other pan. Weigh thus in air (w). Now place the full bottle
beside the specimen in the pan, and determine the joint weight a.
Transfer the specimen to the bottle, remove bubbles with particular
care, replace the stopper, wipe, and weigh again (b). The weight
of water displaced by the specimen = a — b. Then G= — — ,15
CL — O
Mohr's method is susceptible of considerable accuracy. The
gauging vessel is a glass cylinder, which is filled with water to a
standard point formed by a needle projecting from a slip of wood
across the top, the exact level being attained when the point of
the needle and its reflected image in the water coincide. The
weighed substance is then carefully lowered into the cylinder,
when it displaces its own volume of water, with a corresponding
rise of the surface level. The amount of displacement is measured
by drawing the water into a graduated tube or burette until the
original level is restored. A convenient size of graduated tube is
the ordinary alkalimeter used in volumetric analysis containing
1000 grains, and divided into 5-grain spaces, or an equivalent one
with metrical divisions. The level of the water may be adjusted
with great nicety by a simple valve formed of a piece of glass rod
inserted in the indiarubber delivery tube, the aperture of which
can be varied by a slight pressure of the finger upon the tube.
This method, which has the advantage of not requiring a correc-
tion for temperature, is well adapted for taking the specific
gravities of coal, limestone, and similar substances ranging from
2 to 3, which can be used in fragments of about half a pound
weight.12
Walker's balance is the most convenient and portable instrument
of which the geologist can avail himself. A steel bar, A, is
supported in a rest, B, by a knife-edge piece fixed through it
about 3 inches from one end. The remainder, some 18 inches
long, is graduated into inches and tenths, starting from the point
of support.
• The short arm of the bar is notched upon its upper surface,
and a heavy weight, C, can thus be hung from it at a variety of
distances from the fulcrum. The long arm passes through a
SECT. III.] INDOOR WORK. 223
looped upright, D, which checks undue swinging, and, by a mark
scratched on it, serves to indicate when the bar comes to a
horizontal position.
The specimen, which may weigh several ounces, is hung by a
cotton thread, a loop of which passes over the long arm. It is
then slid along the arm until it counterbalances the weight C,
which has been suspended near to or far from the fulcrum accord-
ing to the weight of the specimen used. When the bar indicates
by its swing that it would come to rest in a horizontal position,
the reading a is taken ; i.e. the distance from the fulcrum of the
point of suspension of the specimen.
The weight C is kept in the same position, and the specimen is
immersed in a tumbler of water; to restore equilibrium, the
specimen must now be carried further out along the beam. Let
this new position be b. Then, a and b being, by the principle of
the lever, inversely proportional to the weights in air and water
respectively, G=- .
b — a
The results are accurate to the first place of decimals, and
often compete with the ordinary balance in the second place ;
while for mineral or rock specimens of a fair size they may be
held to be entirely satisfactory.15
Jolly's spring balance is a simple laboratory instrument which
yields excellent results. It consists essentially of a pair of scale
pans suspended one above another ; the upper one is attached to
the end of a coiled steel spring, and the lower one is immersed in
a cistern of water standing on a bracket, whose position can be
adjusted by a sliding movement worked by a rack and pinion.12
The respective weights in air and water can be easily found.
Large masses. — The density of large masses of an approximately
regular figure may be roughly determined by weighing them and
calculating their cubic volume from their measured dimensions.
The specific gravity is found by dividing the weight by the contents
in cubic feet multiplied by 62*4 Ibs., or the weight of a cubic foot
of water. The reverse operation of calculating the weight of a
measured mass from the known specific gravities of its components
is often useful.12
Dense liquids. — If a solution of known density is to hand, and
a specimen, though it has been completely freed from bubbles,
floats upon the surface, while others sink with more or less
rapidity, some idea of their relative specific gravities may be
obtained.
Further, if the liquid is diluted until a particular specimen
swims about in it and remains sluggishly wherever it is placed,
the liquid and the mineral will be of the same specific gravity.
224 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
The specific gravity of the liquid may be determined by throwing
in a series of specimens already determined, until one is found
that will neither float nor sink to the bottom ; or by suspending a
weight from a chemical or Jolly's balance and comparing the
readings given when it is immersed in water and in the liquid
respectively.
Sonstadt's solution is largely used for this purpose. It consists
of a saturated solution of potassium iodide in water, in which is
stirred up as much mercuric iodide as it will dissolve. The
maximum density is about 3'2,15 but the solution may be diluted
as required. This is a useful method of separating mixed minerals
for analysis, when they are so intimately associated as to be
incapable of separation by hand ; as, for instance, the felspars
and lighter silicates in a rock may be roughly divided from the
denser ferrous and magnesian ones by a solution of a specific
gravity of about 2 '75, when the first mineral will float, while the
latter will sink readily. The chief drawback to the use of this
substance is in its extremely poisonous character, and it can
therefore be scarcely recommended except for laboratory use.12
Fracture (see Chapter IV., Section III., p. 65). — In easily
cleavable minerals it is, as a rule, difficult to develop any special
fracture, but it may sometimes be done by striking a fragment a
sharp blow with a blunt point, as that of a rounded hammer or
pestle, when traces of characteristic fractures may occasionally be
obtained, springing across from one cleavage surface to another.12
For Tenacity, Translucency, Colour, Streak, and Lustre see
Chapter IV., Section III.
CHEMICAL CHARACTERS.
Taste and Odour have been referred to in Chapter IV., Section I.,
p. 57. Odour is often most noticeable when the mineral has been
treated with acids ; see below.
Solubility. — This is determined by treating a powdered mineral
with water, acids, or alkalies. The chief solvents used (and the
order in which they are applied) are as follows : —
(a) Water.
(b) Hydrochloric acid : dilute at first, stronger afterwards,
if necessary.
(c) Nitric acid : dilute at first, then strong.
(d) Sulphuric acid.
(e) Aqua regia : a mixture of hydrochloric and nitric acid.
(/) Special solvents, such as oxalic acid, ammonia, etc. ; 39
also citric and tartaric acids.15
To ascertain the solubility of a mineral, a few grains of its
SECT. III.] INDOOR WORK. 225
powder should be placed in a test-tube or watch-glass, and warmed
with a few drops of the solvent. If the substance be freely
soluble — and it is only in such cases that this test is valuable in
determinative mineralogy — the powder will rapidly disappear.
Any effervescence, peculiar odour, change of colour or appearance,
or insoluble residue should be carefully noted.39
In all cases the time of immersion in the acid and the other
conditions of the experiment should be noted where comparison is
desired. As these facts are rarely stated in books on mineralogy,
typical and known specimens should be compared with the
doubtful one under the same conditions. Should complete
solution take place, further qualitative tests may be applied.15
Action of Solvents.— The results may be noted both in cold
acid and after boiling.15
(a) Water. — Sulphates, such as cyanosite, and generally minerals
having distinct taste, are soluble in water.
(b) Hydrochloric acid. — All carbonates effervesce strongly in
warm acid, if not in cold acid. Many oxides, as limonite, dissolve
quietly in HC1, without effervescence or evolution of vapour ;
others, as pyrolusite, give off chlorine, especially when warmed
with the acid.
Some sulphides, as blende, give off vapours of H2S when treated
with HC1; others, as pyrites, are not perceptibly affected.39
The smell of the H2S (sulphuretted hydrogen) will distinguish
sulphides from carbonates.
Some silicates are decomposed by boiling in HC1, particularly
those that are hydrated or with a low percentage of silica. The
silica separates either in a powdery or a gelatinous condition, the
jelly of silicic hydrate being often well seen after partial
evaporation and cooling of the liquid. The mass clings to the
test-tube, but may be removed by boiling with a strong solution
of sodium carbonate. This gelatinisation may be observed in
nepheline (or elaeolite). The great majority of olivine crystals
also gelatinise easily, and may be thus distinguished from pale
pyroxenes, which are not decomposed.15
Titanates are only partially decomposed in HC1, leaving a white
powder (titanic acid) which is insoluble in an excess of the
solvent.
(c) Nitric acid. — This is chiefly used in treating native metals
and metallic oxides and sulphides. Many of the metals, as copper
and bismuth, when so treated decompose the acid and give rise to
red vapours. Sulphides often afford a deposit or floating cake of
sulphur; titanates behave as with HC1. Minerals containing
arsenic and antimony often afford insoluble oxides of these
substances, as white powders.
15
226 GEOLOGY FOR ENGINEERS. [FT. IV. CH. XI.
(d) Sulphuric acid is rarely used as a mineral solvent, but some
silicates, as kaolin, are more readily attacked by it than by HC1.
(e) Aqua regia may be used for the decomposition of obstinate
sulphates and arsenides.39
(/) Organic acids. — Dr Bolton has shown that citric, tartaric,
and oxalic acids effect decompositions for which hydrochloric acid
has generally been thought necessary. Citric acid may thus be
carried about in a solid form, a saturated solution in cold water
may be made at any time, and the ordinary tests for the presence
of carbonic anhydride, or sulphur in certain sulphides, may be
performed with this, hot or cold, in a test-tube. Some silicates
are decomposable, with or without gelatinisation, and in many
cases the solution does not require to be heated. Ordinarily a
rather longer time must be allowed for the action of the acid
than is the case with hydrochloric acid.
Ammonia will serve to precipitate alumina and iron from
solution in HC1.15
Section IV. — Blowpipe Examination.
No geologist can consider himself equipped for determinative
observations until he has systematically examined a series of
typical minerals with the blowpipe and with associated tests. The
instruments and reagents required are few and simple, and may
be had from chemical dealers packed into boxes of very moderate
size.
APPARATUS AND REAGENTS.
For purely qualitative determinations, such as are here
described, the following apparatus will probably be found
sufficient : —
Apparatus. — Blowpipe. — The nozzle is far more important
than the mouthpiece, and its aperture should be clearly circular
and not too large. A platinum nozzle, costing about three
shillings extra, may be added to any blowpipe, and besides being
clean, can never cause coloration in the flame.
Lamps. — Where gas is obtainable the ordinary Bunsen-burner
serves all purposes. A brass tube, flattened at the top and cut
off obliquely, should be dropped into the ordinary Bunsen-tube
from above, preventing the access of air by surrounding the jet
where the gas enters, and at the same time giving a flattened
flame above, the blowpipe being directed along the slit-like
opening.
Where gas cannot be had, any simple spirit-lamp, or the
colourless blowpipe flame, will serve for boiling specimens in acid,
SECT. IV.] INDOOR WORK. 227
etc. The blowpipe lamp may burn oil, and be provided with a
screw-cap for travelling. The wick should be flat. Where space
is limited the best lamps are those filled with grease or solid
paraffin. A small cyclist's head-lamp is not unsuitable, and
excellent work can be done with an ordinary candle.
Platinum wire. — Twelve inches or so should be kept in hand
if much work is undertaken, as it is liable to suffer from the
formation of fusible alloys. A strip or two of platinum foil may
be useful as a support during fusions.
Charcoal blocks, some 10 centimetres long with a section 5 cm.
square, are used as supports for assays (cf. p. 229) and as a reagent.
Forceps. — A pair of steel ones with platinum points, so made
as to be self-closing, is practically indispensable. They should
never be used for metallic-looking substances, or any suspected of
containing arsenic, antimony, lead, zinc, or bismuth, lest the
platinum tips should become fused.
A magnet of any small bar form, small anvil and hammer,
agate mortar and pestle, steel pliers, small triangular file for
cutting glass tubing, open and closed glass tubes, and watch-
glasses or double concave lens will all be found of use, and the
practical geologist will add to the list as occasion requires.15
Reagents. — The following are used : —
Borax. — Powdered crystals.
Microcosmic salt (hydrogen sodium ammonium phosphate). —
Powdered crystals. These two dry reagents are used as fluxes on
platinum wire, characteristic colours being imparted by many
metallic oxides to the glass formed on fusion.
Carbonate of soda. — Powdered crystals of the dry carbonate.
They must be free from sulphur (see Sulphur Test, p. 236). Used
to effect fusions and reductions on charcoal, and as a test for
manganese.
Nitrate of cobalt. — A solution of the crystals in 10 parts of
water, kept in a stoppered bottle. Drops can be taken out with
a glass rod or a tube drawn out as a pipette ; or a little glass
bulb can be made, with a narrow neck. This bulb is heated and
the neck placed beneath the solution, a little of which enters ; on
reheating, so as to convert the water present into steam, and
again immersing the neck, the bulb becomes nearly filled. When
held inverted in the hand, the air within expands and forces out
the liquid in convenient drops (Brush, Determinative Mineralogy).
Hydrochloric acid. — Concentrated in stoppered bottle.
Sulphuric acid. — Concentrated in stoppered bottle. Dilution
must be performed carefully, owing to the heat evolved.
In use, a little of each of these acids must be poured out into
watch-glasses or beakers, since wires, etc., have to be dipped in
228 GEOLOGY FOR ENGINEERS. [FT. IV. CH. XI,
them, and the main store in the bottle must be left absolutely
uncontaminated. This precaution is very simple, but a warning
on the point is often necessary.
Tin-foil. — Used to facilitate many reductions, both in borax
and in hydrochloric acid.
Copper-wire. — (Some workers use cupric oxide) used in testing
for chlorine, owing to its combination with the copper, and the
colour consequently imparted to the flame.
Less important reagents are : — Potassium-bisulphate, fluor-
spar, magnesium-potassium-iodide, sulphur, silver chloride, etc.15
Use of Blowpipe. — Distend the cheeks and breathe in and out
as usual by the nose. Now place the blowpipe between the lips,
or the trumpet mouth against them. Some of the expired air
will pass out by the tube, under pressure from the tension of the
cheeks, and the remainder will pass out through the nose. At
short intervals the cheeks must be redistended in order to
maintain the pressure. In this way a continuous blast can be
kept up without interfering with the ordinary action of the lungs.
Practice is all that is necessary ; most of the difficulties that at
first occur are caused by the endeavour to force all the expired
air out through the blowpipe instead of by its natural exit, and
by allowing the cheeks to fall in too far, so that a sudden
distension is necessary and the blast is momentarily checked.
It is necessary in some reductions to maintain a blast for two
to three minutes, but seldom longer, and when the habit is once
acquired, time makes little difference ; but saliva is apt to
accumulate in the bottom of the blowpipe, during long blowing,
and the expanded part there must occasionally be emptied. In
Fletcher's hot-blast blowpipe, where the tube bearing the nozzle
is coiled round so as to become heated above it in the upper
part of the flame, all moisture is converted into steam before
it can reach the orifice. This form of blowpipe is particularly
adapted for effecting fusions and oxidations, but the hot surface
of the tube is sometimes an inconvenience when laid upon the
table.
Reducing flame (R.F.) — The nozzle of the blowpipe should be
made to touch the outer surface of the flame, which should be
about 1J inches high, and a gentle blast of air directed a little
downwards so as to carry the flame out sideways and produce
a bright yellow cone with a luminous interior. A body placed
well within the interior of the cone is cut off from contact with
the air, and yet, if brought near the point, becomes highly
heated. The result is its reduction, as the yellow flame, not
being sufficiently supplied with oxygen for complete combustion,
has a tendency to take it from any oxidised substance placed
SECT. IV.] INDOOR WORK. 229
within it, or to reduce such substances. Hence it is called the
reducing flame, designated by the letters R.F.
Oxidising flame (O.F.). — The nozzle of the blowpipe is intro-
duced a little way into the flame and a somewhat stronger blast
is sent through it. The interior luminous cone almost disappears
as enough air is introduced to effect the oxidisation of the
glowing carbon compounds. A body placed at the point or a
little beyond this flame becomes heated in contact with the air,
and consequently takes up oxygen according to its affinities.
Fusion-place. — If more heat is required as in the determination
of fusibility, the nozzle is placed as in the production of the
oxidising flame, but the substance is held inside the point of the
visible flame, since here the highest temperature occurs. This
position is termed the fusion-place.15
BLOWPIPE OPERATIONS.
The complete blowpipe examination of a mineral consists of (a)
observation of flame-coloration, (6) observation of fusibility,
and (c) eight or more distinct observations, some of which may,
however, sometimes be omitted without much loss after a little
experience has been gained.1
Assay. — The fragment of mineral operated upon, called the
"assay," should not generally be much larger than a mustard
seed, a small assay being much more manageable than a larger
one.
Observation of Flame-Coloration. — Many volatile substances
impart characteristic colours to the flame. The observation
should be coupled with that of fusibility, but a negative result is
not conclusive. Should no colour be thus seen, the splinter, or
its powder on a moistened wire, should be dipped in a drop of
hydrochloric acid specially placed out for this purpose, and again
be introduced into the flarne. The volatile and decomposable
character of the chlorides thus formed often reveals the presence
of a metal (e.g. barium) that might otherwise remain undetected
throughout the analysis.
Compounds of phosphorus and borax are best treated with
sulphuric acid.
Silver chloride, mixed with the powder of the specimen, is
useful to intensify some reactions, notably those of copper
compounds, the blue flame due to copper chloride becoming at
once apparent.
Gypsum may similarly be used with certain silicates, which
become decomposed when heated with it, the metals present
being rendered volatile in the form of sulphates.
230 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
Often the assay must be held just in the edge of the flame, and
not brought too far within it. The coloration is sometimes
transient, sometimes intensified upon long heating or fusion.
Precautions. — A black background should be used. The
forceps or wire should be cleaned with HC1 until they have no
effect on the flame. The acids must give no colour except the
transient yellow of sodium, which is scarcely to be avoided. The
wire must never be dipped into the acid-bottle, but drops must
be set out for the purpose.
Flame-colorations are as follows : —
Crimson, approaching Purple. — Lithium. — Appears when the
assay is on the very margin of the flame.
Crimson, of Yellower Tinge. — Strontium.
Red to Yellow-Red. — Calcium. — Often similar to that of stron-
tium, other tests distinguishing the compounds of these metals.
Yellow. — Sodium — So prevalent that a strong persistent
flame can alone be regarded as satisfactory evidence of its
presence as an essential constituent of the assay.
Yellow-Green. — Barium or Molybdenum.
Bright Emerald Green. — Copper. — A blue inner flame appears
when hydrochloric acid has been used.
Bright Green. — Boron. — Appears when the assay is on the
very margin of the flame. Sulphuric acid must be used. Borax
is a good example.
Dull Green, inconspicuous. — Phosphorus. — Sulphuric acid
should be used and the flame carefully observed on the entrance
of the assay.
Blue. — Lead, Selenium (rare), or Copper Chloride. — The last
gives the green of the oxide beyond and round it.
Light Blue, smoky. — Arsenic.
Violet. — Potassium. — This flame is very easily masked by
sodium, and entails in most cases the use of the blue glass.
With cobalt nitrate : blue denotes Alumina, and dull pink
Magnesia.^
Observation of Fusibility. — The ease with which a substance
fuses must depend greatly on the strength of flame employed and
on the skill of the operator, as well as on the size of the fragment
employed. Hence it is necessary for each worker to be in the
habit of using splinters of similar size and shape, comparison
being then possible between the results gained by himself from
different substances. The product, after heating, must always be
examined with the lens, and any change of colour, transparency,
etc., also noted. For most purposes the following broad
observations and statements suffice : — (a) Fusible in the unaided
flame of the lamp in fairly large (or small) fragments ; (6) fusible
SECT. IV.] INDOOE WORK. 231
before the blowpipe (bB) with easy formation of a globule ; (c)
fusible bB with easy rounding of the edges; (d) fusible bB in
splinters only ; (e) fusible bB on the edges of thin splinters only ;
(/) infusible bB, even after prolonged heating. The specimens
are held in the flame in the platinum forceps or in a tiny loop of
platinum wire, through which a wedge-shaped splinter may be
slung. The fusion-place is used. To facilitate comparison with
typical minerals, von Kobell proposed the well-known Scale of
Fusibility. The six degrees are formed by : —
1. Antimonite (the most easily fusible member of the scale).
2. Natrolite.
3. Almandine (common), garnet.
4. Actinolite.
5. Orthoclase.
6. Bronzite.
A good blowpipe flarne should fuse the tips of thin splinters of
bronzite into tiny globules. Degrees 1, 2, and 3 correspond
respectively to the verbal descriptions (a), (b), and (c) given
above ; 4 and 5 to (d) ; and 6 to (e).
It must be remembered that the substances styled by the
mineralogist infusible are mostly fusible with ease in the flame of
the oxyhydrogen blowpipe.15
First Operation (Closed Tube). — The assay is placed in a
small tube of glass sealed at one end. This closed tube must be
clean and dry • the assay being placed in it is heated by means of
a spirit-lamp or the flame of a "Bunsen's burner," so that no
coating of smoke may be deposited on the outside. The assay
should be heated gradually, the better to see the changes pro-
duced. At the close of the operation the flame may be urged by
the blowpipe, if little or no change has been observed. The
changes to be looked for are : —
(a) Changes of colour. — Many mineral substances change
colour when heated in the matrass, such as the arseniates and
phosphates of copper, but these usually give off moisture as well.
Carbonate of iron or chalybite turns black when heated, and
becomes magnetic. In this manner it may be readily distinguished
from dolomite, which it sometimes much resembles.
(6) Decrepitation. — The assay flies to pieces with a crackling
noise. This is often observed when wolfram, blende, and other
mineral substances are strongly heated in the closed tube.
(c) Deposition of moisture on the cool part of the tube. — This is
a ready mode of distinguishing between hydrous and anyhydrous
minerals : thus, red haematite gives off no moisture when so
treated ; brown haematite always gives off water, which condenses
in drops.
232 GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI.
(d) Formation of a sublimate or solid deposit on the cool part of
the tube. — This is observable in the case of sulphides, arsenides,
and some other substances. The sublimate of arsenic is white or
black, of sulphur yellow, of sulphur and arsenic together, yellow,
red, or black.
(e) Evolution of a vapour or peculiar odour. — When the odour
is sulphureous or alliaceous, sulphur or arsenic are indicated.
(/) Fusion of the substance. — This will not happen with sub-
stances of a higher degree of fusibility than 2 in the scale of
fusibility.21
Second Operation (Open Tube). — A new assay piece is placed
in a tube about 6 inches long, which is open at both ends, the
tube being held in an inclined position over the spirit-lamp flame.
As before, the flame may be urged by the blowpipe towards the
close of the experiment. The effects to be noted are the same as
in the first operation, but the sublimates will sometimes be
different and the odours more distinct.21
Third Operation (Reactions on Charcoal).— A little of the
coarsely powdered mineral is placed upon the surface of a piece
of charcoal, in a small cavity scooped out for the purpose with a
penknife. The oxidising flame is then directed upon it, and the
effects noted.
(a) Degree of fusibility. — This should be compared with that of
fragments of a similar size from the scale of fusibility.
(b) Evolution of vapour or odour. — This will usually be like
that observed in the second operation.
(c) Deposition of a coating or incrustation on the cool part of the
charcoal. — This will usually be much like that observed in the
second operation, but often much more distinct.
Those most likely to be observed are : —
White, near the assay, garlic odour, indicating arsenic.
White, further from the assay, little or no odour, antimony.
White, yellow while hot, malleable bead of metal in fifth
operation, tin.
White, yellow while hot, no bead in fifth operation, zinc.
Yellow or orange, malleable bead in fifth operation, lead.
Yellow, red, or brown, brittle bead in fifth operation, bismuth.
Dark red, white malleable bead in fifth operation, silver.
(d) Reduction to a bead of metal. — This may occasionally
happen with certain ores of gold or silver.
(e) A non-volatile residue remains. — This may be tested by the
fourth or fifth operation.21
(f) Tinging of the tip of the flame (see Observation of Flame-
coloration, above).
Fourth Operation (Cobalt). — If the residue from the third
SECT. IV.]
INDOOR WORK.
233
operation (e) is white, moisten it with a simple drop of solution of
nitrate of cobalt, and heat it again strongly. Should it turn
green, titanic oxide is probably present; blue, alumina is
indicated ; red or pink, magnesia is present.
If in this second strong heating a bright and intense glow is
observed, it will probably indicate either strontium, lime,
magnesia, or zinc.21
Fifth Operation (with Soda). — If the residue from the third
operation (e) be any other colour than white, add a little
carbonate of soda, and heat strongly with the reducing flame.
The result to be looked for is the production of a bead of metal :
in obstinate cases a little borax or cyanide of potassium may be
added. If the portions of reduced metal be very small, they may
escape observation ; in this case the portion of charcoal around
the assay should be cut out, ground up with water in a little mortar,
and the light carbon and soluble soda washed away. Any shining
particles of metal may then be readily detected.
The metals discovered may be recognised b}r their properties :
thus, gold will be yellow and malleable ; silver and tin, white and
malleable • copper, red and malleable ; lead, grey and malleable ;
bismuth, grey and somewhat brittle ; antimony, grey and very
brittle.21
Sixth Operation (Borax Bead, B.B.). — Make a small loop in the
end of a platinum wire, heat it in the flame of the spirit-lamp,
dip it into powdered borax, hold it again in the clear flame until
the borax has melted into a clear, glassy bead, add to it a very
little of the fine powder of the substance to be tested, heat it
again, put in the oxidising, then the reducing, flame while counting
fifty in each case. If no distinct colour is produced, take a little
more of the assay on the same bead of borax and heat again. Do
this several times if necessary. Should a distinct colour be
produced, it will probably be one of those given in attached
table.21
TABLE VIII. — COLOURS OF BEADS.
Colour in O.F.
Colour in R.F.
Indication.
Brown (violet when
hot)
Yellow (red when
hot)
Greyish on long reduc-
tion, colourless with
tin
Bottle-green
Nickel.
Iron or uranium.
234
GEOLOGY FOR ENGINEERS. [PT. IV. CH. XI,
TABLE VIII. — Continued.
Colour in O.F.
Colour in R.F.
Indication.
Yellow-green
Green
Chromium.
Blue (green hot, and
Brick-red and opaque,
Copper.
if a large quantity
well seen in yellow
is used, when cold)
light of lamp. Facili-
tated by tin or when
a large quantity is
present
Blue
Blue
Cobalt.
Red-violet
Colourless (difficult
Manganese.
with large quantity)
Colourless (yellow,
Yellow to brown
Titanium.
hot)
White and opaque,
Colourless after some
Silver.
turbid with small
time
quantity
BEADS OF MICROCOSMIC SALT.
Yellow
Yellow (colourless after
Nickel.
long reduction with
tin)
Pinkish red (requires
Pinkish red (requires
Iron.
some quantity)
some quantity)
Pinkish red (requires
Darker or crimson-red
Tungsten and
some quantity)
iron or titan-
ium and iron.
Yellow-green
Green
Uranium.
Yellow-green (red
Green (red when hot)
Chromium.
when hot)
Blue
Red and opaque
Copper.
Violet, sometimes
Violet, sometimes blue
Cobalt.
blue
Red-violet
Colourless (easier than
Manganese.
in borax)
Colourless
Violet
Titanium (see
above for Titan-
ium and Iron).
Milky white and
Colourless (after some
Silver.15
turbid
time)
SECT. IV.] INDOOR WORK. 235
Seventh Operation (Microcosmic Salt). — Repeat the sixth
experiment, using microcosmic salt instead of borax. The results
will be generally the same, but the colours will be sometimes
more delicate (see table). By means of this experiment, too,
silica may be readily detected, as it will not dissolve in a bead of
microcosmic salt, but will remain in the bead unchanged as to
form.'21
While a larger quantity of the mineral powder is often required
before a good result is obtained, the reactions are as a whole
cleaner and clearer than those in borax. The salt must be picked
up on the heated wire in small quantities at a time, and fused so
as to expel the water and ammonia after each addition. The
resulting bead drops easily from the wire, but any tendency to
fall during an operation may be generally checked by shifting the
wire to the upper portion of the flame.
Precautions. — The wire must be clean and give no colour.
The bead must be small, so as to be completely enveloped
during reduction. The powdered assay must be added in small
quantity, and increased until it is clear that no good reaction is
obtainable. In the seventh operation larger quantities of the
assay may possibly be required than in the experiments with
borax.15
Eighth Operation. — Hold a new assay piece by means of a pair
of platinum-pointed forceps, or a piece of platinum wire tightly
twisted round it, in the top of the oxidising flame. Observe any
change of tint, as mentioned in the third experiment. This
experiment, too, will afford a convenient opportunity of determin-
ing the degree of fusibility of the specimen, as it may be com-
pared with similar fragments from the scale of fusibility.
The eighth operation is of the greatest use in the absence of
such substances as give sublimates, incrustations, or coloured
beads in the first seven experiments. When, however, such
results have been already observed, the eighth experiment should
be omitted, as the platinum is liable to be injured.21
Should no colour be seen at first, the splinter or its powder or
a moistened wire should be dipped in a drop of HC1 specially
placed out for this purpose, and again be introduced into the
flame. The volatile and decomposable character of the chlorides
thus formed often reveals the presence of a metal (e.g. barium)
that might otherwise remain undetected throughout the analysis.
Compounds of phosphorus arid borax are best treated with
sulphuric acid.
Often the assay must be held just in the edge of the flame and
not brought too far within it. The coloration is sometimes
transient, sometimes intensified upon long heating or fusion. A
236 GEOLOGY FOR ENGINEERS. [rT. IV. CH. XL
black background, such as a charcoal block or a book-cover, should
be used 15
Test for Sulphur. — Because a substance is a sulphate or even
a sulphide, it by no means follows that evidence of sulphur will
be given either in the closed or open tube. The decisive deter-
mination is made as follows : — Fuse thoroughly some of the
powdered mineral with about three times its bulk of sodium
carbonate in K.F., until effervescence ceases. Cut out the slaggy
residue and the patch of charcoal below it, and crush on the
surface of a clean silver coin with a drop of water. Allow it to lie
for about ten seconds and wipe it off lightly. If sulphur has been
present in any form, sodium sulphide will have resulted, which
decomposes on the coin, leaving a brown or black stain of silver
sulphide. This test is delicate and unfailing, and can be per-
formed as a natural sequel to any good reduction with sodium
carbonate, a portion of the slaggy mass being reserved for this
purpose.
The reduction must be thorough, and the charcoal below must
be cut out, owing to its absorption of sodium sulphide.15
PART V.
PRACTICAL GEOLOGY.
IF the reader has made himself acquainted with the facts of
geology, or, in other words, if he understands the nature of the
materials of which the earth's crust is made up, the order of
arrangement of those materials, and the changes undergone both
in the rocks themselves and in the position they occupy, he will
not be inclined to question either the value of such knowledge to
practical men, or the nature of the applications of geology to
practical purposes. Such knowledge must always be available
when anything is undertaken concerning the earth, either as the
basis of operations, or the source whence all valuable materials
are obtained.13
237
PT. V. CH. XII.
CHAPTER XII.
WATER-SUPPLY.
THE several sources of supply known to hydraulic engineering
science are to be regarded merely as stages of the various courses
pursued by water in its passage from the rain-clouds to the ocean.
Whether precipitated through the atmosphere as rain, or flowing
over the earth's surface as stream or river, or percolating the soil
and rocks beneath, the motion of water is to be explained
according to the same uniform physical laws.22
Section I. — "Rainfall and Evaporation.
RAINFALL.
Whether supplies of water are to be drawn from catchment
areas, rivers, springs, or wells, the estimation of the rainfall upon
the area from which the water it is desired to intercept and take is
derived, forms the basis of investigation into the capabilities of
those sources.22
Bain (see Chapter I., Section I., p. 7) as it leaves the clouds is
doubtless pure water, but in its passage through the air it absorbs
certain gases, and carries with it, mechanically, particles of
matter which are floating about in the air. This is the case
more especially with rain that falls after a long drought, that
which falls after a continuance of rain being comparatively free
from them. The substances thus absorbed by the rain in its
passage to the earth are the gases oxygen, nitrogen, carbonic
acid, a little ammonia, and nitric acid, this latter more especially
during a thunderstorm, it being formed by the action of the
electric spark on the ammonia and vapour of water contained in
the air.
The particles floating in the air, which are carried down by
the rain, are for the most part organic. The above would be the
principal, if not the only, impurities found in rain water if it
were collected before it reached the earth in the open country.
238
SECT. I.] WATER-SUPPLY. 239
In or near large manufacturing towns the case is different;
several other substances would then be found in it, as sulphurous
acid, etc., varying with the kind of manufactures carried on near
the spot. Again, if rain be collected after it has fallen on the
roofs of houses, it will be further contaminated with substances
with which it has come in contact, more especially where lead-
pipes or gutters are used.23
The quantity of rain is mainly ruled by the physical con-
figuration of the district, but also, to a certain extent, by the
elevation of the locality, it being found that in many cases the
increase amounts to about 3 per cent, of the total fall at the
sea-level for every 100 feet above it. Much appears to depend
upon the elevation of the country with regard to the region of
the rain-clouds, which may be said to extend to about 3000 or
4000 feet above the sea-level.23
Where the prevailing winds are warm, and heavily charged
with moisture, by crossing a large extent of ocean, the rainfall of
the first high ground encountered by them will be heavy. The
moist air, rising to the altitude of the hills, expands in volume
and is reduced in temperature, in accordance with the adiabatic
law for the expansion of gases and vapours. The cooled air
cannot hold in suspension so large a quantity of vapour as before,
and the latter is deposited in the form of mist, rain, hail, or snow.
The rainfall of a district is likely to be small if the prevailing
winds traverse a wide expanse of land before reaching it, or if
they come from a place of low temperature to a warmer district
of no greater elevation. Under such circumstances the air is
generally in a suitable state for absorbing additional moisture.22
A larger quantity of rain falls on coast-lines on the western
side of great continents in the temperate zones than on the
eastern side or the interior, but in the tropics more on the
eastern side ; more rain falls in tropical than in temperate
climates, though the number of days on which rain falls is
greater in the latter than in the former case. The aspects of the
slopes of the basin, in respect to the direction of the prevailing
winds, affect the rainfall, more rain falling at equal heights on
the windward margin of the basin than on the opposite one.
There are many curious facts connected with the subject of
rainfall and its variation. In districts once thickly wooded, and
now comparatively bare (as, for instance, in colonial settlements),
it is found that the rainfall has considerably diminished from
what it was formerly. Indeed, it would seem to be universal that,
other circumstances being the same, the rainfall is considerably
greater in rugged or thickly wooded districts than in open and
barren plains. In the latter, however, it has been observed that
240 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
the construction of railways influences the rainful to a very
great extent. Instead of continuous drought all along the
Pacific railroad, rain now falls in refreshing abundance.23
Estimation of Mean Annual Fall. — In designing gravitation
schemes, and estimating the compensation to be given to mill-
owners, the mean annual rainfall over the gathering- ground must
be first ascertained. Observations on the ground proposed to be
made available are therefore of the highest importance, and if
none exist, gauges should be placed at the earliest possible date,
and observed with unfailing regularity. But these observations
are of practical use only when a proximate, long-established gauge
exists, and is also regularly noted ; then the determination of the
true fall on the district is a comparatively easy matter. The
proportion of the fall at the newly established gauges to that of
the long-established gauges should be carefully ascertained. If,
then, the recorded fall at the old-established stations be multiplied
by this proportion, a fairly reliable result will be obtained.23
Maximum and Minimum Fall. — The mean fall at any place
being known, an approximate idea of other rainfall elements may
be formed from the following rough rules : — It must be understood
that they are only approximations, and that observed facts are
infinitely preferable where they can be obtained. Where they
cannot be obtained, the departure of extreme years from the
mean may be estimated at 33 per cent, in excess for the wettest
year, and the same amount for defect in the driest. The three
driest consecutive years have ordinarily about 80 or 85 per cent,
of the mean annual fall ; and this value, or its equivalent — five-
sixths of the mean — is generally taken as the basis of calculations
in questions of water-supply. It would appear, from the records
of rainfall at the Greenwich Observatory, that the term of three
consecutive dry years occurs at intervals of about twenty-two
years.
The distribution of the fall over the various months is very
different in mountainous tracts from what it is in flatter and
drier ones. In the former it is greater in the winter months, in
the latter in the summer : in the former January is the wettest
month ; in the latter, July or sometimes October.23
The greatest fall in twenty-four hours is an element of much
importance, and is generally conformable to the following rule : —
With a mean fall of 20 inches it is 16 per cent, of the mean
annual fall (i.e. 3-20 inches) ; for each increase of 4 inches in the
mean annual fall it decreases 1 per cent, until the latter reaches
60 inches ; beyond that point it remains stationary at 6 per
cent., however great the annual fall may be. For example,
Seathwaite, mean annual fall 140 inches; 1-40 x 6 = 8-40 = the
SECT. I.] WATER-SUPPLY. 241
computed maximum fall in twenty-four hours. The greatest fall
yet recorded at that station is 6-60, thus confirming the above
rule.23
This rule is not applicable to India, where the greatest fall in
twenty-four hours sometimes reaches 20 per cent, of the mean
annual rainfall.
EVAPORATION AND ABSORPTION.
Effect on Water-Supply. — Intimately connected with the
subject of rainfall — indeed forming a necessary part of it in its
practical bearing — is that of evaporation. There is the evaporation,
of the rain immediately upon its falling to the ground, and while
being temporarily retained by the latter ; and there is the evapora-
tion from the surface of large bodies of water, such as lakes and
reservoirs. Again, a vast amount of the water which falls in the
shape of rain is absorbed by vegetation : partly to be retained in
the body and fibres of the tree or plant, and partly to be evaporated
from its leaves. In either case, however, it is lost as far as the
purposes of water-supply are concerned. The evaporation from
the ground surface will depend on the temperature, the physical
configuration, and the geological formation of the district, the state
of the drainage, the nature of the surface of the ground, arid the
rate at which the rain falls. The absorption of vegetation will, of
course, depend on the amount and nature of the vegetation. When
in the warmer seasons of the temperate zones the showers come
very lightly on a loose, absorbent soil, they are registered in the
rain-gauge ; but the rain neither sinks into the ground sufficiently
to appear again in the form of springs, nor does it flow into the
rivers and streams, and become available for impounding, but is
evaporated in many instances almost as fast as it falls. On the
other hand, with a steep descent and on an impermeable surface,
the rainfall is less likely to be lost. The extreme cases are — for
the maximum evaporation, a flat spongy district, with a retentive
substratum, as in boggy parts ; and for the minimum, a steep,
bare, and impermeable surface, such as the slated roof of a house,
from which there is scarcely any evaporation.23 The maximum
evaporation is also obtained in sandy plains.1
Loss. — It has been popular in this branch of engineering to
suppose that, on the average, one-third of the rainfall is lost by
evaporation and absorption by vegetation; one-third is drained
into rivers and streams ; and one-third percolates into the ground
to appear again in the form of springs. It would seem, however,
that in a given district the loss by evaporation and absorption is
rather a constant quantity than one directly proportionate to the
16
242 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
rainfall ; for, as the rainfall increases in any season, the propor-
tion of it which is lost will decrease, and vice versa. Indeed, it
is nearer the truth to consider that the proportion of the rainfall
lost by evaporation will vary inversely as some higher power of
the rainfall. The cause is obviously to be found in the diminished
humidity of the atmosphere, and in many cases the increased
temperature in the seasons of less rain ; and it is for the same
reason that the absolute evaporation is greater in those districts
where the mean rainfall is less. Again, the loss will depend
greatly upon the distribution of the rain in the different seasons
of each year ; for, as the proportion of rain falling in the summer
months becomes greater, the loss from evaporation will also be
greater. It has been shown that in the districts where there is
less rain annually the tendency is for the bulk of the rain to fall
in the summer months ; there is thus an additional cause for the
loss to be greater in the districts of less rain.23
The foregoing considerations have reference more especially to
the mean annual evaporation, but it is necessary further to
regard the proportion borne to the annual evaporation by that
which occurs during the drier seasons, when the amount of
available rainfall is of most consequence. The same principles,
however, will be found to obtain, namely, that in the dry season
the proportion lost by evaporation will be enormously increased,
sometimes amounting to 70 or 80 per cent, of the rainfall, even
when taken over a period of five or six months. The available
rainfall of the dry season is measured by the "dry weather flow,"
to which reference will be made hereafter (see below). In
England the loss by evaporation and absorption is found to range
from about 9 to 19 inches per annum, and the average seems to
be about 13 or 14 inches. But, from what has already been said,
it will be seen that in matters affecting the water-supply of towns,
the mean annual loss is, like the mean annual fall, to be used
merely for estimating the quantity that will be actually available
in dry years. It is evident that to speak definitely with con-
fidence on a subject the conditions and circumstances of which
are liable to so much variation, would be folly. It is only from
direct and careful observation, either in the district under con-
sideration or in analogous positions, that any reliable estimates
can be formed.23
Generally, upon permeable soils or upon steep and impervious
land, the loss by evaporation is small. If, however, in permeable
soils the surface of saturation is, owing to the physical features
of the locality, situated near to the surface of the ground, evapora-
tion takes place actively under favourable atmospheric conditions.22
Evaporation from Surfaces of Water. — The discrepancies
SECT. II.] WATER-SUPPLY. 243
between the records of careful observers have been almost
incredible — probably owing to the small scale on which experi-
ments were made.23 In England the loss from evaporation has
been estimated as equivalent to a depth of about 3 feet from the
surface of reservoirs; whereas from reservoirs in India it has
been reckoned as a depth of 4 to 6 feet over the whole area in a
year (cf. Molesworth, 23rd ed., p. 319). In the United States
it has been calculated that the evaporation from surfaces of water
ranges from a minimum of about 18 inches in a year on the North
Pacific coast, up to a maximum of about 100 inches on the
Southern Plateau. The annual evaporation at Melbourne from a
water surface has been found to amount to 40| inches ; whilst in
South Africa it is 39 inches at Port Elizabeth on the sea-coast,
and at Van Wyk's Vley reservoir, in the interior, it reaches 80
inches.24
Dry Weather Flow. — Of more importance, however, than the
mean annual evaporation is the evaporation during the dry
season of the year, when reservoirs are being taxed to their
utmost capacity, and when, therefore, the elements of loss have to
be more closely watched. Mr Burnell, in his Rudiments of
Hydraulic Engineering, says that "the experience derived from
the use of reservoirs on canals appears to indicate that, during the
summer months, it is necessary to allow for an evaporation rang-
ing between one-sixth and one-eighth of an inch per day." In an
important matter like this, it is perhaps advisable, in order to be
on the safe side, to allow for daily loss during the dry season of
not less than one-fifth of an inch.23
Section II. — Underground and Surface Waters.
UNDERGROUND WATER.
Water- Slope. — After falling upon the surface of land in the
form of rain, water, subject to some losses that will be alluded to
hereafter, still continues, under the action of gravity, to seek
lower levels, pursuing those routes in which it experiences the
least resistance to its downward motion. Generally speaking,
this direction is vertical through soil and subsoil, until its pro-
gress is checked by encountering the great body of water that
saturates the subterranean regions at depths depending upon local
circumstances. Here the motion is not arrested, but its direction
becomes inclined at a certain angle, which is determined by the
resistance opposed to the flow by the strata at the place in
question. The inclination of this subterranean water-slope
changes from point to point according to the geological forma-
244 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
tions traversed. In permeable rocks, such as chalk * and gravels,
the slope is naturally flat ; in sandstone it is less so ; whilst in
compact grits the angle of inclination is large in just such degree
as the impervious character of the rocks requires greater hydro-
static force to overcome their resistance to the passage of the
water. The absolute level at any point of the slope also varies
according to the volume of water which, contributed by the rain-
fall, seeks a passage to the ocean.
Thus there extends in all directions a surface of saturation,
or, as it is sometimes termed, plane of saturation, occurring
always where the descending waters assume a definite surface-
slope, in accordance with ordinary hydrodynamical laws.22
Saturation and Imbibition. — The conditions under which water
exists in rocks may be illustrated as follows : — Let us suppose a
large dry block of chalk to be half immersed in a vessel of water.
The part in the water will slowly get as full of water as it can
hold, that is to say, it will become "saturated," every crevice and
hollow, large and small, will be filled with water. When this
state is arrived at, every cubic foot of chalk which is below the
water-level will contain 18 pints of water; i.e. it will contain
35 per cent, of its own bulk of water of saturation. The
upper portion will also be found to have become damp, and an
examination of its condition will show that it contains 10 pints of
water per cubic foot; i.e. 19 per cent, of its bulk. This water has
been soaked up, or imbibed, and is called ivater of imbibition
or "quarry water," inasmuch as it represents the ordinary natural
moisture of the stone when it is first taken from the quarry.
This imbibition is due to capillary attraction, and if the moisture
were removed from the top by evaporation, fresh supplies would
rise by capillary attraction from the saturated portion below. As
in this particular case the water of imbibition is little more than
half the water of saturation in an equal volume of chalk, it follows
that in the upper non-saturated portion there must be spaces
which have no water in them. These are the larger hollows
which are too wide for capillary attraction to fill, and which
therefore remain full of air ; whereas in the saturated part there
is no air at all. The boundary between the saturated and the
non-saturated portions is a plane, which is continuous with the
level of the water in which the chalk is immersed. If, now, a
hole were to be bored with an auger straight down from the top
of the chalk nearly to the bottom, we should find that it would
fill with water up to the level at which the chalk is saturated,
and no higher. Suppose now that some water is gently poured
over the top of the chalk for some time ; some will run off and
* The permeability of chalk is chiefly due to the fissures that traverse it.
SECT. II.] WATER-SUPPLY. 245
some will soak in. The part into which it soaks will be temporarily
saturated ; but the water will gradually sink by gravitation till it
reaches the saturated portion, where its downward course will be
stopped, as there will be no room for it. Near the outside of the
block it will easily make its way into the surrounding water ; but
the water in the centre will not be able to get away so readily, and
for some time the surface of saturation, instead of being flat, will
be curved high in the middle and sloping downward on each side.
The water in the small well will now stand higher than before ;
but gradually, as the water presses downwards and outwards, the
level will sink to its original position, the plane of saturation
gradually becoming less and less curved until it becomes flat as
at first. A fresh watering of the block will raise it again, but it
always tends to the level which is determined for it by the
surrounding water, rising higher when well watered, and sinking
towards its limit when left to itself. This experiment illustrates
the condition of the dry land under the combined influence of the
sea, rain, and evaporation. The saturation line never sinks much
below the level of the sea ; below that line the rocks are always
saturated. The height to which they are saturated above that
line depends on their porosity, the rainfall, and distance from the
sea. At the coast the saturation line coincides with the sea-level,
from which it rises gently or abruptly according to the nature
of the rocks, combined with the rainfall. After a long dry period
the water of imbibition in the upper part of the strata passes off
into the air in the form of vapour or is abstracted by vegetation ;
but more is drawn up by capillary attraction from the lower
portions, so that however dry the actual surface may be, it is
never dry very far down.5
Capacity of Rocks for Water.— The amount of water, either of
saturation or imbibition, which any rock will contain depends on
its composition and texture. The looser the texture, and the
more numerous and the larger the cracks, the greater the
quantity of water the rock will contain when saturated ; but the
rock which takes a great deal of water to saturate will not
necessarily contain a large quantity of water above the line of
saturation, because the cavities between its particles may be too
large for capillary attraction to act. Thus a coarse loose sand
will contain a large quantity of water of saturation, but it never-
theless makes a very dry soil, because, in the first place, the water
can very readily make its way downwards through it, and because,
secondly, capillary attraction is weak because of the large size of
the spaces between the grains.5
Rocks vary greatly in the quantity of water they retain, in the
way in which they retain it, in the relative facility with which
246 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
they absorb or part with it, and in the degree of accidental inter-
ruption that can interfere with the free course of the water
beneath the surface. Thus sands, if loose, allow water to perco-
late freely through them ; if hardened, they conduct water very
badly or not at all ; if broken, they offer natural channels, per-
mitting a very perfect but partial transmission. So limestones,
under certain circumstances, are good conductors, and under
other circumstances, very bad conductors of water : and this is
governed by the nature of the rock, its condition, its position,
and generally by those facts observed and described by the
geologist. Even clays, although generally tough and quite
impermeable, retaining water to any extent, are sometimes broken
by permeable joints, and sometimes mixed with so much sand and
lime as not to be absolutely close.13
Sands and gravels may be considered the most open of the
different kinds of rocks, but both require careful examination if
we would discover their true condition. Thus, many sand
rocks, although themselves loose and containing much water with
which they would readily part, have undergone a partial consolida-
tion, or are traversed by a multitude of crevices, and sometimes
by systems of faults parallel to each other filled up with clay,
quartz, or oxide of iron, and crossed by others at right angles to
them. The whole mass of rock is thus divided into compartments
or cells which have little communication with each other, and if
one such compartment is drained by pumping, others at a distance
are not necessarily affected. When part of a rock of this kind is
covered with gravel, little difference might be anticipated ; but if
this surface-gravel covers up and conceals boulder-clay of a stiff
and tenacious character — and this is by no means uncommon in
various parts of England — the compartments above alluded to will
be very differently supplied with water in various parts of the
same district.
Loose sand rocks, alternating with bands of marl and not inter-
sected by impermeable bands such as form the great mass of the
New Red Sandstone series in the middle and south of England,
usually allow water to percolate freely to their base, the marl beds
forming mere local interruptions, and retaining the water at the
surface only so long as it is running towards some natural vent.
Harder sands and sandstones, such as the Millstone grit, form an
almost impassable barrier for water, and conduct it to some other
more permeable rock.
Clays when of considerable thickness and extent do not allow
water to pass downwards into the earth, and often by their level
and easily smoothed surface retain large pools and sheets of water
to the great injury of the soil. When there is a natural fall to
SECT. II.] WATER-SUPPLY. 247
the sea, however small, there is always a possibility of greatly
improving the condition of such land by drainage, while springs
of water are neither required, nor if required would they be easily
found without sinking. It may happen — and the geological
structure of the district would show whether this is likely or not
— that the clay covers up permeable and very wet beds which,
if borings were made, would rise to the surface in artesian wells.
On the other hand, it may happen that by opening a way into
the lower beds the surface-waters would be drained off.
Calcareous or lime rocks differ a good deal in their containing
power with reference to water, and much doubt has long existed
as to the true state of such rocks in particular cases. They may
be divided into two groups — the one partaking more or less of a
spongy nature, and the other hard and semi-crystalline. The
Oolites offer a kind of intermediate condition. The first of these
groups is illustrated by chalk, of which the soft upper beds are
exceedingly porous and absorbent of water. The lower beds of
chalk, though not as soft as the upper, are usually, when
penetrated by sinkings, found to be exceedingly wet, and a large
quantity of water is yielded freely, though the replacement seems
to take place but slowly. In addition to the ordinary sources of
water in the mass of the rock, there is no doubt of the existence
of numerous fissures and crevices, and frequently large cavities,
in chalk and all other lime rocks, and these are often filled with
water at considerable pressure.13
Water-bearing Strata. — Permeable strata are found at very
different and variable depths ; for in some places the surface layer
consists of recent deposits, and at other parts, owing to denuda-
tion or geological disturbances, older strata, and even primitive
rocks, appear at the surface. Igneous rocks and fissured un-
stratified strata do not afford facilities for the storage of water,
but in the Magnesian Limestone and Lower Red Sandstone, con-
stituting the upper portion of the primary series of rocks, large
quantities of water are often found. In the Secondary and Tertiary
formations the permeable strata are interspersed with impermeable
strata which occasion the retention of the water percolating
through the outcrop into the permeable strata overlying them.
Drift, consisting of the debris of rocks carried down and
deposited by flowing water in valleys and depressions in the
ground, and sometimes on the lower slopes of hills, having been
washed down by rain from the higher ground, is very irregular in
thickness, and often discontinuous. The porosity of the drift
depends on the nature of the materials of which it is composed,
which are usually gravel and sand, but sometimes consist of less
permeable materials brought down from the adjacent hills.
248 GEOLOGY FOR ENGINEERS. [FT. V. CH. XII.
Alluvial deposits are very similar in their origin to drift, but
they are more regular and extensive ; they are usually composed
of materials brought from a greater distance, often filling up
ancient lakes and river-beds, and they consist mainly of sand,
gravel, and stones, together with clays and marls. Sometimes
these permeable strata form the surface layer, and receive
their supply of water by the direct percolation of the rainfall ;
but they are often partially overlaid by an impervious stratum,
under which the ground-water flows for considerable distances.
Sand furnishes the most porous stratum, being capable of absorb-
ing from one-third to nearly one-half its volume of water ; whilst
gravel and sand can contain from one-quarter to three-tenths their
volume of water. Numerous wells have been sunk into these
upper permeable stata for supplying water to large towns in the
United States.
The Chalk is the principal water-bearing stratum for a consider-
able part of the southern portion of England, with its good
thickness and large outcrop, absorbing almost 30 per cent, of its
volume of water on the average, whilst the Greensands furnish
large volumes of water, more uniformly distributed throughout
them than in the Chalk ; and both these formations yield good
supplies to wells sunk into them.
The New Red Sandstone or Trias, though less extensive in area
in England than the two above-mentioned strata, traverses the
more rainy western districts, stretching from the Channel on the
south coast of Devonshire to the Solway Firth, and therefore may
be regarded as quite as suitable for wells. Moreover, although
wells have to be sunk to a considerable depth in the New Red
Sandstone to reach water, the volume is abundant when found,
and is less hard than water from the Chalk. This stratum,
known as Trias abroad, extends over considerable areas in Europe,
and also for long distances in North America.
Other sandstones yield large quantities of water proportionate
to their extent, outcrop, available rainfall on them, and porosity,
which ranges from at least 28 to 7 per cent, in volume in the
sandstones of the United States, according to their compactness,
the porous Potsdam and St Peter sandstones having been largely
resorted to for deriving water-supplies from wells.
Limestones. — Water is also drawn from wells sunk in the
Oolitic, Lias, and Magnesian Limestones, both- in England and
North America, but not with the same certainty and facility as
from sandstones, since limestones only yield water when exten-
sively fissured, and the underground flow is liable to be obstructed
by faults.
Dip, outcrop, and slope. — The absorption of rainfall by stratified,
SECT. II.] WATER-SUPPLY. 249
water-bearing strata at their outcrop is largely affected by their
dip, their freedom from a surface covering of an impermeable
nature, and the flatness or depression of the ground. A consider-
able dip facilitates the descent of the water into the stratum
along the interstices between the successive layers, but if con-
tinued for some distance causes the stratum to descend to too
great a depth below the surface. The inflow of the rain is
dependent on the permeable outcrop being free from obstruction
at the surface by an impermeable layer of overlying drift, and
the rain is adequately retained for percolating into the porous
stratum when falling on fairly flat ground, and still more on a
valley or depression, whereas it would be liable to flow away
down a steep slope, and be to a great extent lost to the permeable
stratum.24
Yield of Water. — The quantity of water which any particular
rock may yield does not depend simply on the quantity it can
contain when saturated. The water capable of being drawn off is
that which it contains over and above what it is able to hold as
water of imbibition. For instance, in the case of the before-
mentioned chalk (cf. p. 244), which contains 18 pints per cubic foot
when saturated and 10 pints of water of imbibition, the amount
of water which it would yield per cubic foot would be 8 pints, the
difference between 18 and 10.
For practical purposes of water-supply another important factor
is the readiness with which any given rock will give off this
surplus water. There may be plenty, but the rock may part
with it very slowly. Loose sand or a well-jointed and cracked
sandstone will part with its water with the greatest ease ; but if
any clayey material is present the case is very different, partly on
account of the strong affinity clay possesses for moisture, and
partly through the crevices getting choked with it. Chalk, again,
has a very large capacity for water, but a solid lump of chalk,
when once saturated, is not at all ready to surrender the water
again, as may be seen by trying to drain the water from it. In a
well sunk in the chalk the water issues chiefly, not from the
chalk itself, but from the cracks and joints. For this reason it
is usual, where a large supply is needed, to drive headings
through the chalk in various directions from the bottom of the
well in order to tap as many of these fissures as possible.5
Porosity of Rocks. — Tables showing the absorbent power of
various rocks as deduced from laboratory experiments are given
in part iii. of Rivington's Building Construction and other works,
from which it may be seen that the quantity of water absorbed
by the different strata is very variable. It is small in compact
sandstones and limestones, large in soft sandstones and oolites
250 GEOLOGY FOR ENGINEERS. [FT. V. CH. XII.
and largest in pure quartzose sands. But the full absorbent
power of a rock, which represents both the water of imbibition
and of saturation, does not represent its value as a water-bearing
stratum. Clay can absorb a large quantity of water ; but trans-
mits none. Chalk absorbs freely; but transmits slowly and in
small quantities. A sand of the Upper Greensand, although it
held when saturated 3 gallons per cubic foot, only transmitted, in
consequence of the presence of a small relative proportion of
argillaceous matter, 3J gallons per hour; whereas purer sand of
the Lower Greensand, although only holding when saturated 2 to
2| gallons, transmitted at the rate of 8 to 14 gallons per hour.
Laboratory experiments, moreover, are made on compact,
unfissured places of the several rocks, whereas in nature the
chalk, oolites, and sandstones are traversed by joints and fissures
which hold and transmit water freely. Even compact, imperme-
able limestones, for this reason, will form high waterless tracts,
with strong springs issuing in the valleys. The value of the
strata as water-bearing strata is in direct ratio of capacity of
saturation, and in inverse ratio of power of imbibition. Thus,
although solid chalk and loose sands may hold the same quantity
of water, the resistance to the free passage of water in the former
is to the latter in the proportion of about 600 : 1. In imperme-
able strata, such as quartzites, slates, granites, clays, etc., satura-
tion and imbibition are more or less nearly balanced.
If, with strong imbibition, the rocks are also compact, percola-
tion is very slow, as in the case of deep-seated and undisturbed
chalk ; but, if they are fissured, the cracks and fissures serve as
channels and conduits to facilitate the passage of the water. In
oolitic strata and soft sandstones fissures and joints prevail as a
rule.4
Bournes. — It sometimes happens that a permanent spring
issues at a certain point generally low down in a valley. At
intervals of two, three, or more years it suddenly bursts out 2
or 3 or more miles further up the valley, and continues to
flow for some time, when it again as suddenly ceases. This is due
to the " saturation line " (cf. p. 245) being temporarily raised owing
to exceptionally heavy rainfall, and the phenomena of a bourne
is thus caused.4
Quality of Water. — Absolutely pure water is not to be obtained
in nature ; and fortunately it is not essential nor even desirable
for the purposes of animal and vegetable life. In ordinary cases,
rain water contains ammonia, and in or near towns is always
tainted with various impurities, introduced into the atmosphere
when large numbers of human beings and animals are collected
together, and especially where household fires, and manufactories
SECT. II.] WATER-SUPPLY. 251
of various kinds, involve the combustion of very large quantities
of mineral fuel. Spring water contains numerous mineral sub-
stances, chiefly salts and gases, obtained from the rocks passed
through ; and as water is an almost universal solvent, the variety
of these is very great. In ordinary cases the salts of lime and
soda are chiefly abundant ; but salts of potash and magnesia are
also common. The salts include chlorides, carbonates, sulphates,
and phosphates. Iron, silica, and very small quantities of organic
matter are occasionally found.
River water contains, in addition to the various substances
obtained from springs, and from rocks over which the stream
passes, a quantity of organic matter, both of animal and vegetable
origin, which in the neighbourhood of large towns usually includes
much sewage matter.
It might be supposed, and has often been stated, that where
this deposit is constantly stirred up by the periodical passage of
the tidal wave the water cannot be in any other than an unwhole-
some state and unfit for general use. There are, however, causes
at work tending to purify the water by simple exposure. The
decomposing animal and vegetable matter is rapidly removed
from a mischievous condition partly by aeration and partly by
those myriads of animalcules which are often spoken of as being
themselves impurities, but which really collect the offensive
particles and reintroduce them into the realms of life. River
water is freed from its impurities, even of the worst kind, in a
wonderfully brief space of time, and with the aid of a little filtra-
tion it is admirably adapted for household use.
Spring ivater is generally the purest as far as regards admixture
with organic matter; but on the whole, and for most economic
purposes, the best water is that obtained from mountainous or
hilly districts, where there is abundant rainfall, and where the
rain is collected on a surface of hard rock containing little lime-
stone and no other soluble mineral.13
SURFACE WATERS.
The surface of saturation ordinarily coincides with the "free-
level " of the subterranean waters at every point in the district,
although in synclinal basins overlaid by extremely impervious
formations this is not necessaiily the case. In a district the
geological structure of which is of a compact and impervious
nature, the surface of saturation is often situated at no great
depth underground, and may at times, when the rainfall is heavy,
become raised until it coincides with the land surface, such a
condition resulting evidently from the permeability of the land
252 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
being barely adequate to meet the demands of such increased
quantity of water for a passage through it. The land is then
said to be " waterlogged." 22
Conditions of Flow. — There are in general two conditions of
which the immediate result is the establishment of flow upon the
surface.
Case I. — When the surface-slope of a considerable tract of
land is less than the hydraulic gradient required to force the
entire volume of water through the earth as rapidly as it falls
upon it, the surface of saturation of the district rises above the
surface of the land, until a hydraulic gradient is formed adapted
to the circumstances of the case, and part of the flow takes place
over the ground. The "hydraulic surface," as the free surface
thus formed may be conveniently designated, does not differ much
from the ground surface, because the water flowing above ground
is comparatively free from frictional resistance, and a slight fall is
enough to produce considerable velocity, and to effect discharge
off the surface as fast as the rain falls upon it.
In the special cases of rain falling upon frozen ground, or fall-
ing very heavily, the resistance of the surface to the passage of
water through it may be so high as to prevent any considerable
portion of it from penetrating the earth, and abnormal flow may
b>0 established upon the surface, although the true surface of
saturation is at the time situated at some depth beneath. Those
who have experience of severe tropical rains, or of floods caused
by the sudden melting of large accumulations of snow, must have
been astonished to observe the current and the depth of water
which may prevail temporarily over wide areas of land into which
ordinary rainfall disappears at once.
A similar effect is produced when permeable material has
accumulated in hollows on more or less impervious rock with a
sloping surface. Rainfall in the rocky surface is absorbed by the
permeable accumulation, and reappears at its lower edge on the
surface of the rock ; this a casual observer might take to be
the level of the surface of saturation, whereas the real saturation
level may be much lower.
When the surface of saturation is high, the smallest depression
in the land may be sufficient to cause it to issue therefrom, since
the water-slope in any direction is determined by the facilities
afforded to the passage of water in that direction. Any hollow
below the surface of saturation presents to the water in the
adjacent ground a course of diminished resistance which is
naturally taken advantage of. In proportion to reduced resist-
ance the surface of saturation becomes flattened, until, in the
hollow, it issues above the ground as a true hydraulic surface.
SECT. III.] WATER-SUPPLY. 253
Case II. —When, at any place, the surface-slope of the land is
of higher inclination than the hydraulic gradient required by
the flow of the percolating subsoil waters through the rocks,
the surface of saturation naturally issues above the ground in the
manner described in Case I. Illustrations of this action
frequently occur in the streaming vertical faces of sandstone
quarries, and in the marshy areas often found on steep hillsides.
Some of the water that enters every ditch is contributed in like
manner from the adjacent subsoil.
The rills on every hillside, no less than rivers and lakes, owe
their origin and maintenance to such causes ; and it is due
jointly to the high position of the surface of saturation and to
their undulating character that districts of hard and impervious
geological structure lend themselves so readily to yield " surface-
water " — that is to say, water which, after falling upon the earth,
is almost at once directed by its own gravitating impulse to flow
in channels on the surface of the land.
Forests have an important effect in acting as regulators which
retard the flow of the rain into the streams, thus tending to
prevent excessive rise of the latter after storms.2'2
Section III.— Springs and Wells. «•>
SPRINGS.
When water falls from the clouds in the form of rain or snow,
sinks into the ground and percolates until it reaches an imperme-
able stratum, appearing again at the surface at a lower level, the
outgush is called a spring. The general conditions under which
springs are met with in nature are necessarily most varied,
dependent as they are on the geological structure of the locality,
the alternation and inclination of pervious and impervious strata,
and their endless contortions, dislocations, and faults. Water-
bearing strata are such as are of an open, porous, or absorbent
nature, and overlie other strata of an impermeable quality, the
latter serving to retain the water in the former.23
Ordinary Springs. — Pervious or impervious. — The simplest case
under which springs are met with is where a pervious stratum
overlies an inclined impervious one, as in fig. 75, the rain falling
upon the surface of the former being delivered at S as a land or
shallow-seated spring.
If the impervious substratum be depressed into a hollow or
basin, the water will necessarily accumulate in the same, and the
lower part of the porous stratum will become permanently
saturated. Fig. 76 illustrates such a case, A B S being the line of
254
GEOLOGY FOR ENGINEERS.
[FT.
V. CH. XII.
saturation • and inasmuch as the water is sustained partly by
capillary attraction, it will be seen that this line need not
necessarily be horizontal.
It would at first sight appear strange that the water does not
rather ooze out as a sand-soak along the junction of the impervious
FIG. 75. — Spring at outcrop of permeable stratum.
with the pervious bed, than make its appearance at certain places
only on this line, and then in the form of continuous gushing
streams. This, however, is explained by the fact that on the
surface of the impermeable bed numerous irregularities exist
similar to those on the exposed surface of the land, and these
FIG. 76.— Hollow collecting water.
conduct the water in definite channels and courses. Rents and
fissures acting as subterranean drains assist in the concentration
of the flow of water at certain points.
Pervious between two impervious beds. — Springs are sometimes
found at the lower outcrop, C (fig. 77), of a permeable bed, A,
FIG. 77. — Spring arising from water falling on outcrop.
lying between two others, B, B, which are impermeable ; the
supply, however, is limited to the rainfall on the basset or
exposed surface of higher outcrop, D, and as much of the drainage
from the upper impermeable stratum, D, as flows down the sides
of the hill and is intercepted by the stratum A.23
SECT. III.] WATER-SUPPLY. 255
Intermittent Springs.— Where the overlying pervious stratum
is comparatively shallow and of small extent, the springs issuing
from it will generally be of an intermittent character, being
limited by the variations of the rainfall ; but, on the other hand,
where it is of considerable extent and depth, it acts as a natural
storage reservoir, and the rain falling at intervals on the upper
surface is delivered with a uniform flow. Friction and capillary
attraction, acting in opposition to gravity, are the chief agents in
bringing this about.
Syphon action. — There is a class of intermittent springs
phenomenon of which is attributed to an action similar to that of
the syphon. In Fig. 78 B is a permeable stratum lying on an
impermeable one C, and having a layer of an impermeable
material above it. The layer B may for a moment be conceived
as a tube. Rain falling on the basset E F will penetrate and
descend into the pervious stratum B, and will accumulate in the
FIG. 78. — Syphon action.
subterranean reservoir C until it attains to a level sufficient to
overflow at G, appearing in the form of a spring at S. If the
part S G C of the impervious stratum be regarded as a syphon
tube, it will be understood that the water which has accumulated
in the basin will be drawn over the ridge in the impermeable
bed until the water-level has been lowered to a point at which
the syphon will cease to act, and water will not again issue from
the spring until the reservoir has received a supply sufficient to
bring the syphon again into action. A well-known example of
such a case may be seen beside the road leading from Buxton to
Castleton.23
Line of Saturation. — Other conditions under which water
occurs are illustrated in figs. 79 and 80. In fig. 79 A is an
impermeable cap of clay, resting on a porous bed, B, which in its
turn rests on an impermeable stratum, C. The water which falls
on the surface of B, and perhaps some of that which falls on A,
will sink into the porous stratum, B, and accumulate nearly to the
level of a b, at which level it is drained by springs, breaking out
at c. In wells sunk at e and / the water will rise to the level of
the line a b ; also, in borings made at d, the water will probably
GEOLOGY FOR ENGINEERS.
[FT.
V. CH. XII.
rise through the bore-hole and overflow the surface, forming what
is called an overflowing artesian well. It is evident, if the mass
A covered the permeable strata to a higher level than c, namely,
to as high a level as the edges of the bed C, then the line of
saturation would correspond with that upper level — a distinction
which will be sufficiently understood by inspection of fig. 79,
without the aid of another diagram.
FIG. 79. — Water at outcrop of permeable between two
impermeable beds.
Fig. 80 represents the case of a basin drained by a river and
having an inclined line of saturation. Here A, B, and C represent
the same succession of strata as in fig. 79. At a is a river,
where the water lodged in B finds the means of escape ; and hence
the line of saturation and the height to which water will rise in
wells become the line a b, drawn from the outcrop of C to the mean
level of water in the river at a.
It is evident, if any part of the surface of B should lie below a 6,
FIG. 80. — Inclined line of saturation.
that we may expect to meet with springs breaking out on the
surface ; and so, if any part of the surface at A should lie below
ab, then we may expect to find overflowing artesian wells, as
in fig. 79.
It is probable that the line of saturation a b is not invariably
a straight line, but in dry seasons is depressed into a hollow curve
beneath the straight line, while in wet seasons it swells into a
convex curve above the straight line. If we conceive it to swell
SECT. III.]
WATER-SUPPLY.
257
in wet seasons to such an extent as to cut the surface D at any
point to the right of the mass A, we shall have for a time a spring
flowing at that point. This is one mode of accounting for
intermittent springs.
Fig. 81 shows an arrangement of strata which often prevails in
nature, the impervious mass C cropping out at very different
levels, a and b. Here the line of saturation also will be inclined
from b to a, and at this level the water will stand in wells sunk
between a and 5.25
Fault Springs. —Fig. 82 is a section across a valley, B, looking
up the same, in the neighbourhood of a fault. The hills A, C
are supposed to be formed of a permeable stratum a a a", resting
on an impermeable bed of clay b b' b". Between these two hills
is a valley of denudation, B, towards the head of which the
junction of the permeable stratum a a with the clay bed bb'
FIG. 81. —Inclined line of
saturation.
FIG. 82. — Origin of two kinds of springs.
produces a spring at the point S ; here the intersection of these
strata by the denudation of the valley affords a perennial issue to
the rain water which falls upon the adjacent upland plain, and, per-
colating downwards through the porous stratum a a, accumulates
therein until it is discharged by numerous springs in position
similar to S, near the head and along the sides of the valleys.
The hill C represents the case of a spring produced by a fault
H. The rain that falls upon this hill between H and D descends
through the porous stratum a" to the subjacent beds of clay b".
The inclination of this bed directs its course towards the fault
H, where its progress is intercepted by the dislocation edge of the
clay bed b', and a spring is formed at the point /. Springs
originating in causes of this kind are of very frequent occurrence,
and are easily recognised in cliffs upon the seashore.
Three such cases may be seen on the banks of the Severn, near
Bristol, in small faults that traverse the low cliff of red marl and
lias on the north-east of the Aust passage. In inland districts
the fractures which cause these springs are usually less apparent,
17
258
GEOLOGY FOR ENGINEERS. [FT. V. CH. XII.
and the issues of water often give to the geologist notice of faults
of which the form of the surface affords no visible indication.23
Figs. 83 and 84 show one of the most common modes of
occurrence where the fault X has caused a dislocation of the
strata and brought down the impermeable bed A in contact with
the porous stratum B. Fig. 83 shows the spring breaking out in
the valley at X, but the same effect sometimes takes place near
the tops of hills or on high tableland, as at X, fig. 84, especially if
the beds in B dip towards X.
It has been observed by geologists that the occurrence of
springs in limestone districts is one of the best indications of the
existence of faults. In the Carboniferous district of Gower the
limestone is traversed by a succession of nearly parallel faults,
which range across the limestone at right angles to the coast-line.
The lines of these faults are invariably marked on the surface by a
series of springs breaking out at different levels from that of the
FIG. 83.— Spring in valley
caused by fault.
FIG. 84.— Spring on hill
caused by fault.
sea, up almost to the summit of the country. The lower springs
are far more copious, and some of those near the level of the sea
never cease to flow, while those at the higher levels are readily
affected in dry seasons, and often cease for months together to
yield a drop of water.
Springs arising from faults, unlike those caused by alternation
of strata in valleys of denudation, are by no means confined to
combs or valleys. On the contrary, they often appear on table-
lands and other high elevations. The great boundary fault of
the Dudley coal-field, in the neighbourhood of Wolverhampton,
where the magnesian limestone and Red Sandstone marls are
brought down in contact with the Coal Measures, gives rise to
numerous springs almost at the summit of an elevation district
along the margin of the coal-field. Many of these springs burst
up in an almost vertical direction, and may be seen in several
cases breaking through the hard surfaces of roads and flowing
over into the gutters.25
Dyke Springs. — Springs are occasionally thrown out by dykes
or thin layers of impermeable material intersecting a water-bear-
SECT. III.]
WATER-SUPPLY.
259
ing stratum, as in fig. 85. The water will accumulate between
the impermeable substratum and the dyke, until it makes its
appearance on the surface at S.23
Artesian Springs. — In fig. 86 A and C are beds of clay or
FIG. 85.— Spring thrown out by a dyke.
other impervious material, and B is a water-bearing stratum.
Water will accumulate in the hollow of the lower impervious
stratum until it is pressed upwards against the under side of the
FIG. 86. — Water held down in porous bed by superimposed
impervious stratum.
upper one by hydrostatic force. If, therefore, a well be sunk or
a hole bored, say at K, the water will rise to a level determined
by this hydrostatic pressure. Such wells are called artesian, from
FIG. 87.— Natural fissure giving rise to artesian spring.
the French province of Artois, where they are very common, and
were executed with the greatest success as far back as the twelfth
century. If J* the upper surface of the impervious stratum be
below the level determined by the hydrostatic force just mentioned,
260 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
a bore-hole through the impervious stratum at that point will give
rise to an overflowing artesian well. A natural fissure in the
impermeable stratum will, under similar circumstances, give rise
to an artesian spring. In fig. 87 these conditions are illustrated.
The rise of the water from the bore-hole at A, or the spring at S,
will be seen to depend on the elevation of the outcrop of the
pervious stratum at B.23
Springs as a Source of Supply. — Long-continued observation is
the only safe guide for ascertaining the relationship which subsists
between the flow of a spring and the rainfall upon the area from
which the water is drawn. Springs may more frequently be
utilised as contributing to a supply than as the sole source.
Sometimes, however, two or more springs, too small independently
for the demand to be met, may be led into a common reservoir,
serving also, perhaps, as a service or town reservoir. One advan-
tage to be drawn from the joint utilisation of waters from different
springs is, that probably their least separate discharges will not
occur at precisely the same season of the year. Difference in the
extent, nature, situation, elevation, and distance of their respective
drainage grounds, and also difference in the lithological characters,
massif, and inclination of the respective strata, may bring this
about, but always with the advantageous result that the periodi-
cal diminution of flow in any one spring will be more or less
neutralised by the more liberal flow from the others.23
WELLS.
Wells are either shallow or deep, as explained below ; they may
also be divided into ordinary and artesian.
Shallow Wells. — Wells which are sunk comparatively but a
short distance into a superficial water-bearing stratum are known
as shallow wells. They are supplied by the infiltration of rain
and other water which falls on the adjacent surface of the ground,
or which is drained from ponds, cesspools, sewers, rivers, or other
reservoirs and channels. The numerous wells sunk for domestic
purposes in many villages and towns are, as a rule, of this kind.
They are highly objectionable when situated in the immediate
neighbourhood of towns, cemeteries, highly cultivated lands, and
other sources of organic matters ; but localities may frequently be
discovered where the conditions are favourable for sinking them,
and where at the same time the water will be wholesome and
comparatively pure.
The quantity derivable by these means will depend upon the
depth of the well, the nature and position of the water-bearing
stratum in which the well is sunk, and the disposition, of the
SECT. III.] WATER-SUPPLY. 261
impermeable stratum below. If the well be sunk in a permeable
stratum, as in fig. 75, the water derived from it will be simply
that which, in percolating downwards through the pores and
fissures, flows in through the, sides of the well, because of the
diminished resistance to its passage, more quickly and from a
larger surface than it can filter away through the bottom of the
well. This drip-water is an element in the yield of all shallow
wells and of some deep ones. In the case of fig. 76, if the well
is carried down below the line of saturation A B S, the supply will
no longer be limited to the drip-water, but will be drawn from
the subterranean reservoir formed by the depression in the under-
lying impervious stratum. The distance from the ground surface
to the line of saturation will sometimes vary considerably, even in
closely adjacent sites. Irregularities or undulations of the reten-
tive substratum may divide the geological basin into different
reservoirs with different lines of saturation, and thus render the
selection of the most favourable site a somewhat doubtful task.
Shallow wells are frequently sunk in the vicinity of rivers and
lakes, and are supplied by the water filtering through the sands,
gravels, or rocky detritus which forms their margin.23
Deep Wells. — Wells which are supplied by water which has
had to percolate and filter through large masses of the earth's
crust are known as deep wells. The difference between shallow
and deep wells consists rather in the greater or less distance of
the source of the water which flows into it than in the actual
depth of the well ; for a deep well, or more properly a deep-seated
well, may be formed by sinking through a moderately thin bed of
clay or rock into a water-bearing stratum, whose nearest drainage
area or outcrop is at a considerable distance.23
Causes of Success or Failure. — The conditions which affect the
success of a well, as far as the yield of water and its level are
concerned, are so varied, that any attempt to illustrate them with
an approach to completeness would be futile. The cases which
are given here must be regarded only as a few types.
One of the most frequent causes of either success or failure is
the existence of faults in the strata in which a well is sunk.
Referring to fig. 77, let it be supposed that the fault there shown
has been filled with an impervious material, forming a dyke which
serves to retain the water in the permeable stratum lying above
it. A well sunk, say, at A, in the latter would yield a supply
more or less abundant according to the extent of the exposed
surface of that part of the water-bearing stratum, while one sunk
on the other or lower side of the fault would evidently be a failure,
as far as the yield is concerned. If, however, the fault were filled
with the detritus of the adjacent strata in such a manner as to
262 GEOLOGY FOR ENGINEERS. [PT. V. CH. Xlt.
freely admit the passage of the water, it is obvious that the most
favourable site would be one below the fault, carefully selected
with regard to the position of the fault on plan, and also in such
a manner that the fault would be intersected by the well ; for the
water from a comparatively large extent of the stratum would be
drained into the fault and thence into the well. Should the fault
not be struck in the vertical line of the well, a tunnel or heading
driven from the well into the fault would have a similar result.23
Wells as a Source of Supply. — The waters of " shallow " wells
are frequently unfit for human consumption (see p. 260). The
waters of "deep" wells will depend for their characteristics upon
the nature of the strata through which they have percolated and
the soluble matters contained therein ; they are more free from
organic matters than river waters, as they undergo a more or less
complete natural filtration ; the greater the depth of the well, or
rather the longer the time which the process occupies, the more
complete will be the oxidation of the organic matters.
When comparing different sources on the ground of purity,
note must be taken of the possibility of contamination at future
periods, such as by mineral workings in mountain districts, or by
the cultivation of the land, or the increase of population in the
district. Of all sources, deep wells are least liable to have the
quality of their water injured by such causes, because of the great
depths of natural filtration which the waters undergo.23
Quality of Water. — Springs and Wells. — The quality of water
is much affected by the rocks through which it passes, although
it is not always safe to conclude what the result will be without
actual investigation. Thus water obtained from surface deposits
is almost sure to contain in solution some of those organic sub-
stances which in cultivated land must always abound, and which
are usually carried down to some little distance by the descending
supply of rain; water from ferruginous rocks, whether sand or
otherwise, being generally chalybeate, and that from calcareous
rocks holding carbonate and other salts of lime in solution. The
salts of soda, potash, magnesia, and other substances will also be
taken up, while the very action of water and the decompositions
otherwise going on produce sulphuric acid and thus again act
upon the containing rock, or alter combinations already in solu-
tion in the water. Thus it results that in all wells, however the
water is obtained, there will generally be found a certain propor-
tion of saline and other ingredients, although the actual quantity
is frequently less in amount in deep than in shallow wells in the
same locality. The nature of the impurity is often very different
from what might be anticipated in the case of water obtained
from great depths 13
SECT. IV.] WATER-SUPPLY. 263
Section IV.— Elvers.
Flow of Water. — Rivers are channels that maintain a perennial
though ever-varying discharge. The formation of a river is due
to precisely the same cause as that of the smallest rill. It owes
its maintenance to the rainfall of its district preserving the level
of the surface of saturation above the natural hollow that forms
its bed. The occurrence of river-valleys, small originally, but
ever widening and deepening by the erosion due to the scour and
fretting of their currents (see Chapter L, Section III.), offers to
the water percolating through adjacent land a course of less
resistance than that of the interior of the rocks ; the subterranean
waters gravitate towards the bottom of the valley ; the surface of
saturation is depressed in the vicinity, rapidly at first but
flattening as the river is approached (see fig. 88), and emerges
from the ground coincident with the hydraulic surface of the
V
FIG. 88. — Surface of saturation near a river.
river. The water flowing in rivers is contributed in three ways :
directly, by adventitious surface-flow and by rain ; indirectly, by
rivulets and ditches, which tributaries derive their own flow as
miniature rivers ; and, normally, by the percolating land- water
that enters their beds under the hydraulic head of the neighbour-
ing subterranean waters. The last-mentioned form of contribu-
tion is sometimes peculiarly marked by the evident increase in
the size of rivers, without the apparent cause that is afforded by
the junction of the tributaries. Thus, in defining the watershed
or catchment area of a river, it is necessary to consider not only
the superficial extent of land that discharges surface-water into
it, but, further, the area from which underground water is
contributed to it — two elements that are seldom coincident.22
Quality of Water dependent upon Strata.— The water found
in rivers, streams, and lakes is either that which has been
immediately drained into them from the surface of the land or,
having been previously absorbed by porous strata, has fed them
in the shape of springs ; or, thirdly, that which has drained into
264 GEOLOGY FOR ENGINEERS, [PT. V. CH. XII.
them by artificial means. In any case, however, the nature of
the foreign matter contained in river water will depend upon the
nature of the strata through which it has percolated, and over
which it has flowed.
Where the rain falls on impervious strata, such as granite, it
runs off the surface without encountering any substances which
it can dissolve to any great extent ; it therefore remains com-
paratively free from foreign matters. The water from rivers and
lakes in such districts approaches more nearly the nature of rain
than any other natural water. It is the softest of river water,
and its solvent powers are therefore comparatively high.
The next waters are the rivers which have passed over or
through districts containing carbonate of lime in some form or
other. They vary but little in the nature of their inorganic
constituents (consisting principally of carbonate of lime, sulphate
of lime, carbonate of magnesia, and chloride of sodium), but vary
very considerably in the total quantities of these substances, and
the proportions of them one to the other in the several waters.23
Much has been said in favour of a supply from large rivers on
sanitary grounds. The water is usually softer than that derived
from wells, springs, and small streams, and contains a less amount
of mineral salts than either of these, at the same time that it is
commonly more impregnated with organic matter. A large river
flowing over many geological formations and many different
varieties of soils may naturally be expected to take up in solution
a variety of mineral matters, and therefore to present a greater
number of ingredients than water derived from a more limited
area ; and this is generally found to be the peculiar character of
river water.25
It must be remembered, however, that rivers which drain large
areas of cultivated land, and into which the sewage of towns on
their banks must sooner or later, and in either a crude or modified
form, find its way, are always open to suspicion.23
The self-purification of streams during their flow has engaged
much attention ; and, although it must be conceded that such
action does take place, it is infinitely less effective than the natural
processes of filtration and distillation.22 Further information
with regard to this subject must be sought elsewhere l (cf. p. 251).
River Schemes. — In these, water is drawn from a stream or
river whose flow is greatly in excess of the quantity to be
abstracted. This excess makes one of the chief differences
between river schemes and impounding or gravitation schemes ;
inasmuch as, in the latter, storage reservoirs, to equalise the
supply and demand, are essential, whereas, in the former,
reservoirs for such purposes are, except in very rare cases, quite
SECT. IV.] WATER-SUPPLY. 265
unnecessary. It is sufficient that the smallest dry-weather flow
of the river is so large as not to be injuriously affected by the
withdrawal of the quantity required for the works.
The great experience and careful observation necessary for the
success of large gravitation works may be here largely dispensed
with — that is, as far as ensuring an abundant supply is concerned.
The larger the stream, the smaller, proportionately, will be the
variations in its flow at different seasons. The greater extent of
the drainage area will alone be a moderator of the effects of
irregularities in rainfall ; and even more so will be the existence
in that drainage area of absorbent strata serving to retain the
rain water only to yield it again in the form of perennial springs.
And thus it is that droughts which would threaten the complete
failure of impounding works need scarcely be regarded in
connection with river schemes.23
Flow of Streams and Elvers. — The discharge of watercourses,
which constitutes the available rainfall of the basins which they
drain, with the exception of any springs flowing straight into the
sea, or any water which may be drawn off from underground
sources, varies with the conditions which, as already pointed out,
affect the flow of the rainfall off the ground. The strata forming
the upper portion of the basins of rivers on high ground are
generally impermeable, the fall of the upper river is large, and
the rainfall greater than on the lower ground. Accordingly, the
flow of streams draining the higher portions of river basins is
usually very irregular, the streams rising rapidly in high flood
during rainy weather, and running almost dry in dry weather.
In the lower part of a river-basin, on the contrary, the ground is
commonly somewhat alluvial, and therefore permeable, the fall of
the river is reduced, and the discharge being derived from a much
larger area, is much more uniform, and less liable to sudden varia-
tions from great fluctuations in rainfall usually limited in extent.
Rivers, consequently, in the lower part of their course, besides
having necessarily a much larger discharge, possess a more
regular flow ; and even in tropical countries, the main rivers
draining large basins subject to varied meteorological conditions
still maintain a discharge in the dry season. Moreover, some-
times rivers rising in mountainous districts with a large rainfall,
eventually in their course to the sea traverse almost rainless
districts, bringing water to these arid tracts, which would be
uninhabitable without them, of which the Nile and the Indus
furnish typical instances.24
Rivers, however, do not necessarily have a larger discharge in
the lower parts of their course, e.g. the Nile, where enormous
quantities of water are lost in the Sudd regions (see Sir W.
266
GEOLOGY FOR ENGINEERS. [FT. V. CH. XII.
Willcock's Report on Assuan Dam and Egypt Fifty Years Hence).
Again, sometimes rivers disappear underground, or a large portion
of the discharge will flow underground to appear above ground
again further down. Some rivers of considerable volume in hilly
country dry up and disappear altogether in the sandy deserts
lying at the foot of the hills.1
The following particulars of the summer discharge of rivers,
taken from Mr Beardmore's Annual, are of value in connection
with this subject, as showing the powerful influence of retentiveness
in the geological character of the drainage ground acting even in
opposition to the moderating effect of extent : — 23
TABLE IX. — SUMMER DISCHARGE OF RIVERS.
Summer Discharge.
Rivers.
Drainage
Area.
Annual
Rainfall.
Total.
Per
• i
Per
1000
Equiv.
Annual
sq. mile.
acres.
Rainfall.
Nene at Peterboro ;
sq. miles.
c. ft.
per min.
c. ft.
per min.
c. ft.
per sec.
Inches.
Inches.
oolites, Oxford
clay, and lias
620
5,000
8-45
•22
1-88
23-1
Thames at Staines ;
chalk, greensand,
Oxford clay,
oolites, etc.
3086
40,000
12-98
•338
2'93
24'5
Loddon ; greensand
222
3,000
13-53
•352
3-01
25-4
Mimram ; chalk .
50
1,200
24-0
•625
5-5
26-6
Wandle ; chalk .
41
1,800
43'9
1-147
9 93
24-0
The larger percentage of summer discharge in the case of the
chalk rivers may be explained as follows : — Rivers flowing in a
clay basin are only fed by the rain falling within the actual
basin ; and as this rain evaporates very slowly in winter and very
rapidly in summer, such rivers are subject to great winter floods
and to severe summer droughts. The flow in chalk districts is,
however, much more uniform, because the rivers are fed by
springs as well as by surface drainage ; hence the water stored up
in the subterranean reservoirs is discharged by chalk rivers even in
the driest seasons. In fact, they draw their supplies from areas
beyond their actual basin, and their discharge is much more
uniform throughout the year than in most other rivers.25
SECT. V. WATER-SUPPLY.
267
Section V. — Lakes and Impounding Reservoirs.
Comparative Advantages. — The purest supplies of water are
obtained from lakes in hilly districts, and from impounding
reservoirs formed by dams enclosing the valleys of mountain
streams, especially where the lands draining into them are devoid
of habitations and culture. Moreover, the rainfall in mountainous
districts is, under ordinary conditions, considerably greater than
on lower ground \ and as the hills are commonly formed of imper-
meable strata, and the slopes of their sides are steep, a large
proportion of the rainfall flows down them into the valley below.
Accordingly, with a large available rainfall out of a considerable
total fall, the flow of a given drainage area is much greater in
such regions than elsewhere ; whilst the loss from evaporation,
both over the land and the reservoir, is reduced by the comparative
coldness of high altitudes. The catchment basins of mountain
streams are, indeed, necessarily very much smaller than those of
rivers in the lower portion of their course ; but lakes converted
into reservoirs for water-supply, and artificial impounding
reservoirs, possess the very important advantage of storing up the
surplus flow in flood-time for use during dry weather. These
reservoirs of water, moreover, when situated in high, mountainous
country, enjoy the further merits of being free from sources of
pollution, and of being at a sufficient elevation above the district
to be supplied, for the water to be conveyed by gravitation to the
service reservoirs.24
DRAINAGE AREAS.
The source of supply in gravitation works is the rainfall upon
the gathering-ground or catchment basin, a tract of land more or
less completely bounded by ridge lines or more properly watershed
lines. This latter distinction is necessary, because the hydro-
graphical basin is not necessarily coincident with that traced from
surface contours. Valleys of denudation on an anticlinal axis,
for instance, where permeable strata are superimposed, would
show from surface contours a gathering-ground larger than the
drainage area really available for the impounding of water, and
vice versa. In impervious or rocky districts the case is simplified
to one of surface observations.23
Size of Catchment Area. — Unusually heavy falls of rain are
the determining causes of the excessive floods that occur on
catchment areas ; and, as might be supposed, the relative magni-
tude of such floods is greater in the smaller areas.
There are two reasons for the decrease of the rate of flood-dis-
charge as the catchment area increases: (1) Extremely heavy
268 GEOLOGY FOR ENGINEERS. [PT. V. CH. XII.
falls only last for a short time, and rain falling in the remote
portions of a large watershed takes appreciably longer to flow
to the place of discharge than does the rain precipitated at more
central parts ; so the duration of the flood is prolonged, whilst
its intensity is diminished. (2) Heavy falls of rain, occurring
only locally over limited areas, naturally affect but slightly the
discharge from extensive watersheds.
It is useful to remember that 1 inch of rainfall per twenty-four
hours over 1000 acres is approximately equivalent to 42 cubic feet
per second. Also that a fall at the rate of 1 inch per hour
corresponds with a discharge of 1 cubic foot per second off an area
of 1 statute acre.22
Available Rainfall. — The gathering-ground having been
determined, and its area ascertained, an estimate has to be made
of the available rainfall upon that area.
The available fall is a quantity more or less short of the mean
fall — how much so remains to be seen. The mean annual fall is re-
ferred to in Section L, p. 240, and the first deduction from this is
one rendered necessary by the variations in the amount of fall. The
extent of the variations, as already stated, is found to be about
two-thirds of the mean fall — that is, one-third in excess, and one-
third in defect. Were the whole of the rainfall (neglecting for a
moment the loss by evaporation) to be impounded, and a uniform
quantity, equal to the mean fall, to be discharged from the
reservoir, the storage capacity of the reservoir would have to be
far greater in proportion to the supply than has hitherto been
found economical. The greater the mean supply (rainfall) com-
pared with the mean demand, the less will be the storage capacity
required to ensure the demand being regularly met ; and it is
now the practice to consider as available no more than the mean
fall for three consecutive dry years, and to secure a gathering-
ground correspondingly large. Where an extension of catchment
area presents difficulties, and an increase of storage capacity unusual
facilities, a modification of this practice may be advantageous.
The mean fall in three consecutive dry years is found to be,
with remarkable regularity, one-sixth less than the mean fall,
and this deduction is therefore always made; the one sixth
passes away in floods which the reservoir is not large enough
to impound.
The next deduction is for the loss by evaporation and absorp-
tion, which varies in this country from about 9 to 19 inches per
annum (vide Section I., pp. 241-2) ; an estimate of it for any case
can be formed only from careful observation and by experienced
judgment. The actual loss for a particular period may be found
by comparing the gaugings of the stream or streams fed from the
SECT. V.] WATER-SUPPLY. 269
drainage ground with the returns from the rain-gauges for the
same period. The Difference will, of course, give the loss for that
period. If the period of stream-gauging be one in which the
rainfall has proved to be less than the mean annual fall, the
proportionate loss shown by the gaugings will be greater than
the proportionate mean loss, and vice versd. Gaugings for short
periods require to be treated with the greatest caution, and in
inexperienced hands would be almost sure to lead to erroneous
conclusions.23
Ratio of run off a catchment area to total rainfall on the catchment.
— A table of the proportion of rainfall running off into outfalls,
from observations at Nagpur by A. Binnie, is given in Molesworth's
Pocket Book, 23rd ed., p. 319. Captain A. ff. Garrett, R.E.,
states that he has tested these in the Central Provinces, India,
and found them remarkably correct — generally within 5 per cent.
Probably similar percentages would hold in other parts of the
world. In Rajputana, he says, they generally took for new pro-
jects 10 per cent, run-off from sandy catchments and 20 per cent,
from hilly ones, though in exceptional cases as much as 70 per
cent, has been obtained off bare rocky catchments. It is, how-
ever, almost impossible to make an accurate estimate unless there
are previous records to go on.
In the case of the Tendula project, just sanctioned in the
Central Provinces, for a storage tank to impound 18 square miles
of water, Captain Garrett worked out the probable supply as
follows : —
Catchment area is over 300 sq. miles in two valleys. Rain-
gauges were established in centre of each valley and read for
two years and compared with readings from the gauge at
Dhamtari, some 30 miles distant, of which there were records for
thirty years. By taking proportions, the mean rainfall of the
Tendula catchment for past thirty years was then worked out,
taking the Dharntari records as basis. At the same time the
Tendula river was carefully gauged daily for two years — the
gauges being read every four hours during high floods. From
the results of these gaugings the percentage of run-off was
calculated and a curve was plotted showing the percentage run-
off after 20, 25, 30, 35, 40 inches of rain had fallen. From this
curve the yield for each of the thirty years was then deduced
from the calculated mean Tendula rainfall. Again, from the
rainfall statistics it was possible to see about when water
would have been required for irrigation. Allowing for water
drawn off for irrigation and 5 feet loss annually for evaporation
and absorption, it was possible to completely trace the working
of the whole scheme, supposing it had been in existence for thirty
270 GEOLOGY FOB ENGINEERS. [FT. V. CH. XII.
years, and thus to form a very fairly reliable forecast of its
working in the future.1
Capacity. — The geological structure is extremely important in
estimating the capacity of a drainage area. It is not alone the
rain which falls on the sloping surface of the hills and finds its
way by gravitation to the lower levels, but the effect of springs is
also often very great in augmenting the quantity of water. Mr
Beardmore relates an instance where an oolitic district was found
discharging a very large quantity of water with scarcely any
drainage area lying above or beyond it. In this case the porous
strata, with a very small dip cropping out on the sides of the
valley, were delivering the water which filtered into them far
beyond the limits of the drainage area, as indicated by the levels
of the surface. In fact, many districts will be found to have a
geological drainage area as well as a surface drainage ; and it
often happens that the former is far the most important of
the two.25
LAKES.
Natural reservoirs are provided by lakes, formed generally by a
depression in a mountain valley through which a river flows, in
which the water is retained by a ridge of rock across the valley at
its lower end, and over which it has to rise before the river
flowing in at the upper end can continue its course down the
valley below. The lake, in regulating the flow, stores up to some
extent over its large area the flood discharge of the river above ;
and it also acts as an immense settling basin, in which all the
sediment brought down by the river is gradually deposited as the
current is checked on entering the lake. A notable example of
this result is furnished by the River Rhone, which enters the
Lake of Geneva as a very muddy, glacier-fed river, and emerges
at Geneva as pure and blue as the waters of the lake. The value
of lakes as storage reservoirs depends upon the discharge of the
river flowing into them, together with the flow of their own
gathering-ground, and the freedom of the drainage area and the
shores of the lake from sources of pollution.24
Advantages. — Lakes, by their very existence, prove that the
strata forming their basin are thoroughly water-tight, which is an
essential condition in a reservoir. Another advantage possessed
by lakes for conversion into reservoirs is the existence of a rocky
barrier across their outlet, which is a cause of their existence ;
for the water discharged from them would have worn away any
soft obstruction,24
SECT. V.] WATER-SUPPLY. 271
IMPOUNDING RESERVOIRS.
Sites. — The valleys of mountain streams draining uninhabited
and uncultivated districts afford the most favourable sites for
impounding reservoirs, owing to their freedom from pollution,
and because, from their situation, they are exposed to a heavy
rainfall, a large proportion of which, falling on very sloping,
impermeable strata, finds its way into the watercourse draining
the valley. The area to be covered by the reservoir must be
adequately impermeable and continuous throughout so as to be
perfectly water-tight, or capable of being readily made impervious
in small defective places by layers of clay puddle. A narrow
part of the valley should, if possible, be selected for the dam, so
as to reduce its length, and a site where the valley widens out
considerably above the gorge for some distance so as to provide
an extensive area for the reservoir.24
Suitable sites for dams are often found just below the junction
of two or more streams, as in such cases two or more valleys are
available as storage basins.
It is also most important in selecting sites for storage reservoirs
to see that a suitable position for the waste weir is available so
that floods may be discharged harmlessly.1
Geological Features. — Simultaneously with the favourableness
of the site for capacity, and for the formation of the bank in point
of dimensions, the geological features must be carefully regarded,
in order that a water-tight reservoir may be constructed. If any
porous strata be intersected, it will be necessary to study their
dip, for if it be away from the valley, such strata will only drain
the reservoir of its contents ; but if the valley be on a synclinal
axis, the porous strata, if any, dipping towards the reservoir will,
on the other hand, serve to augment its waters by the inflow of
springs which most likely will be perennial. Cracks and fissures
in rocks are frequently sources of leakage from reservoirs, and
special means should be taken to stop all such as are discovered,
by the introduction of concrete and puddle. The reservoirs of the
Manchester Waterworks, situated on the Lower Coal Measures and
the Millstone Grit, presented many difficulties in this respect.
The mountain limestone also is full of fissures, by which the water
is almost sure to be drained away. Where excavations are con-
ducted in the interior of a reservoir, care must be taken not to
cut through a sound water-tight bottom, and expose a pervious
stratum into which the impounded water may escape.23
PT. V. CH. XIII.
CHAPTER XIII.
BUILDING-STONES.
THE importance of a practical knowledge of geology when dealing
with building-stones i is so obvious that it would appear quite
unnecessary to dilate upon this theme. Unfortunately, however,
petrology, or the study of rocks, is a branch of study which is
frequently neglected by many architects and engineers.
Disregarding private dwellings, on which such various materials
are employed, according to the motives that lead to their erection,
it may be fairly stated that a knowledge of the general structure
of rocks, and the situations whence the best materials may be
obtained, is essential to those who are either charged with or
direct public works. A stone which may be sufficiently durable if
plunged beneath water, may not be so when kept alternately wet
and dry by the rise and fall of water in a river or on a tidal coast,
or when wholly exposed to the action of the atmosphere. A some-
what porous sandstone, for instance, may do well when kept
constantly under water ; but the same rock, when exposed to the
atmosphere, more particularly in climates subject to frost, might
gradually crumble away from causes referred to in Chapter I.
The observer desirous of selecting a stone to be exposed to
atmospheric influences would do well to study the mode in which
it is weathered in the locality whence it is obtained. He may
there learn which part, if it be a compound rock, is liable to give
way before such influences, and the conditions under which it
does so. Granite generally is considered a proper material for
national monuments. Some granites, however, though they may
be hard and difficult to work when first taken from a quarry, are
among the worst building materials, in consequence of the facility
with which the felspar in them decomposes when exposed to the
action of a wet atmosphere, in a climate which may be warm
during part of the year, and cold during the other.19
It is therefore abundantly clear that a careful investigation of
the geological history and structure of the rocks of his district
will frequently enable the engineer to avoid such expensive
272
SECT. I.] BUILDING-STONES. 273
mistakes as importing materials which can be obtained, of similar
quality and at a low price, on the spot.
Moreover, a knowledge of the physical properties and weather-
ing qualities of building-stones is of the highest importance, and
these should be studied at the quarry site and not merely deduced
from carefully selected samples.1
Section I. — Granites and Granitoid Rocks.
GRANITES AND SYENITES.
The lithology of these rocks is fully described in Chapter VII.
It is only necessary to add here l that granites are described as
fine-grained, medium-grained, or large-grained or as porphyritic,
when, like that of Shap in Westmoreland, they contain large and
independent crystals of felspar scattered through the mass.11
Constituents of Granite (p. 106). — Not only do granites vary
greatly in the relative proportions of their mineral elements, but
they also exhibit considerable variation in their constituent minerals.
For although we may use the general formula of quartz, felspar,
and mica to describe the rock, yet the felspar or mica may be almost
any member, or members, of these families of minerals, and they
may be supplemented or partly replaced by minerals which are no
essential component of granite, and are local in their development.
And when the chemical composition of granite is examined, the
variation is almost as remarkable ; for although we may regard
the normal composition as including silica, alumina, peroxide and
protoxide of iron, lime, magnesia, soda and potash, and water, yet
sometimes in addition to these there are perceptible quantities of
oxide of manganese, phosphoric acid, litnia, and fluorine, while not
infrequently the protoxide of iron, or even all the iron, may be
absent, as may be the magnesia and the water. Even in British
granites the percentage of every constituent is very variable ; thus
the silica ranges from as low as 55*20 in the granite of Ardara to
as high as 80-24 in the granite of Croghan Kinshela; so that,
judged by this test, the Ardara rock might be termed basic, while
the Croghan Kinshela rock is typically acidic.
The alumina varies from 11*14 per cent, at White Gill, Skiddaw,
to 20 per cent, in the granite of Glen in Donegal. The peroxide
of iron ranges from -23 at Botallack to 7*3 in some of the granites
of Leinster ; whilst the protoxide of iron, which is so frequently
absent, amounts sometimes to upwards of 2 per cent. The lime
varies from J per cent, in some of the Cornish granites to up-
wards of 5 per cent, in some of those from Donegal. The
magnesia, which may be a mere trace, amounts to 3J per cent, in
18
274 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
the granite of Ardara. Soda may be but J per cent, in Cornish
granites and 5£ per cent, in some of the Leinster rocks. Potash
is less than J per cent, in one of the Leinster granites and more
than 8J per cent, in the granite of Chywoon Morvah in Cornwall.
The manganese never quite amounts to 1 per cent., and the water
is never more than 2 per cent.6
Qualities. — The granites are quarried, for the most part, from
hillsides and other rising grounds, have little or no superficial
covering, are blasted for smaller purposes, but split with wedge
and mallet for larger blocks and monoliths. In most quarries the
rock has a rudely jointed or tabular structure, but in some
instances it is massive and capable of yielding blocks of large
dimensions. Like other rocks it can be squared and dressed with
greater facility when newly raised and in possession of its " quarry-
sap," and this, according to the texture of the rock, may vary from
5 to 1 per cent, of its weight. Some granites of open texture are
capable of absorbing as much, it is said, as from 2 to 3 gallons
per cubic yard, and those -absorbing the most are the least to be
relied upon for their durability. The specific gravity of ordinary
granites ranges from 2*6 to 2'8, a cubic foot weighs from 164 to
169 Ibs., and from experiments on inch cubes, the crushing force
varied, according to the texture and composition, from 3000 up to
13,000 Ibs.11
Durability. — Although all granites are similar in structure, the
difference in the proportions of their constituent substances occa-
sions great difference in their enduring and useful properties.
Some varieties are exceedingly friable, and liable to decomposition,
while others, including that known as syenite, suffer but imper-
ceptibly from moisture and the atmosphere.26 Owing to the
substitution of hornblende for mica in its composition, syenite is
often more durable than true granite.1
The ultimate chemical analysis of a granite gives no idea either
of its colour, texture, resistance to pressure, or durability. The
silica is partly free, partly in the felspar and mica ; the lime, soda,
and potash partly in the felspar and partly in the mica ; and the
magnesia in the mica. The colour, texture, susceptibility of
polish, resistance to pressure, and durability, depend upon the
size and arrangement of the several ingredients — the granites
most liable to decay being those containing an excess of lime, iron,
or soda in the felspar and mica. Those containing large crystals
of mica are unfitted, of course, for building purposes ; and the
same may be said of varieties in which soda-felspar, and very deep
red (iron) felspar, predominate.11
Geological Age of Granite. — Although it was once supposed
that granite is the oldest of rocks, it is now known, from observa-
SECT. I.] BUILDING-STONES. 275
tions extending over large tracts of the earth's surface, that
granites have been formed at several geological periods from the
Silurian down to, at least, the close of the Cretaceous period.
Thus it is known that the granite of Cornwall and Devon is more
recent than the Carboniferous period, as also that of Arran ; that
the granite of the Alps of Savoy is more recent than the Jurassic
period ; and that the granite of the Eastern Pyrenees is more
recent than the White Chalk. On the other hand, there are
granitic rocks of great antiquity, such as some of those found in
Scandinavia, the Highlands of Scotland, Donegal and Galway, all
of which are older than the Devonian ; some, than the Upper
Silurian periods.27
The age of granite is always newer than the rock which it
penetrates, and older than a stratum deposited upon it. It is
rare to be able to fix both of these limits of age. But the more
ancient or Silurian granites are found in the Harz, Thuringerwald,
Saxon Erzgebirge, Vosges, Christiania in Norway. The protogine
granite of the Alps is newer than the Lias.6
Syenite. — This term was formerly applied to hornblendic
granite, but is now usually reserved for the rock described in
Chapter VII., Section I., p. 108. The engineer will, however,
frequently meet with the older nomenclature, and for this reason
syenites are classed with granites as regards distribution.
The hornblende gives syenite a darker colour and adds to its
durability.1
British Granites and Syenites. — England. — Granite occurs in
Devonshire and Cornwall, also in North Wales, Anglesea, the
Malvern Hills, the Channel Islands, Charnwood Forest in Leicester-
shire, and in Cumberland and Westmoreland. Granite blocks are
found in the beds of some of the rivers in the north-west parts of
Yorkshire, and in clay pits in Cheshire and Lancashire, at great
distances from any quarries where the stone is available.26
In Cornwall and Devon the granite forms bosses of several
square miles in extent, and rises at its culminating point at
Dartmoor to the height of 2050 feet. It is generally light-
coloured ; and in some parts of Cornwall it contains large crystals
of white felspar (porphyritic granite), and at other places numerous
crystals of black tourmaline (schorlaceous granite).4
There are six principal masses of granite besides smaller patches.
The granite of Dartmoor is coarse-grained, with the mica some-
times white, sometimes black. It is schorlaceous where it joins
the slates. After the mica disappears the felspar vanishes and
the rock at last consists of quartz and schorl, as at Holm
Lee.
The granite of the Bodmin Moor or Brown Willy district is
276
GEOLOGY FOR ENGINEERS.
[PT. V. CH. XIII.
TABLE X. — ANALYSES o
Cornish Granites.e
Lake
District^
Scotch Granites. 6
Botallack.
Chywoon
Morvah.
Gready in
Luxullian.
White Gill,
Skiddaw.
Syenitic
Granite of
Buttermere.
Peterhead.
Ross of Mull.
Bell's Grove,
Strontian.
|
Silica .....
74-54
70-65
69-64
75-22
71-44
73-70
74-48
62-09
67-0
Alumina .....
14-86
16-16
17-35
11-14
15-34
14-44
16-20
17-60
17'2
Ferrous oxide .
0-23
0-52
1-94
1-77
1-10
1-49
0-74
0-4
Ferric oxide
2-53
1-53
1-04
trace
1-23
0-43
0-20 4-78
31
Lime . . . .'
0-29
0-55
1-40
1-62
1-06
1-08
0-13 4-95
2-9
Magnesia . . . • ' , . .
0-21
1-08
0-72
trace
0-27 : 3-17
1-2
Soda V . . . •••'.
3-49
0-54
3-51
3-99
3-95
4-21
3-78
4-08
3'2
Potash ... .
3-73
8-66
4-08
4-51
4-43
4-43
4-56
3-25
3-9
Manganese oxide
Lithia ....
Phosphoric acid . .
trace
trace
trace
trace
' »
trace
0-40
••
trace
0-14
0-11
trace
Water, hygrometric . . .
Water, combined
0-87
trace
0-83
0-89
0-13
0-59
0-21 ^
0-40 /
0-60
Loss . ... . .
0'50 , 0-58
••
••
••
similar, consists of quartz, felspar and two micas, is often porphy-
ritic, but not particularly schorlaceous, except near St Cleer.
The granite of St Austell or Hensborough is much more variable
and much richer in schorl than those of either Brown Willy or
Dartmoor, and is more decomposed. The Carn Menelez or
Falmouth granite is occasionally porphyritic. It is poor in
schorl. At the Land's End the granite abounds in schorl and
often passes into schorl rock. That of the Scilly Isles is somewhat
coarse, two micas being present ; schorl is rare.
Leicestershire. — Syenite occurs in Charnwood Forest. The
rock is rather coarsely crystalline and contains dark-green
hornblende with pink and greenish felspar with small masses of
yellowish-green epidote and occasional grains of pyrites. When
the rock is more finely crystalline it is generally of a red colour.
The Mount Sorrel granite is usually pinkish or grey. It is
occasionally slightly porphyritic and consists of quartz, felspar,
SECT.
BUILDING-STONES.
277
GRANITES AND SYENITES.
Donegal Granites.6
N.E.
Ireland.6
Leinster.6
1
1
1
X
a
1
1
1
.
Ardara.
I
Dunlewy.
1
1
|
£
£
M
Enniskerrj
Croghan
Kinshela.
Grey Grani
| 1 Syenite, We
oo |
f
'a
•
ll
£
1
r
55-20
58-44
68-80
75-24
70-48
64-60
70-38
74-24
80-24
75-46
56-36
58-05
60-97
62-52
19-28
0-46
6-08
20-00
2-05
6-44
16-40
0-65
2-60
13-36
14-24
14-64
1264
13-64
12-24
11-89
3-52
15-60
2-57
575
20-05
7-96
17-71
8-29
16-44
10-58
14-13
7-38
0-60
3-72
6-04
3-16
1-40
0-72
5-08
4-72
1-75
2-25
1-48
3-16
2-84
1-48
0-89
1-25
6'52
7-22
5-81
5-14
3-36
3-66
1-57
0-85
0-14
0-40
2-80
0-53
trace
0-08
8-40
4-12
2-07
1-80
1-50
4-63
3-81
3-75
4-86
3-66
4-02
3-13
2-72
5-58
2-56
2-92
1-70
3-24
0-80
3-05
3-17
0-96
2-82
5-31
3-27
4-26
3-15
5-90
3-95
0-40
4-40
3-80
274
2-98
3-41
0-08
6-25
0-64
/
1-12
2-24
0-62
1-34
1-03
1-20
1-59
1-13
1-16
1-20
\
black mica, and dark-green hornblende, occasionally with pyrites
and epidote.6 Its warm rose-tint renders it suitable for ornamental
purposes. It is, however, very hard and consequently expensive,
but is capable of being extracted from the quarry of any required
dimensions, and may be moulded to any desired form. It is
also well suited for paving.26
Channel Islands. — Large quantities have been raised and
exported from the quarries of Mount Mado and La Perruque in
Jersey, as well as from Guernsey and the little island of Herm.26
North Wales. — The reputed granite of Anglesea, which is
probably granitoid gneiss, where best developed is composed of
quartz, felspar, and black and silvery mica, but is usually coarse,
with the felspar not well crystallised and the mica often absent.
Lake District. — The Shap granite, with its large flesh-coloured
or reddish-brown crystals of felspar, is the best known ; the
felspar is partly orthoclase and partly triclinic.
278 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIlI.
Scotland. — Typical granite occurs in the Grampians, where the
lower portions of the masses are exposed by extensive denudation,
and is well seen in the Ross of Mull, but in the higher peaks the
granite becomes more hornblendic and then graduates into a
more or less porphyritic felsite.
The central mass of Ben Nevis is hornblendic. The granite of
Loch Etive is a fine-grained rock in which the felspar is mainly
anorthic ; that of Strontian is dark and coarse-grained, with red
orthoclase, white felspar, quartz, a large proportion of black mica
and hornblende with crystals of sphene and perhaps zircon. The
granite of Goat Fell and principal mountains in Arran is a large-
grained variety in which felspar predominates and mica is
comparatively rare.
Ireland. — The granite in Donegal has a stratified structure, the
beds being nearly vertical. In the Mourne Mountains the granite
is fine-grained, and abounds in cavities filled with crystals of the
minerals which form the granite. The rock consists of smoky-
brown quartz, opaque-white orthoclase, albite, and dark-green
mica. Granite also occurs in the Carlingford district, in Leinster,
Gal way, and Mayo.6
European Granites. — The principal granitic districts in Europe
comprise : —
France. — Eastern part of the Vosges, much of the high land of
the Auvergne, the district between Nantes and Parthenay, the
Pyrenees, and Brittany.
Germany and Austria. — West of the Schwarzwald, in the
Odenwald, south of the Thuringerwald, in the Harz, much of the
Fichtelgebirge, several areas in the Erzgebirge, Oberlausitz, in
Bohemia, the Riesengebirge, the Sudetic Alps, the highest peaks
of the Tatra in the Carpathians, the Bohmerwald.
Switzerland and Italy. — Mont Blanc, St Gothard, etc., Velteline
Alps, Trientine Alps, etc., Corsica and Elba.
Spain. — North- West Province of Galicia, the Sommo-Sierra, the
Guadarrama Mountains, the Sierra Morena.
Scandinavia. — A large part of the peninsula.
Russia. — East side of the Ural, and a large area in the south.6
European Syenites. — Among the European localities for
syenite are Plauen, near Dresden, many places on the southern
slope of the Thuringerwald, in the Odenwald, Meissen, in Saxony ;
in Moravia it extends 30 miles from south of Kienitz, through
Brunn, to north of Boskowitz ; in the mountains of Lower Silesia
a large mass of syenite extends from Glatz to Ullersdorf. A rock
of syenitic character, classed by Zirkel as a syenite-granite-
porphyry, stretches from north to south in the east of the Banat
from Kudernatch to Moldawa. A somewhat similar rock occurs-
SECT. I.] BUILDING-STONES. 279
in the Bihargebirge in South-East Hungary, penetrating Neocomian
rocks. In the Vosges, massive syenite appears between Windstein
and Ballow, north of Geromagny. In the Tyrol it forms the centre
of the eruptive mass at Predazzo, and the great mountain mass of
Monzoni, characterised by red orthoclase, white oligoclase, with
films of hornblende and brown mica. In the south of Norway
syenite is seen around Christiania, penetrating slates and lime-
stones, and in Finland it occurs near Viborg.6
GRANITOID ROCKS.
Gneiss. — Gneiss (vide Chapter VII., Section III., p. 125) is a
foliated crystalline aggregate of the same minerals which consti-
tute the different varieties of granite ; typically, of orthoclase, plagio-
clase, quartz, and mica. These minerals are arranged in more or
less distinct layers or foliae which are approximately parallel to one
another. The mica especially forms very distinct, although thin,
bands, and it is to this arrangement of the mica that the schistose
and often fissile character of the rock is due. Sometimes the mica
is a potash, sometimes a magnesian mica, and at others both kinds
are present. Gneiss varies in colour, the orthoclase in some
varieties being red, while in others it is white or greyish. There
is a marked chemical difference between red and grey gneiss, red
gneiss containing from 75 to 76 per cent, of silica, while the grey
variety contains only 65 to 66. 16
Syenitic or hornblendic gneiss has the same mineral constitution
as syenitic granite. The felspar is, in great part, represented by
oligoclase. It is a rock of very extensive occurrence, and passages
have been observed from hornblende gneiss into hornblende
schist.16
Granitic Gneiss. — When the foliation of gneiss becomes indistinct
the rock approximates lithologically to granite. Rocks of this
vague character are not infrequently met with, and it is hard to
say whether they should be called gneiss or granite. Such
intermediate forms are styled granitic gneiss.7
Occurrence.— Gneiss has a very wide and irregular distribution,
rising in bosses chiefly amidst Palaeozoic strata, and in vast bands
generally coincident with mountain ranges ; more rarely in dykes.4
Gneisses generally occur among the so-called Archaean rocks in
Central France, Scandinavia, United States, Canada, etc.1
Porphyry (see Chapter VII., Section I., p. 108). — Certain rocks,
closely allied to and sometimes called granites, are quartz-porphyry,
felsite-porphyry, felstone, and felsite. These are composed of a
mixture of orthoclase and quartz as the essential constituents,
little or no mica being found in their composition. When the
280 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
quartz forms conspicuous crystals the rock is called quartz-
porphyry, but when the felspar and mica are intimately mixed so
as to present a homogeneous matrix the rock is termed felsite-
porphyry, felstone, or felsite ; they are generally very compact in
texture or even flinty in appearance. Triclinic felspars occasion-
ally occur in these rocks, while the minerals mica and hornblende
accompany the essential constituents, apatite, magnetite, and
pyrite being at times met with as accessories. The colours of
these rocks vary from flesh-red, purple, yellow, to slate-grey,
depending chiefly on the felspar, while dark-grey, brown, and
greenish tints are imparted by the presence of mica or horn-
blende.28
The antique porphyries were of several varieties. The stone
originally known as porphyry was quarried by the Egyptians in
the granite found between Suint and the Red Sea. It was dark
crimson or purplish, hence the name. A similar stone of a green
colour was quarried in Greece between Sparta and Marathon.10
The porphyries generally occur as dykes (cf. p. 35) and eruptive
masses intersecting the older schists and slates, and are usually
much fissured and jointed, and for this reason incapable of being
raised in massive monoliths like the granites.
Occurrence. — They are found cutting through the Cambrian,
Silurian, and Devonian rocks of Ireland, Wales, Devon, and
Cornwall, the Lake District, the Southern Uplands and Northern
Highlands of Scotland. Both varieties appear in many tints —
red, flesh-coloured, fawn-coloured, black, bluish black, and bluish
green. Incapable of being raised in large blocks, they are polished
only for minor ornaments, their principal use in Britain being
for causeway stones and road metal, for which their hardness and
toughness render them specially suitable. Though chiefly used
for road material, in some districts they are employed in the
building of country mansions, farmsteads, and walls, and when
properly dressed and coursed make a very fair structure (especially
the fawn-coloured sorts) and are perfectly indestructible.11
Serpentine. — A siliceo-magnesian rock of metamorphic origin,
arising apparently from the transmutation of magnesian lime-
stones or other closely related strata. Its average composition
is 40 per cent, of silica, 40 of magnesia, and 13 of water, with
varying proportions of iron-peroxide and traces of other colouring
matter.
Serpentine is not adapted for outdoor use, especially in towns,
for it is acted on by hydrochloric and sulphuric acids, but it is
very suitable for indoor decoration.
Occurrence. — Serpentines occur in Cornwall and Anglesea in
England ; in the counties of Banff, Aberdeen, Perth, and Forfar
SECT. I.] BUILDING-STONES. 281
in Scotland ; and in Galway and Donegal in Ireland, as well as
among the metamorphic or crystalline rocks of most countries —
France, Germany, Italy, Greece, the Urals, Egypt, India, Canada,
and North America yielding many varieties.11
Crystalline Schists. — These old rocks generally occur in a slaty
or fissile state, and are better adapted for roofing, paving, and
other slab purposes than for building ; and yet some of the
compacter beds of the Silurian (the greywackes) make not a bad
building-stone (Keswick, Kendal, Hawick, Galashiels), being flat-
bedded and easily squared and jointed. Where obtainable, a
frontage of this sort is greatly improved by light-coloured sand-
stone dressings. In some districts, where sandstones and lime-
stones are scarce, the mica schists, gneisses, and chlorite schists
are employed for building purposes; but, though tough and
durable, they seldom produce anything like a satisfactory effect.11
TRAP ROCKS.
The basalts and felstones or claystones, as well as the rocks
known as greenstones or whinstones, are often all included under
the name of trap rocks, but the term trap is more properly applied
to the dark compact greenstones or basalts of which the successive
streams have flowed in great horizontal sheets and have given
rise to a step-like structure, as in the case of the lavas of the
Faroe Islands, the Deccan, Norway, etc.4
Greenstone is an old name for the dark-green, fine-grained rocks
known as Diorite, Diabase, Gabbro, and Aphanite. The name is
sometimes confined to diorite, but the more general designation
is sufficient for practical purposes. These rocks1 all occur as
dykes and veins, chiefly in the more ancient rocks. Their green
colour is derived partly from their hornblende and partly from a
small quantity of chlorite which is generally present. Gabbro is
coarse-grained, diabase and diorite are fine-grained, and aphanite
is very compact and fine-grained. They are all occasionally
amygdaloidal, and are, no doubt, varieties of the same rock
solidified under slightly different conditions.5
Diorite, S.G. 2-6 to 2-9, contains silica 47-58 per cent. It is
found amongst Silurian, Cambrian, and metamorphic rocks,
generally in the form of dykes, often assuming a bedded aspect
and a columnar structure. It is generally extremely hard and
tough, and is consequently well suited for road material and
paving.27
Mica trap or Minette occurs in a manner precisely analogous to
diorite; it is generally tough, and weathers rusty brown. It
occurs in the form of intrusive dykes amongst the Silurian rocks
282 GEOLOGY FOR ENGINEERS. [PT. V. OH. XIII.
of Wicklow and Mayo and the Southern Uplands of Scotland. It
is of frequent occurrence amongst the Lower Silurian strata of
Cumberland and Westmoreland. Except as a material for mend-
ing roads it is useless for any economical purpose.27
Whinstone. — Any very hard dark-coloured rock that is not
easily broken up in excavating, as basalt, chert, or quartzose
sandstone, is called a whinstone locally.10
Basalt (cf. p. 1 11). — These lavas have a dark colour on the newly
fractured surface, varying through shades of greyish brown, blue,
and greenish black ; but when the external surface is weathered,
the rock is commonly a pale drab, though the tint varies with
chemical and mineral composition and texture. Basaltic rocks
have a high specific gravity and basic composition. Their silica
rarely sinks below 40 per cent.; a lower percentage of silica is
usually associated with large percentages of iron, and sometimes
of lime. The silica rarely exceeds 56 per cent. The alumina has
no necessary relation to the silica, though the average amount
ranges between 1 1 per cent, and 28 per cent. The lime, magnesia,
potash, and soda all vary in amount, and on this variation depends
the mineral composition of the rock. Basalt abounds in labra-
dorite and augite, generally contains magnetite and olivine, and
sometimes may have a little quartz and sanidine.6
The basalts vary considerably in structure : the coarsely
crystalline varieties, and those in which the different mineral
constituents are sufficiently well developed to be distinguished by
the naked eye, are termed Dolerites; those in which the con-
stituents are too small to be recognised without a magnifying
power, but in which a crystalline texture is yet clearly discernible,
are styled Anamesites', while the still more compact varieties,
which to unassisted vision present a more or less homogeneous
appearance, are called Basalts (basalts proper) or basal tites.16
Dolerite includes rocks which were once termed "greenstones,"
e.g. diabase, etc.
Basalts and dolerites occur under three general modes: (1) as
vertical dykes ; (2) as sheets or beds intruded amongst older
rocks ; (3) as tabular sheets poured over the surface and forming
horizontal or inclined beds, often interstratified with volcanic
ashes, agglomerates, and bands of bole.27
Vertical dykes are extremely numerous over the north-east of
Ireland, the north of England, and the centre of Scotland,
traversing rocks of different ages from the Silurian to the Oolitic.
Intrusive sheets are prevalent amongst the Carboniferous rocks of
Ayrshire, the Clyde basin, and other parts of Scotland. Tabular
sheets occupy a considerable area in the north-east of Ireland.
In texture and composition these basaltic rocks are extremely
SECT. I.] BUILDING-STONES. 283
variable. In some places they are soft, earthy, and amygdaloidal ;
in others compact, or highly crystalline. They are among the
most effective rocks for resisting crushing force. Basalt weighs
171 to 181 Ibs. per cubic foot, absorbs less than 4 oz. of water per
cubic foot, and is extremely durable. With these qualities it is
admirably adapted for street-paving, for foundation and curb
stones, and for road metal ; but it is generally objectionable for
building purposes owing to its gloomy and heavy appearance.27
Occurrence. — Basalts occupy large areas in the Southern Eifel
and Northern Bohemia as well as in many parts of Saxony,
Bohemia, Moravia, Styria, Hungary, and Transylvania, North
Italy, the Auvergne, and south of Sweden. Basalts are well known
in Greenland, Iceland, the Faroe Islands, and Inner Hebrides; at
Paranagua in Venezuela, in the Galapagos Islands, Sandwich
Islands, north of Melbourne in Victoria, St Helena, the Isle of
Reunion, at Funchal in Madeira, at Patna, and at Cruz in
Teneriffe ; also in many parts of North America.6
Lavas. — The term "lava," properly speaking, includes all the
molten rocks of volcanoes (see Chapter II., p. 27) ; but for practical
purposes the basaltic rocks, which have been already described,
are excluded and " lavas " denote only the lighter varieties, such as
trachyte, rhyolite, andesite, and obsidian.1
Trachyte (p. 27). — The varieties of this rock, consequent on
changes in chemical composition or the presence of accessory
minerals and different rates of cooling from a molten state, are
endless ; for while on the one extreme we have a crystalline
granular rock, resembling granite, on the other we find the same
constituents passing into obsidian (or volcanic glass) or pumice-
stone so porous as to float on water. In this condition it passes
beyond the category of building-stones.27
Rhyolite (p. 110). — A similar rock to the Hungarian and Tran-
sylvanian lavas, to which the name of rhyolite was first given,
occurs in the Lipari Islands, Euganean Hills, in Rhenish Prussia, the
Auvergne, Iceland, the Rocky Mountain region of North America,
the Northern Island of New Zealand, and several of the islands of
the Greek Archipelago.6
Andesite (p. 109). — Among the more important European locali-
ties for andesites are Schemnitz, Kremnitz, the St Andra-Vise-
grad Mountains, near Buda-Pest in Hungary ; the Transylvanian
Erzgebirge, the south ^f Servia, the Smrkouzgebirge in Styria ;
near Banau in Moravia. In the Auvergne hornblende andesites
are seen in the lavas. In Italy andesites occur in the Euganean
Hills, at Monte di Ferro di gran Pietra, Monte della Croce, and
Teolo. In the Andes of Ecuador, at Palulagua, the andesite is
almost free from augite. A similar rock occurs at Toluca in
284
GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
b|z;o
H
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II
co Td n
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SECT. I.]
BUILDING-STONES.
285
TABLE XL — ULTIMATE CHEMICAL ANALYSES OP SOME
GRANITOID AND TRAP ROCKS — Continued.
Schists and Shales, Malvern.6
Schist.
Schist.
Shale.
Shale.
Shale.
Silica ....
43-61
45-82
49-37
53-97
64-37
Alumina ....
19-34
16-39
21-47
23-24
18-62
Oxide of iron .
17-02
16-20
13-39
9-51
2-07
Manganese
0-15
0-45
0-30
Lime ....
2 '31
1-46
0-75
1-58
1-70
Magnesia.
7-10
7-81
1-00
3-66
0-77
Alkalies and loss
4 96
5-68
7-06
8-04
8-86
Loss on ignition
5-51
6-19
6-66
3-84
Gneiss.4
Mica Schist.4
Chlorite
Schist.4
Erzgebirge,
Saxony.
Pargas,
Finland.
4
PI
^
£
o
1
i
&
£
i
!
N
a
Silica
65-19
68-66
75-90
65-13
82-38
71-26
33-72
Alumina .
14-75
15-03
12-95
18-16
11-85
20-03
19-81
Iron peroxide .
6-88
1-92
1-31
...
...
1-10
...
, , protoxide .
3-09
...
5-27
2-28
3-61
24-83
Lime
2-50
2-03
1-48
0-32
...
0-28
0-60
Magnesia .
2-04
1-97
0-16
270
1-00
trace
12-01
Potash .
4-77
2-47
5-12
2-99
0-83
2-48
trace
Soda . . .
1-99
0-64
2-39
0-53
3-80
0-59
trace
Titanic acid
0-89
...
...
...
...
Water .
1-01
0-64
0-40
3-73
0-77
1-63
9-27
286
GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
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Portillo, Teneriffe.
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Silica . % .
Alumina .
Peroxide of iron
Protoxide of iron
1 1
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III!
Titanic acid
Phosphoric acid
SECT. IT.] BUILDING-STONES. 287
Mexico. Hornblende andesite is met with in the Caucasus near
Kasbek.
Dacite occurs in Hungary and Transylvania, also at Neu
Prevali in Carinthia and Monte Alto in the Euganean Hills and
in America.
Augite andesite. — The variety which is free from quartz is
found chiefly in lava streams. It occurs in Iceland, at Portillo
in Teneriffe, and in Transylvania and Hungary, as well as in the
Auvergne ; also in North and South America, Victoria, Australia,
and Java.6
Phonolite > occurs occasionally in the form of lava-flows, but
more commonly in conical masses or hills. It sometimes exhibits
well-marked columnar structure, and has a very general tendency
to split into slabs or slates, the more finely cleavable varieties
being used for roofing purposes in certain localities. In advanced
stages of weathering the rock passes into an earthy condition.16
Uses. — In Italy, Auvergne, and the Rhine district lavas of
closer texture have been employed in building ; but their main
use now, as in former years, has been as materials for streets and
roadways.11
Section II. — Sandstones, Limestones, and Argillaceous
Rocks.
Weathering Properties of Sandstones and Limestones (see
Chapter VII., Section IV.). — The decomposition of stones employed
for building purposes is greatly influenced as well by the chemical
and mechanical composition of the stone itself and by the nature
of the aggregation of its component parts as by the circumstances
of exposure. The oolitic limestones will thus suffer unequal
decomposition unless the brittle, egg-shaped particles, and the
cement with which they are united, be equally coherent, and of
the same chemical composition. The shelly limestones, being
chiefly formed of fragments of shells, which are usually crystalline
and cemented by a calcareous paste, are unequal in their rate of
decomposition, because the crystalline parts offer the greatest
resistance to the decomposing effects of the atmosphere. These
shelly limestones have also, generally, a coarse laminated structure
parallel to the plane of stratification, and, like sandstones formed
in the same way, they decompose rapidly when used as flags,
where their plane surfaces are exposed ; but if their edges only
are laid bare, they will last for a long period.
Sandstones, from the mode of their formation, are frequently
laminated, and more especially so when micaceous, the plates of
mica being generally deposited in planes parallel to the beds.
288 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
Hence, if such a sandstone, or shelly laminated limestone, be
placed in a building with the planes of lamination in a vertical
position, it will decompose in flakes, more or less rapidly, accord-
ing to the thickness of the laminae ; whereas, if placed so that the
planes of lamination are horizontal, that is, as in its natural bed,
the edges only being exposed, the amount of decomposition will
be altogether immaterial. The sandstones being composed of
quartzose or siliceous grains comparatively indestructible, they
are more or less durable according to the nature of the cementing
substance ; while, on the other hand, the limestones and magnesian
limestones are durable in proportion rather to the extent in which
they are crystalline, those which partake least of the crystalline
suffering most from exposure to atmospheric influences.
The chemical action of the atmosphere produces a change in
the entire matter of limestones, and in the cementing substance
of sandstones, according to the amount of surface exposed. The
mechanical action due to atmospheric causes occasions either a
removal or a disruption of the exposed particles ; the former by
means of powerful winds and driving rains, and the latter by the
congelation of water forced into, or absorbed by, the external
portions of the stone. These effects are reciprocal, chemical
action rendering the stone liable to be more easily affected by
mechanical action, which latter, by constantly presenting new
surfaces, accelerates the disintegrating effects of the former.18
Brandts Test. — To determine the weathering properties of
stones ; especially adapted to oolites and other calcareous rocks.
Cannot be applied with any certainty to other rocks.
1. Several specimens should be selected from a block of stone
to be tried, taking, for instance, those which present differences
of colour, grain, or general appearance.
2. These fragments should be cut into 2-inch cakes, with
sharp edges, and each marked carefully.
3. A saturated solution of Glauber's salt (sulphate of soda) is
then to be boiled and the cubes submerged, and retained in the
boiling liquid for half an hour. If a longer period elapse the
effects exceed those of ordinary atmospheric action and frost.
4. The specimens are then withdrawn and hung up in the air,
and beneath each is placed a vessel containing a quantity of the
solution in which it has been boiled, care being taken that it
contains no fragments of the stone detached during the boiling.
5. If the weather is not too wet or too cold it will be found
that the surface of the stones, twenty-four hours after they have been
suspended, are covered with small white acicular crystals of salt.
When these appear, the cubes are to be plunged into the vessel
below them, to get rid of the efflorescence ; and this is to be done
SECT. II.] BUILDING-STONES. 289
repeatedly, as often as crystals of the salt are thrown out during
the experiment.
6. If the stone resist the decomposing action of damp and frost,
the salt does not force out any portions of the stone with it, and
neither grains, laminse, nor other fragments of the stone are
found in the vessel. If, on the other hand, the stone yield to
this action, small fragments will be perceived to separate them-
selves, detached, even from the first appearance of the salt, and
the cube will soon lose its angles and sharp edges. The cubes are
weighed at the end of the experiment and the difference noted.
The experiment should last four days.13
SANDSTONES.
Lithological Character. — These rocks consist essentially of
grains of silica. They either occur as superficial accumulations
of loose sand forming desert tracts, or low-lying districts on sea-
coasts, where the wind piles the sand up in dunes ; or they may
occur as beds of loose sand, interstratified with coherent beds of
rock. They are also met with in a state of more or less imperfect
consolidation, the grains being feebly held together by an iron
oxide or by calcareous matter ; or they may be excessively hard
and compact, the constituent grains being cemented by either
silica, carbonate of lime, iron oxides, or carbonate of iron. In
some few cases there even appears to be, according to Professor
Morris, no cementing matter present, as in some of the New Red
Sandstones, the constituent grains being apparently held together
merely by surface cohesion superinduced by pressure.
Grits. — The rocks called grits vary considerably in lithological
character. The term " grit " appears indeed to be very ill-defined.
The Millstone Grit, which may be taken as one of the leading
types, is more or less coarse-grained, while some of .the Silurian
rocks, such as the Coniston and Denbighshire grits, are frequently
very fine-grained and compact in character. Under these
circumstances it seems that a grit may best be defined as a
strongly coherent, well-cemented, or tough sandstone, usually, but
not necessarily, of coarse texture.16
Colour and Texture. — Sandstones appear in all colours — white,
black, grey, greenish grey, red, brown, fawn-coloured, and yellow ;
and these colours sometimes fade, and sometimes become intensified
by exposure to the weather.
In texture they occur in every degree of fineness from particles
scarcely perceptible to the naked eye to grains as large as a pea —
in other words, from fine-grained soft sandstones to coarse-grained
siliceous grits ; but see above as to grits.
290
GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
Composition. — As mixed rocks sandstones consist of several
ingredients, and, as the case may be, are spoken of as siliceous,
quartzose, micaceous, calcareous, argillaceous, ferruginous, bituminous,
carbonaceous or, if derived from the decomposition of felspathic
rocks, felspathic.
In chemical composition the sandstones vary extremely, and
no two strata even from the same quarry will yield perhaps the
same results. The following are analyses of some well-known
varieties, as given in the Report of the Commissioners for the
selection of stone for the new Houses of Parliament. Other
analyses are given at the end of this section : —
TABLE XII. — ANALYSES OF SANDSTONES.
®
3
g
a
1
|
T3
3
•
>,
"ti
«
1
1
*§
W
o
M
o
0
P
^
Silica .
98-3
95-46
96-40
95-1
93 1
49-4
Garb, lime .
1-1
•61
0-36
0-8
2'9
26-5
Garb, magnesia
Iron alumina
0-0
0-6
•69
2-04
o-o
1-30
o-o
2'3
o-o
4-4
16-1
3-2
Water and loss
0-0
1-2
1-94
1-8
0-5
4-8
Specific gravity
2-23
1-96
2-62
2-29
2-24
2-23
In specific gravity the sandstones and grits vary from 2 to 2'6 ;
in weight per cubic foot from 130 to 160 Ibs. ; in absorbent
power from 1 to 11 Ibs. of water per cubic foot, sandstones of
ordinary softness and porosity absorbing from 5 to 6 Ibs. ; in
crushing weight from so low a figure as 500 Ibs. to 14,000 Ibs. for
the cubic inch.11
Selection for Building. — Many of the sandstones, from their
softness and rapidity of disintegration when exposed to the
weather, are altogether unfit for building, while others are so
hard and siliceous as to be better adapted for road metal than for
building.
In selecting sandstones, the finer-grained, the more homogeneous
in texture, the least absorbent of water, and the freest from lime
and iron should be preferred. All blocks containing balls or
nodules of sulphide of iron (iron pyrites) should be carefully
rejected, as in a few years such nodules oxidise, become blackish
brown with unsightly stains, and finally weather out into cavities.
The builder cannot have a better test of the durability of a sand-
stone than by observing it in the face of exposed cliffs and old
SECT. II.] BUILDING-STONES. 291
quarries ; its absorbent nature he can test by experiment ; and
in the case of a new variety he may subject it to Brand's pro-
cess n (cf. p. 288). ^
Cambrian and Silurian. — The grits are for the most part very
tough, closely compacted sandstones, frequently containing minute
fragments of felspars and sometimes scales of mica. Their con-
stitution implies that they were formed, at all events to some
extent, from the detritus of pre-existing eruptive rocks. They are
generally traversed by numerous joints, so that they are seldom
used for building purposes, except locally in the construction of
rough walls. They are, however, well suited for road metal, and
in some places good flagstones are quarried, but these are, for the
most part, rather to be regarded as sandy shales and slates than
true sandstones. The flaggy sandstones are generally micaceous,
and to this circumstance their fissile character is often due.16
Old Red Sandstone. — Both building stones and flagstones are
quarried. They are mainly employed in the districts where the
stone is procured. It is often of a deep reddish-brown or purple
colour, owing to the presence of peroxide of iron ; at other times
it is greyish or yellowish, occasionally with a greenish tinge. The
stone, if judiciously laid, is generally durable; but in some old
buildings, such as Chepstow Castle and Tintern Abbey, it has
suffered considerably from the weather.16 The flagstones of
Caithness are well and widely known, and many of the Perth and
Forfar rocks form good and durable material for building, and the
same may be said of those of Cork and Kerry. The main objec-
tion to them is their dull rusty-grey tints, and the frequent
embedding of pebbles or nodules of foreign matter. As they are
tough and strong, however, and can be raised in blocks of any size,
they are well fitted for harbours, sea-walls, and heavy structures,
as may be seen in the docks of Dundee.11
Carboniferous. — The sandstones including those of the Yoredale
series, the Millstone Grit, and the Coal Measures afford good
material for building and paving.16 The building-stones of the
Millstone series occur either as coarse massive grits, finer siliceous
grits, or flaggy sandstones, suitable respectively for foundations,
bridges, piers, engine beds, ordinary building-stones, paving and
flagging. The stone is generally hard, durable, and of greyish or
light-brown colours, and is used in many parts of the north of
England to a large extent, notably at Bramley Fall near Leeds.
The stone is admirably adapted for resisting the effects of the
smoky atmosphere of the large manufacturing towns, as very
little lime enters into its composition.
In Scotland Carboniferous sandstones occur low down in the
series, taking the place of the Mountain Limestone of England.27
292 GEOLOGY FOR ENGINEERS. [FT. V. CH. XIII.
The fine-grained pale-brown and grey sandstones from Craigleith,
near Edinburgh, and the Binnie quarry in Linlithgowshire are
extensively employed for buildings ; 16 the whitish sandstones of
Glasgow and the yellower sandstones of Stirlingshire are very
durable; while those of Fifeshire are softer, but harden on
exposure to the weather.11
In Ireland yellowish and reddish sandstones capable of pro-
ducing a good building-stone are distributed throughout parts of
Londonderry, Tyrone, and County Antrim.27 The Carlow flags
are perhaps the most important sandstones of Ireland ; they are
sometimes more or less micaceous and are of dark-bluish or grey
colour.16
The Gannister Beds produce excellent flagstones known as
Yorkshire flags, which are generally micaceous, evenly bedded, and
parted by bands of shale.27 Some of them absorb water readily ;
consequently, in very exposed and damp situations they are liable
to flake, particularly if placed in positions where they are unable
to part with their moisture.16
The sandstones of the Coal Measures are generally of a more
destructible nature, containing as they do more argillaceous
matter, as well as iron, than is the case with those just described.
They are also rather softer, generally of purple, yellow, or
greyish colours, and are very apt upon exposure to become iron-
stained. The Pennant Grit Sandstones, however, of Somerset-
shire and South Wales more nearly resemble those of the Millstone
Grit.27
Permian. — The sandstones are but little used in England,
except locally, for building-stone, as in some parts of Cumberland,
Staffordshire, Nottinghamshire — the reddish - brown and almost
white varieties at Mansfield are said to be durable — and Yorkshire.
As a rule the Permian sandstones are not well suited for building,
being very absorbent and liable to decay. These rocks have
mostly a deep red colour, due to the presence of peroxide of
iron, which together with dolomitic matter constitutes their
cement.16
Triassic, — Those belonging to the Upper Trias or Keuper are
the most important, the Lower Keuper Sandstones being extensively
used in the midland and north-western counties of England. It
is of pale red, brown, and yellow colours, sometimes almost white,
and is mostly fine-grained and easy to work. This stone has
been largely used in the cathedrals of Chester and Worcester.16
It is not, however, comparable to the Carboniferous Sandstone
in texture and durability.11
The sandstones of the Lower Trias or Bunter are, as a rule, too
loosely cemented and friable in character for building purposes,
SECT. II.] BUILDING-STONES. 293
but are useful for moulds in foundries. They are often variegated
and mottled and frequently exhibit false bedding 16 (cf. p. 38).
Jurassic.— The rocks of the Jurassic period are for the most
part limestones, but good sandstone is quarried at Aislaby near
Whitby 16 from the Lias, which is used not only in the locality
but at London, Cambridge, and other towns.11 In Lincolnshire,
Northamptonshire, and Dorsetshire sandstone belonging to the
inferior Oolite is employed for building.16 A hard and fine-
grained calciferous sandstone is found at Tisbury in Wiltshire.11
Cretaceous. — Those of most importance belong to the Hastings
sand series. This sand-rock is not a very coherent stone when
first dug, but it hardens on exposure and is used locally for
building, though it is not very durable. A calcareous sandstone
occurs at Godalming in the upper part of the Hythe beds. The
rubbly sandstones in these beds are termed "hassock." The
Folkestone beds of the Lower Greensand afford hard sandstone
and grit suitable for building and road-making. In the Upper
Greensand, at Godstone and Merstham, a pale calcareous sand-
stone called fire-stone occurs, which is well suited for the floors
of furnaces and is also a durable building-stone.
Tertiary. — Although, in England, beds of sand are of constant
occurrence in the Tertiary formations, they are not, as a rule,
sufficiently coherent to be of value for building purposes, except
for making mortar. There are, however, a few very hard sand-
stones in the Woolwich series and Bagshot beds which are used
for building and paving. In some parts of the world Tertiary
sandstones attain great importance.16
LIMESTONES.
Lithological Character. — Limestone of several varieties is
largely employed as a building material. These varieties depend
very much on differences of origin and composition, and correspond
to successive geological periods. Amongst the oldest formations,
limestones are comparatively rare, at least in the British Isles and
Europe, but in each successive period they gradually assume a
higher importance. This gradual augmentation in volume, as
compared with the associated sedimentary strata consisting of
various forms of sand or clay, appears to be intimately connected
with the development of those classes of marine animals which
form for themselves calcareous shells or skeletons by the vital
process of assimilation ; by which the calcareous matter dissolved
in the waters of the ocean by carbonic acid is seized upon, and
converted into the stony skeletons of the inhabitants of the
deep.27
294 GEOLOGY FOR ENGINEERS. [PT. V. CH. XITI.
Qualities. — A family consisting of such members as chalk,
oolite, dolomite, compact limestone, and crystalline marbles must
necessarily vary much in density, absorption, and resistance to
pressure ; and hence such experiments as have been made must
be received as applicable only to the rocks to which they relate.11
In structure the limestones are often jointed, and incapable of
being raised in large blocks ; in texture they vary from earthy to
compact and subcrystalline, but, owing to their organic origin,
uniformity of texture is frequently interrupted by the remains of
corals, shells, encrinites, and other exuviae. Many, however, of
the Devonshire, Derbyshire, Yorkshire, and Westmoreland lime-
stones are thick-bedded and homogeneous, and can be raised in
blocks of great size and solidity.11
Marbles. — Any rock susceptible of a fine polish is termed
"marble" by the stone-cutter; hence we hear of "Connemara
marble," which is a true serpentine; and of "Sicilian marble,"
which is often a brecciated lava. The term, however, should be,
and is, restricted by geologists to limestones capable of receiving
a polish, and frequently exhibiting a variety of colours in veins
and blotches. We have thus uni-coloured marbles, such as pure
blacks and whites ; and parti-coloured sorts, deriving their tints
from accidental minerals, from metallic oxides, giving them a
veined or clouded appearance, or from shells, encrinites, corals,
and other organisms which impart a variety of " figure " as well
as of hue.
The following are a few of the better-known and more esteemed
varieties, ancient and modern : — Carrara, pure white, saccharoid,
and semi-transparent ; highly esteemed for statuary purposes.
Parian, of a waxy cream colour, also crystalline and employed in
statuary. Giallo-antico, yellow and mixed with a small pro-
portion of hydrate of iron ; used for ornamental purposes.
Sienna, a rich yellowish-brown, with lighter veins and cloudings.
Rosso-antico, a deep blood-red, more or less veined. Mandelato, a
light red, veined and clouded. Verde Antique, a cloudy green,
mixed with serpentine, or serpentine itself. Cipolino, a mixture
of talcose schist with white saccharoidal marble. Bardiglia, a
bluish-grey variety with bold black veins and cloudings. Luna-
chello or fire-marble, a dark-brown variety, having brilliant
chatoyant reflections, which it owes to the nacreous matter of
enclosed shells. Black marbles like those of Derbyshire, Dent,
and Kilkenny, deriving their dark colours from bitumen.
Encrinal marbles, like those of Dent in Yorkshire and other
Carboniferous districts, deriving their " figure " from the stems
and joints of encrinites. Shell marbles, like those of Purbeck and
Petworth in Dorset and Sussex, and Kingsbarns in Fife, receiving
SECT. II.] BUILDING-STONES. 295
their "figure" from the component shells of univalves and
bivalves.
The marbles are among the most varied and useful of rocks
whether for external structures or for internal decoration. They
are sufficiently durable in dry and pure atmospheres; can be
raised, for the most part, in blocks of any size ; and are easily
tooled and polished. As building-stones they are unsuited to our
climate ; hence their use is chiefly for interior decoration.
Statuary marbles of the finest hue and texture are brought from
Italy and Greece (Carrara and Paros), as are also many of the
parti-coloured varieties for internal decoration. Some beautiful
marbles are also obtained from Belgium and France, but several
useful sorts are derived from the formations of our islands, as
shown below.11
Archaean. — The limestones and marbles of Archaean age are
found chiefly in the Scottish Highlands, and are usually greyish
crystalline varieties, or bluish- and greenish-veined varieties. None
of them are used as building-stones, but only for mortar and
agricultural purposes.11
Silurian. — Developed chiefly in Wales and of comparatively
little value, except for mortar and agricultural purposes.11
Devonian. — Mainly restricted to Devonshire. The calcareous
beds of the Old Red Sandstone proper are limited and irregular,
often siliceous and concretionary, and seldom quarried, unless on
a very small scale for mortar and agriculture.11 The Devonian
limestones are, however, extensively used for building and paving,
and some of them are well adapted for ornamental purposes on
account of the richly coloured mottling and veinings which they
frequently exhibit.16
The South Devon marbles, which are worked at Plymouth, St
Mary's Church, Babbacombe, Totnes, Newton Bushel, and other
places, are of various shades of grey, with veins of white and
yellow, occasionally reddish or flesh-coloured, with deeper veinings,
and not unfrequently coralline or "madrepore." The North
Devon marbles, though not so extensively quarried, present some
useful varieties, having a black ground irregularly traversed with
bold white veinings.11
Carboniferous. — In England the limestones of this system are
largely developed both in thickness and extent, comprising the
main portion of the Lower division of the series. Several of the
limestones are used as ornamental marbles, notably the black
marbles of Ashford, Matlock, and Dent, the brown of Bake well,
the encrinal of Dent, and the grey-shelly and encrinal of Poolwash ;
the great bulk of them are quarried for the blast-furnace, for
mortars, cements, agriculture, road-making, bleaching, tanning,
296 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIIL
gas purification, and other industrial purposes; while only a
small proportion is raised for building. They vary extremely in
composition — some containing upwards of 90 per cent, of
carbonate of lime, with minor proportions of silica, alumina, and
oxide of iron ; some containing from 10 to 15 per cent, of
carbonate of magnesia and passing into dolomites ; and others
embodying such a large proportion of silica and alumina as to
pass into cherts and hydraulic limestones. The unattractive
colours of these mountain limestones, and the difficulty of tooling
them, is against their wider adoption ; but many of them make
strong substantial structures, and would be more generally
employed were it not for the abundance of available sandstones
with which they are associated in Carboniferous districts.
In Scotland the beds are thin and irregular and the stone is
employed for the manufacture of mortar or smelting iron ores —
there being no other calcareous strata — and is far too valuable to
be used for building.
The Carboniferous Limestone occupies the greater part of the
central plain of Ireland and has been largely used both in the
ancient and modern buildings of this region. The Lower and
Upper divisions produce a good crystalline greyish limestone,
sometimes dolomitic, and in a few instances oolitic ; but the
Middle or Calp division produces a dark carbonaceous or earthy
grey limestone which is liable to rapid decay.27 Several excellent
marbles occur : black in Kilkenny and Gal way ; grey, coralline,
and encrinal in Cork, King's County, and Tipperary ; reddish and
variegated in Armagh ; red and mottled in Limerick ; and other
veined and mottled varieties in several other counties.11
Permian. — The limestones are mainly magnesian — that is,
consist of carbonates of lime and magnesia, with varying pro-
portions of silica, alumina, and iron. If the silica is in excess
they become calcareous sandstones, generally of hard and close
texture ; but when it constitutes only a small percentage, they
are regarded as magnesian limestones. Many limestones in other
formations contain small amounts of magnesia, but only those
containing above 15 or 18 per cent, are entitled to the name of
" magnesian." These limestones derive their warm yellowish
tints from the oxide of iron, assuming deeper tints as that
ingredient prevails. In specific gravity they vary from 2 to 2 '6 6,
are much more absorbent of water than the sandstones, weigh
from 128 Ibs. to 152 Ibs. a cubic foot, and in the more crystalline
varieties withstand a considerable crushing power.
In England they occupy considerable areas in Durham,
Yorkshire, Derby, and Notts, and appear in many varying beds
(earthy, laminated, compact, concretionary, and crystalline). In
SECT. II ] BUILDING-STONES. 297
Durham they are seldom used as building-stones. In Yorkshire
they are employed in various structures with varying results. In
Derbyshire the Bolsover Moor stone employed in the new Houses
of Parliament has proved to be of varying quality — some wasting
and becoming worthless, and others being fairly durable. The
celebrated quarries of Mansfield in Notts yield a siliceous dolomite
of hard, close-grained texture and enduring quality.
Texture and Durability. — The Commissioners on stone for the
Houses of Parliament concurred in stating that in proportion as
the stone is crystalline does it appear to resist the decomposing
effects of the atmosphere ; and Professor Daniell observed that
the nearer the composition of magnesian limestones approaches to
equivalent proportions of carbonate of lime and carbonate of
magnesia, the more crystalline and better they are in every
respect.27
Few rocks vary so much in texture and durability as the
magnesian limestones of England. In the same quarry, beds of
tried excellence are frequently associated with others which look
as well, but are worthless ; hence the skilled and watchful care
that is requisite in selection. It is not only that they differ in
composition — the magnesia ranging from 45 down to 10 per cent,
and under — but that they vary in textural aggregation from hard,
compact, and crystalline beds to others that are so soft and earthy
as to yield readily to the nail.
Permian limestones do not occur either in Scotland or
Ireland.
Jurassic. — The Liassic limestones are argillaceous, and are
burnt for hydraulic lime16 (see Chapter XIV., p. 318).
The Oolitic limestones are so numerous and constitute such
valuable building-stones, that it is only possible to mention a few
of those principally employed.16 They occur in four series of the
system.
The Inferior Oolite, which is largely developed in the Cotswold
hills, yields some fine-grained compact white or yellow beds.
The Bath Oolite is still more largely quarried along the
Somerset and Wiltshire hills, and yields a fine, close-grained,
whitish stone which can be raised in blocks of any size, and
though soft enough when first extracted to be cut with the saw,
yet soon hardens on exposure. As this zone trends eastwards
through Oxfordshire, Northamptonshire, and Lincolnshire it
assumes browner and richer tints n — the Ketton stone in the
latter county being an exceedingly valuable building-stone,
possessing great tenacity, working freely and resisting atmospheric
influences, even when placed in unfavourable situations. The
Ancaster stone is also very durable.16
298 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
The Coralline Oolite, being inferior in texture and durability,
is seldom used as a building-stone.
The Portland limestone has been long and largely quarried,11
and constitutes one of the most important building-stones of the
country.
The Purbeck limestones, which, unlike the preceding, are of
fresh-water origin, have been used for paving ; while in the
upper part of the series a compact limestone is known as Purbeck
marble and has been used for architectural decoration for some
centuries.16
Texture and Composition. — They vary in texture from compact,
small-grained roe-stones to pea-stones, and from pea-stones to
coarse-grained shelly and coralline rag-stones. They differ in
structure from the limestones of older and more recent date,
in that they are usually aggregates of little spherical deposits
of carbonate of lime, which have formed in concentric crusts
round nuclei. These nuclei consist sometimes of a granule
of sand, sometimes of the remains of a minute organism. The
little spherules are seldom much bigger than a pin's head, and
they are also cemented together by calcareous matter. Oolitic
structure is not exclusively peculiar to limestones of Oolitic age,
for it occurs in certain beds of Carboniferous limestone near
Bristol, while it is also developed in the coarser pisolites or
pea-travertines of recent date.16
In specific gravity the oolites vary from 2 to 2*5 ; a cubic foot
weighs, when dry, from 125 to 150 Ibs. ; when dry they absorb
from 8 to 10 per cent, of their weight of water, and in composition
they are nearly pure carbonates of lime with minor proportions of
carbonates of magnesia, silica, and iron.
Durability. — When carefully selected and not exposed to the
carbonated atmosphere of towns, many of these limestones are of
fair durability ; but even the best of them are not to be compared
in this respect with the siliceous grits and sandstones.11
Cretaceous. — Kentish Rag, which is derived from the Hythe
beds, is mostly a very hard sandy limestone, and contains more or
less dark-green glauconite, generally in fine, occasionally in
coarse, roundish grains. Glauconite is stated to sometimes form
the cementing medium in these rocks, but more or less carbonate
of lime is always present in this capacity. By decomposition the
protoxide of iron in the glauconite is converted into peroxide of
iron, and the rock, under these circumstances, assumes a reddish-
brown tint. According to Ehrenberg, the glauconite grains often
fill, invest, or replace the tests of foraminifera. These rocks form
very durable building-stones.
Limestone, either as ordinary chalk or as subordinate beds of
SECT. II.] BUILDING-STONES. 299
compact limestone, represents a considerable part of the Cretaceous
series of rocks, while most of the Cretaceous sandstones are very
calcareous. The chalk attains a great thickness in some parts of
the kingdom ; the lower portion, termed the Grey Chalk or Chalk
Marl, is generally glauconitic at the base. The Upper Chalk con-
tains numerous nodules, and occasionally bands of flint, which
follow the stratification, although at times vertical bands of flint
occur filling up what once were open fissures. Chalk, besides
being largely burnt for lime, is also locally used for building.
Certain hard beds occur in the chalk which are better suited for
this purpose than the softer material.16
Tertiary. — In the British Isles these are but poorly repre-
sented. The Binstead limestone, occurring in the Bembridge
beds in the Isle of Wight, has, however, been extensively
quarried, and has been employed in the construction of some of
our early churches. In other parts of the world Tertiary
limestones often attain great thicknesses, and constitute impor-
tant building-stones. The Pyramids, for example, are built of
Nummulitic limestone.16
ARGILLACEOUS ROCKS (SLATES, SHALES, AND CLAYS).
Lithological Characters. — These rocks are, chemically speaking,
impure hydrous silicates of alumina. Sometimes the impurity
consists of sand, sometimes of carbonate of lime ; and more or
less carbonaceous matter is in many cases present. Their
coarseness of texture is mainly dependent upon the coarseness of
the sand which often occurs in them. When free from sand,
they are usually of fine texture. They have all originally been
deposited as mud, in most instances at the bottom of the sea,
in others at the bottoms of lakes or as deltas, and exceptionally
over land, when temporarily flooded by the overflow of rivers,
as in the case of the Nile. Clay deposits often have a well-
laminated structure, and, in the older geological formations,
have assumed a more or less indurated character, frequently
accompanied by a tendency to split along the planes of bedding.
Very often another and more strongly marked fissile structure
is superinduced in directions cutting across the planes of
stratification at various angles. This is slaty cleavage, described
in Chapter VI., Section III. (cf. p. 103). Those argillaceous rocks,
which split parallel with the planes of lamination or bedding, are
called shales or flags, but the term flag is applied to a rock of
any character which splits along its bedding into large flat slabs,
and consequently it is common to find the term used to denote
300 GEOLOGY FOR ENGINEERS. [PT. V. CH. XIII.
sandstones which are sufficiently fissile, when quarried, to yield
slabs or flags.
Slate. — To the argillaceous rocks which split in directions
other than that of bedding, the term slate is given. Still, in
this case the term is also applied to rocks which differ widely
from ordinary slate. The Collyweston slates, calcareous sand-
stones of the inferior Oolite, and the green slates of the Lake
District, which have been mapped as volcanic ash by the
Geological Survey, are examples of the application of the term
slate as indicative of fissile structure, and not of lithological
character.16
Qualities. — A good slate is little absorbent of water, cuts
freely but toughly, weighs from 160 to 180 Ibs. per cubic foot,
and should resist a crushing weight of from 20,000 to 25,000 Ibs.
For thinness, lightness, and straightness the Welsh slates are
unequalled, but the Irish and the Lake District varieties are
harder, heavier, tougher, and more durable ; while for strength
and solidity the Scotch are perhaps superior to either, but
sometimes contain iron pyrites.11
The best slates are obtained from various parts of North
Wales, near the coast; from Delabole, Tintagel, and elsewhere
on the north coast of Cornwall ; from various parts of Cumber-
land ; and from the west coast of Scotland, generally from
quarries of great magnitude. The best slate slabs are from
Wales. The finest slabs and flagstones (not argillaceous) are
from Yorkshire and Caithness (see Sandstones). Excellent foreign
slates are obtained in France, chiefly from near Angers, and in
Brittany ; in Belgium from the Ardennes ; in Western Germany
from the Duchy of Nassau ; and in the east of Europe from other
places. Slates and slabs are also found in America.13
Cambrian. — The Cambrian slates are very important rocks,
affording compact roofing-slates of admirable quality, mostly of
a dark purple or greenish colour, and capable of being split into
very thin and large slates exceedingly free from pyrites, which is
common in many slates, but, from its decomposition, is most
detrimental to them as roofing material. The slates of the
Penrhyn and Bangor and of the Din or wig or Llanberis quarries
in North Wales are of Cambrian age.16
Silurian. — The Skiddaw (Lower Silurian slates of Cumberland)
are black, or dark-grey rocks, which are often traversed by many
sets of cleavage planes, causing them to break up into splinters
or dice, so that no good roofing-slate can, as a rule, be procured
from them. The best Lower Silurian slates of North Wales are
quarried in the Llandeilo and Bala beds. They are black, dark
grey, and pale grey. Ffestiniog, Llangollen, and Aberdovey are
SECT. II. J BUILDING-STONES. 301
among the principal quarries. The cleavage in these rocks is
often wonderfully perfect and even, so that occasionally slates
10 feet long, 6 inches or a foot wide, and scarcely thicker than a
stout piece of cardboard, are procured. These remarkably thin
slates are tolerably flexible. The Upper Silurian rocks also
afford good slates and flags in certain localities, while the rough
material serves for local building purposes. Silurian slates are
quarried in Scotland in Inverness-shire, Perthshire, and Aberdeen-
shire ; also at Killaloe and some other localities in Ireland™
Devonian. — Slates of a grey colour are worked in Cornwall,
at the Delabole and Tintagel quarries, and in Devonshire in the
neighbourhood of Tavistock, at Wiveliscombe and Treborough in
Somersetshire, and in other parts of the United Kingdom.16 The
slates of Valencia in Ireland somewhat resemble those of Killaloe,
but have a greener tinge.27
Carboniferous. — Flags are quarried for roofing and paving
purposes at several places in Yorkshire, Lancashire, and other
counties where Carboniferous rocks occur, and are mainly pro-
cured from the Coal Measures. They are of dark-grey colour or
black, and are principally used in the neighbourhoods where they
are quarried.
There are no true clay slates of later age in Great Britain, but
in other parts of the world slates of even Tertiary age occur.16
Selection of Quarry. — It is not usual to find slates and slabs
in good condition near the surface, where long exposure to the
weather has usually disintegrated and even destroyed the
texture, and often, by partial hardening, obliterated or obscured
the cleavage. As it is, however, entirely from the superficial
rock and its geological condition that a judgment must be
formed, a certain amount of experience, combined with a know-
ledge of the material, enables the geologist to judge well of the
chance of a valuable quarry. Uniformity of texture and con-
dition of the rock for considerable distances, the nature and
condition of the cleavage, the direction of the cleavage-planes,
the nature of the small veins of other material pervading the
slate (of which there are always many), the presence or absence
of iron pyrites, the direction and magnitude of the joints — these
are the chief points concerning which careful investigation is
necessary. But any or all of these are altogether insufficient to
communicate value to a property unless the essential point of
cheap and ready conveyance to a large market can be secured,
and the quarries are so situated that the waste can be disposed
of, and the valuable part of the slate laid bare without great
expense, 12
302
GEOLOGY FOR ENGINEERS.
TABLE XIII. — ANALYSES OF SANDSTONES, LIMESTONES,
AND CLAY-ROCKS.
brian Grit,
irmouth.'
roredale
ndstone.6
.stone Grit
ndstone.6
,1 Measure
ndstone.6
iter Sand-
stone.6
imian Sand-
Hunstanton.6
Sandstone
r), Shiffnall.6
e Sandstone
3), Horderley.6
1
g -~
r>
5 a
o
3
o
u->
""3* °
^
s
rS
pq
8 a
K3
CO
PH 5
PQ
Ij
Silica
80-60
75-75
87-40
85-55
87-15
49-81
96-31
92-49
9775
Alumina .
9'20
8-22
3-99
7-57
3-94
5-17
0-80
2-47
)
Ferric oxide
trace
10-52
...
...
1-35
29 17
1
Y 0-50
Ferrous oxide .
2'37
1-35
l'-36
1'91
0-35
h 1*30
4*62
J
Ferric persulphate .
Carbonic anhydride .
1-02
0-30
1-87
075
o;'io
1-20
Sulphuric ,,
trace
0-17
0-06
trace
0-09
Phosphoric ,,
0-07
0-15
trace
0-07
trace
0-42
...
...
...
Manganese oxide
0-23
trace
0-27
Lime
1-33
0-53
1-93
0-58
2-68
2-43
6-35
Magnesia .
1-28
0-36
0-68
0-61
1-08
0-95
075
...
Potash .
1 64
1-05
0-74
0-91
1-27
0-48
Soda
T37
1-28
033
1-11
0-84
0-84
...
Water hygrometric .
,, combined
0 12
0-93
0-15
070
0-05
1-29
0-40
0-85
0-15
0-30
3'85
6'56
} 0-65
0-42
175
l«
*|
*
g
G g
.§
g
'8
fn Cft
3
•gJ|J
bo
a
| .
"1^
e
O °
u«_
••^
£
o
Chalk with flints
,—
J — ,
(Shoreham, Kent)4.
98-40
0-08
1-10
o-
42
100-0
Hydraulic limestone
1
(Kimmeridge)4
75-7
15-0
8-
2
1-1
100-0
Lithographic limestone
(Solenhofen)4 .
96-24
0-21
2-02
98-47
phosphate
carbonates
of lime
alkaline
Jurassic limestone
(Geneva)4 .
91-52
171
1-41
1-
58
3-77
BUILDING-STONES.
303
Specific
Gravity.
Crushing Load
per sq. ft. in
tons.
Absorption in
percentage of
is Dry Weight.
CO TJH •**• CO CO OO CO •<*
CO rH rH rH CO rH CO rH
CO CO CO CO CO CO rH CO
CO
Weight per
cub. ft. in Ibs.
0 0
«o : iis
CO • CO
Bitumen.
Water and
Loss.
rH iO iO i— I OO •«* CO
eo <p cp co rH p t~t^ os oo »p CO
00 rH rH CO CO CO CO fH rH CO COT*
Alumina and o £ £ £ £ oog^S: §?§ § co o co os
Iron. rH O rH rH 00 rHOOOCO
CM CO OS CO CO 03 O
Silica woscop. cpoooo
rHCOrHCOCO COCOOO^CO -'rH • ^OC« O
s*l v ^ CO CO CO ^i v£i t>» iO O O O O O rH
Carbonate ot ^tMco»prr' coco^cpt^cp oiursco r- 1 cot»rH co *•»
Magnesia corncooos orHosrHcoci cocorH -^ «>co* co
•«<J< Tfl CO Tfl CO Tji
Phosphate of . . ^
Lime. : b
Sulphate of . g= « ^
Lime.
/-(•i f COCOCOOO OS O»«OsCOCOt>.O
oaroonate 01 poo>posp rHrH>o^oo«p ipfT1 T1 ^P?5 *P °°
Lime »0^^»£>tO rHTtlt>.OCOrH C*0-*0 CO OSOSJO M<
OSOSOSOSCO lOiOOOCOiO OSOSOS OS i>.t^.OS rf< kQ
J
I
% J • • • • • • a
1 1 J
s||,
^rl ^ rl J"^ 'ig'^rSj'!^^'!^^ Jlg^H If J |
304
GEOLOGY FOR ENGINEERS.
TABLE XIII. — ANALYSES OF SANDSTONES, LIMESTONES, AND
CLAY-ROCKS — Continued.
Srf
*4
si
4
«
05
0
0 •§
O °
S B
|S,C
& a
o be
'S §
Alumin
II
o
||
pS
.§
JC
o
C8
"S '^
1
0
o
Crystalline limestone
•
(Carrara)4 .
98-1
0-9
I'D
100-0
x
Crystalline (Tiree)4 .
94-94
1-13
3-19
r
0-54
"*
99-8
Dolomite (Italian
Alps)4 .
58-4
44'2
fl'4
100 -C
Roofing- slate ( Wai es)4
Roofing-slate (Camel-
ford)4 .
0'39
1-10
22-04
2-57
6-96
58-35
2-45
1-23
0-23
4-60
99-9
Cambrian Slates.6
Welsh
Slate.
Chiastolite
Slate, How
Gill.
Skiddaw
Slate, Red
Pike.
Silica
60-50
65-75
54-48
Alumina ....
19-70 14-18
2072
Protoxide of iron .
7-83
7-30
8-18
Peroxide of iron .
trace
0-98
Manganese oxide .
...
...
Lime .....
I'll
1-17
T62
Magnesia ....
Soda
2-20
2-20
2 34
• 1-98
1-94
6-21
Potash
3-18
3-26
3'20
Sulphuric acid
0-29
Water and loss
3-30
3-73 2-06
CH. XIV.]
CHAPTER XIV.
BRICKS AND CLAYS.
CLAYS.
IN the common acceptation of the word, clay is used to denote
any earthy substance which can be worked up with water into
a plastic mass, that is, a mass which may be pressed into any
form and will retain the shape given to it. It is also generally
understood that it will retain its shape when dried by heat,
though this is very imperfectly the case with many of the
substances that would be called clays in common parlance. A
clayey substance will often hold together as long as it is damp,
but falls to powder when all the water is driven off.7
Kaolin and Felspathic Mud. — The clay of ordinary language
includes two substances of totally different character. The one
is kaolin or china clay (see Chapter V., p. 85) ; the other clayey
substance is composed of felspar (p. 76) or a felspathic mineral re-
duced to a very fine powder, but not decomposed. It is eminently
clayey in many of its properties ; so finely divided that when
mixed with water it takes days to settle to the bottom ; fairly
plastic, though seldom to the same degree as kaolin ; and it will
sometimes hold together moderately well when baked. But it
is anhydrous ; if it be first dried at 100° C. to drive off the
mechanically mixed water, it gives off no water in a closed tube
at higher temperatures. It is not a simple silicate of alumina,
but has approximately the same composition as the mineral from
which it was derived, a complex silicate of alumina, alkalies,
alkaline earths, and perhaps of other substances, as the case
may be ; and it has been produced not by the chemical decom-
position, but by the mechanical trituration of a felspathic
mineral. This substance may be called Felspathic Mud. Hence
clays may be divided into two classes, viz. — Clays (cf. p. 114), com-
posed essentially of kaolin with admixtures of other substances ;
Mudstones (cf. p. 116), composed essentially of felspar with mix-
tures of other substances. In practice a great many clayey rocks
305 20
306 GEOLOGY FOR ENGINEERS. [PT. V.
cannot strictly be placed under either of these heads, because they
contain both kaolin and felspathic mud in variable proportions.
Method of distinguishing Clay and Mud. — Kaolin and felspathic
mud are most certainly distinguished by analysis, but the
following method will often suffice : — The clay is elutriated
with water till all grains of sand or foreign matter are removed.
The residue is boiled in dilute hydrochloric acid to dissolve off
the coating of oxide of iron which colours the grains ; it is then
filtered and well washed; a small portion is pressed with the
blade of a knife into a thin plate with a sharp edge, and
dried. If the clay be kaolin, this plate will be infusible before
the blow-pipe ; if felspathic mud, its edges at least may be
rounded.
It may be here noted that it is the combined water which
gives kaolin its plasticity. If this be driven off by strong heat
the residue is no longer plastic. This water, for instance, is
expelled in the burning of bricks, and though powdered brick
will absorb a great deal of water, it is impossible to make it in
the least degree plastic by any amount of water. The degree of
plasticity seems to depend largely on the fineness of the
particles.7
Loam, Shales, Marl, etc. — Loam (cf. p. 115) is a mixture of clay
and sand, the latter being present in sufficient quantity to allow
of water percolating through the mass and to prevent its binding
together. Clayey rocks which split into layers along planes of
bedding are called Shale ; Bind, Blue-bind, Plate, Shiver are
other names applied by miners to the same rock. Shales con-
taining a sufficient quantity of iron pyrites are used for the
manufacture of alum and are called Alum Shales. When there
is a good deal of sand present, the rock is called Arenaceous or
Sandy Shale, or Stone-bind, or Rock-bind. These forms pass
gradually into argillaceous sandstones and common sandstone.
Shales stained dark by vegetable matter are called Carbonaceous
Shale, Bass, or Bait, When such shales contain sufficient bitu-
minous matter to be used for the manufacture of paraffin, they
are called Oil Shales. Such shales pass gradually into cannel
coal occasionally. The streak of oil shales is usually brown. Marl
(cf. pp. 1 15, 116) is a clay containing carbonate of lime ; if the rock
splits into plates, it is called calcareous shale or Marl Slate.
Balls and irregular lumps of clay, ironstone, and iron pyrites are
very common in clays and shales. Crystals of selenite are not
uncommon ; they are generally found in clays containing iron
pyrites and some calcareous matter. The oxidation of the iron
pyrites produces sulphuric acid, and this acts on the carbonate
of lime and produces sulphate of lime.7
CH. XIV.] BRICKS AND CLAYS. 307
British Clays. — Of the clays used in this country for economic
purposes may be mentioned the china clays or kaolins of Corn-
wall, which have been formed from the decomposition of the
felspathic constituents of granite ; the Watcombe clay, which
occurs in the Trias, and is now used in the manufacture of
pottery ; the calcareous Liassic clays, used for brick-making and
burning for lime and hydraulic cement; the various clays of
Oolitic and Neocomian age, some of which are used for brick-
making, etc. ; the Gault, the clays of the Woolwich and Reading
beds, and the London clay, all of which are used for bricks ;
the celebrated Poole clay, dug at Wareham, which belongs to the
Bagshot series, and is extensively used for pottery. The clays
of the Bovey beds, large quantities of which are annually shipped
at Teignmouth, afford good pottery-clays and pipe-clays. There
are also many brick-earths and clays of post-Tertiary age which
are extensively used for brick-making and other purposes. The
river-mud in the Medway and at the mouth of the Thames is
largely used in the manufacture of Portland cement, after being
artificially mixed with chalk and burnt.16
Colouring. — The varied colouring of clays (and other rocks) is
due to the presence of iron in various states of oxidisation, and
to organic matter. The latter colours the clay from light grey
to black. The former, in the state of anhydrous peroxide, im-
parts the deep reds which, on becoming hydrated, change to
bright yellow, while intermediate conditions and concentration
of the iron give shades of brown and purple. The grey clays so
largely developed as clunches and fire-clays in the Coal Measures
owe their colour, in addition to the presence of Carbonaceous
matter, to carbonate of the protoxide of iron in a fine state of
subdivision, and occasionally to the presence of finely divided
bisulphide of iron. In the white and light-grey clays iron occurs
principally in the form of carbonate of the protoxide. It has
also been shown that many clays contain a notable proportion of
titanic acid.4
Qualities. — All the clays are essentially hydrous silicates of
alumina, more or less mingled with mineral impurities, and
coloured by the presence of metallic oxides and organic matter.
Generally speaking, they are soft, sectile, and plastic, and emit,
when breathed upon, a peculiar odour, known as the clayey or
argillaceous. The majority are superficial deposits occurring in
estuaries, desiccated lake-sites, river-valleys, and upraised sea-
beds, or scattered over the surface as drifts or boulder-clays.
They are also found in Tertiary formations sufficiently soft and
plastic for the purposes of the potter and brick-maker; but in
the older formations, with the exception of some beds in the Lias
308 GEOLOGY FOK ENGINEERS, [PT. V.
and Oolite, they become more compact, and pass into the texture
and consistency of shales and clay slates.11
As sedimentary deposits, resulting from the waste and decom-
position of pre-existing rocks, clays occur in various states of
purity and plasticity — some being pure, unctuous, tenacious, or
long clays as they are termed, and others impure, meagre, and
short, or deficient in tenacity. Whatever their natural characters,
they are all improved by being dug in summer, laid out in heaps
of moderate thickness, and exposed to the action of air and frost,
during which they undergo a kind of fermentation or internal
decomposition. This ripening or tempering, as the workmen
term it, greatly improves their quality, and is no doubt the
result partly of chemical change, as the decomposition of lime,
pyrites, etc., and partly of mere mechanical disintegration.
Besides this mellowing most of the clays have to undergo various
processes of washing, crushing, pugging, and admixture, according
to the fabric for which they are intended — a clay fit for a common
brick being unfitted for a fire-brick, and a clay suited for common
or brown earthenware being altogether unsuitable for porcelain
or china.
Refractory Qualities. — Pure clay (silicate of alumina) is re-
fractory — that is, capable of resisting intense heat ; and one
essential requisite in a good clay is, that it should not contain
iron oxide, lime, or other alkaline earth in such proportions as to
render it in any degree fusible. According to the experiments of
E. Richters (1868), the refractory qualities of clay are least
influenced by magnesia, more so by lime, still more by oxide of
iron, and most of all by potash.11
Brick and Tile Clays are widely diffused. The thickest and
most extensive beds are the so-called "brick clays" (cf. p. 138) of
the Glacial or immediately post-Glacial period, and which are
generally fine in texture, and red, blue, yellow, or grey, according
to the rock formations from which they have been derived, or
with which they are associated ; but abundant supplies can also
be obtained from estuary silts, from the clays of the Tertiary
system, and occasionally from the outcrops of the argillaceous
beds of the older systems.11
Brick-clay of the better kind consists of a tolerably pure
silicate of alumina, combined with sand in various proportions,
and free from lime and other alkaline ingredients, of which there
ought not to be more than 2 per cent. The relative percent-
ages of silica and alumina do not seem extremely important, and
there is always a variable proportion of water present, which is
also of little consequence. It is clear that, for use, the clay must
be tolerably free from large stones and coarse particles ; and, as
CH. XIV.] BRICKS AND CLAYS. 309
the principal process of manufacture before burning consists in
mixing the clay with water and sand, or ashes, to a uniform
consistency, anything that would interfere with this process is
injurious.
A certain proportion of iron compound is commonly present, and
this, when the brick is burnt, usually passes into the state of per-
oxide and gives the brick a dark-red colour. Too large a quantity
of iron compound renders the brick liable to run into glass in the
kiln.13
FIRE-CLAYS, FIRE-BRICKS, ETC.
Fire-clays derive their name from their highly refractory or
infusible nature — a property they possess from their containing
little or no lime, protoxide of iron, or alkaline earths, that would
cause them to yield to intense temperatures. Unlike the other
clays, which are mainly superficial deposits, the fire-clays are
obtained from the Coal Formation, where they occur in beds from
1 to 5 feet in thickness, and for the most part as the floors
or " under-clays " of coal-seams. Being more expensive to raise
and manipulate than ordinary clays, they are chiefly employed in
the fabrication of fire-bricks, furnace- linings, grate-backs, oven-
soles, gas-retorts, coke-ovens, crucibles, and other objects which
have to endure exceedingly high and long-continued tempera-
tures.11
As far as infusibility goes there is no widely diffused substance
that will resist heat better than silica, and if finely divided
silica were plastic we could not have a better material for making
bricks capable of standing heat without being fused. But before
we can make silica into bricks we must have some vehicle to bind
the grains together, and this vehicle must be itself infusible.
Such a vehicle we find in kaolin, and hence a theoretically
perfect fire-clay is either kaolin or a mixture of kaolin and silica.
Kaolin shrinks and cracks in drying and firing too much to
allow of its being used for brick-making, even if it were plentiful
enough to be employed for this purpose. But nature has
furnished us with rocks which approach a mixture of kaolin and
silica in composition very nearly, and these make the best fire-
bricks.
It must not be assumed, however, that a clay, because it has
a theoretically suitable composition, will necessarily make a good
fire-brick. There are other conditions to be satisfied ; the brick
must not crack and fly when exposed to a sudden rise or great
extremes of temperature, it must support great pressure at high
temperatures without crumbling, and it must resist the corrosive
action of some of the slags produced in metallurgical operations.
310 GEOLOGY FOR ENGINEERS. [PT. V.
Chemical and mineralogical examination will often enable us to
say that certain clays will assuredly not make fire-bricks ; and it
will enable us to say that other clays are promising enough to
make it worth while trying them ; but nothing short of making
a test-brick, and subjecting it to the heat that it will be required
to stand, will settle the question.7
Dinas Bricks. — Fire-bricks are in some cases made out of
substances composed almost entirely of silica. The Dinas brick
is made of pounded or weathered gritstone which contains
between 98 and 99 per cent of silica. About 1 per cent, of lime
is mixed with the sand, and the mixture pressed into moulds,
dried, and strongly fired. The lime causes the outside of the
quartz grains to fuse and adhere together.7
Firestones. — Any stone that stands heat for a considerable
time without perceptible injury is entitled to the designation of
a Firestone. The term, however, is usually applied to certain
sandstones of the Greensand, Oolitic, and Coal formations, employed
in the construction of ovens, glass furnaces, and similar erections
subjected to high and oftentimes to intermittent temperatures.
The Upper Greensand of Kent and Surrey (Reigate) yields a stone
of this description which was at one time much prized ; some of
the soft yellow sandstones of the Tyne have also been employed
in furnace structures ; and the sandstone of Craigenbank, near
Borrowstounness, has been shipped to St Petersburg for furnaces,
ovens, and similar purposes. Such sandstones, however, are all
but superseded by fire-clay fabrics. The firestone of Nevada, U.S.,
is described as a light, porous, siliceous rock, having a specific
gravity of 1*49, capable of being sawn into blocks of any form,
and able to resist intense and intermittent temperatures.11
A stone called gannister (cf. p. 292) is used for making fire-bricks
and linings to furnaces and Bessemer " converters " in Yorkshire
and Lancashire. It contains up to 96 per cent, of silica, and it
is probable that the ferric oxide, lime, and alkalies present in it
play the same part as the lime in the Dinas process.7
Floating Bricks. — Light mealy deposits composed of the
siliceous shields of infusoria and the frustules of diatoms — known
as diatomaceous, infusorial, and microphytal earths — have been
employed in the manufacture of floating bricks by mixing the
fossil flour with a paste of lime and clay. As these bricks are
only one-sixth the weight of ordinary bricks, and unaffected by
the strongest heat, they are suitable for use as fire-proofs on board
ship. These siliceous earths are by no means rare — the
" polishing slate " of Bilin, the " mountain meal " of Sweden and
Tuscany, and the "Richmond earth " of Virginia being examples
on a large scale.11
CH. XIV.] BRICKS AND CLAYS. 311
Terra-cottas. — These "baked earths" of the Italians are
merely unglazed wares — vases, bricks, tiles, mouldings, and other
architectural ornaments — prepared from the finest fire-clays.
Extreme care is bestowed in the selection and manipulation of the
raw material — the object being to secure a substance that will
contract equally, and so avoid all warping or distortion in the
finished article. Italy and France have long enjoyed the
supremacy in terra-cottas ; but recently Staffordshire and Lanark
have produced shafts, vases, statuettes, and the like of unrivalled
symmetry and elegance.11
SCIENCE OF BRICK-MAKING.
Choice of Clay. — The brickmaker deals with natural clays only,
the constitution of which, when more or less ascertained in respect
to his object, he may modify by the addition of other mineral
bodies, such as sand, ashes, etc., or by the mechanical extraction
of naturally mixed matter, as sand, pebbles, pyrites, etc., and
whose physical qualities he may alter by mechanical means —
grinding, " slip- washing," etc.
The choice of a clay that shall answer well for the brickmaker's
use cannot be made before trial, by any amount of examination,
unless we also possess a chemical analysis of the natural material.
Aided by that, it is quite possible, upon tempering a ball of the
clay, observing its plasticity and body, and then further wetting
a little bit, and rubbing it between the thumb and forefinger, to
tell with a great degree of certainty whether it will make good
bricks or not, either alone, or, as is almost always the case, mixed
(and so altered) either with more sand or more tough clay, and
occasionally with coarsely ground coal, or breeze, or ashes, etc.29
Clays. — They are essentially chemical compounds, and this is
true whether they be or be not always mere mud from dis-
integrated rocks, as some geologists have supposed (see Chapter
VII.). They are, in fact, true hydrates, and have the general
constitution (Si02 + A1203) + H20 + RO, the last or accidental
base or bases being usually oxides of calcium, magnesium,
manganese, or iron, or more than one of these ; and they may be
divided into four great classes. Pure aluminous clays and pure
magnesian clays, both hydrated : these are rare, the latter
especially so — when indurated, constituting meerschaum. They
do not require further notice here, as they belong to the porcelain-
maker, not to the brickmaker.
More widely spread for our use, we have the ferruginous clays,
which have generally the combination Si02 4- (A1208 + Fe203) ±
FeO + (Na20 + K20) + H20 ; and the calcareous clays (Si02 +
312 GEOLOGY FOR ENGINEERS. [FT. V.
(A1203 + Fe203) + (CaO + C02 + MgO + C02) ± FeO + Na20 + K20)
+ H20. Either of these may be mixed with more or less siliceous
sand, and when this is in considerable proportion the clay is a
loam.
They lose more or less of their hygroscopic water at 212° F. ;
most of their combined water at a red heat; and at a bright
yellow or white heat, or rather below it, they bake into pottery or
brick. While many of the clays rich in alumina, silica, and iron
oxide do not fuse, or but very slowly, at the melting-point of cast-
iron, most of the calcareous clays melt at or below this tempera-
ture, or at least agglutinate, assuming a vitreous texture if the
heat be long continued.
Clays should, if possible, be delivered into the brickyard in
their moist natural state, for when they have been permitted to
dry up under a scorching sun or drying wind, they shrink and
harden greatly, and the labour of mixing into good brick "stuff"
is greater, and the plastic mixture not so free and nice as
before.
Analyses of various clays are given in the annexed table29 (p. 317).
Foreign Bodies. — Most clays, as found in nature, contain some
organic matters and pebbles of foreign bodies. Unless these are
of hard pyrites or limestone, they are unimportant. Flinty
pebbles can generally be crushed in the clay-mill, or taken out by
the screen or sieve.
Whether a natural clay contains much or little sand naturally
is not important. Every clay requires more or less grinding and
mixing, and when sand in a separate form is at hand, it is easiest
and best mixed in such proportions as we may require in the
pug-mill. Clays naturally very rich in lime or the alkalies
(derived from felspar) are the worst, and in fact a clay that
contains more than about 5 per cent, of lime is scarcely fitted for
good brick-making.
If the lime be in the state of carbonate, it is so much the worse,
and if it exist in the state of diffused limestone or chalk pebbles,
it is worst of all, for these burn into caustic lime in the kiln, and
then, when the brick absorbs moisture and carbonic acid, the
nodules of lime " slack " and swell in their places, and so burst
the brick to pieces.
Iron pyrites also is a not uncommon accidental product present
in clays, and unless separated, durable, to say nothing of well-
coloured brick can never be made of the clay. The pyrites is but
partially decomposed in the kiln ; oxide of iron and basic sulphides
of iron remain. When these are exposed later on to air and
moisture, which are absorbed to all depths in brick, oxidation
takes place, sulphate of iron, and frequently also sulphates of lime
CH. XIV.] BRICKS AND CLAYS. 313
or alums (sulphates with double bases), are formed, and, crystal-
lising within the mass of the brick, split it to pieces.
Common salt is nearly always present in minute quantity in
clays ; but when these are taken from the seashore, from beneath
the sea-washes, or from localities in and about the salt formations
(Trias), they frequently, though in all other respects excellent
clays, are unfit for burning into good brick. Chloride of sodium
is not only a powerful flux when mixed even in very small
proportion in clays, but possesses the property of being volatilised
by the heat of the brick-kiln, and in that condition it carries with
it, in a volatile state, various metallic compounds, as those of iron,
which exist in nearly all clays, and also act as fluxes. The result
is that bricks made of such clays tend to fuse, to warp, twist, and
agglutinate together upon the surfaces long before they have
been exposed to a sufficient or sufficiently prolonged heat to burn
them to the core into good hard brick. " Place bricks " can be
made of such clay, but nothing more ; and these are always bad,
because never afterwards free from hygrometric moisture.
Much carbonaceous matter naturally mixed in clays is also in
certain states objectionable, for when not burnt completely and in
the kiln, which is sometimes difficult with the denser clays, the
bricks are of a different colour in the exterior and interior, and
will not bear cutting for face- work without spoiling the appear-
ance of the brick-work. But, worse than this, such bricks, when
wetted in the wall, occasionally pass out soluble compounds like
those absorbed from soot by the bricks of the flue, and like these
(when used again in new work) discolour plastering or stucco-
work.29
Normal Constituents. — The normal constituents of brick clays,
then, may be said to be oxides of the earthy metals, and of a few
others, hydrated or not, with silicic acid, and with small amounts
of the alkalies, potash and soda, also present, together with
several other chemical compounds occasionally, but uncertainly,
present in minute proportions, with which we need not concern
ourselves.
Silicic acid, the great electro-negative element of clays when
combined with the oxides of the earthy bases, singly or in com-
bination, and exposed to high temperatures in certain proportions,
forms glass or enamel (i.e. opaque glasses).
Alumina, though in a less degree, also plays the part of acid
towards the earthy bases, though itself a base with respect to
silicic acid. As regards the oxides of the earthy metals, alumina,
lime, magnesia, etc., these, in accordance with the general law of
chemistry that bodies in the same range combine, oxides with
oxides, etc., also combine at high temperatures. The most
314 GEOLOGY FOR ENGINEERS. [PT. V.
powerful bases, such as the alkalies or oxides of potassium, and
sodium and the oxides of iron, combine more readily with silicic
acid than do the earthy oxides. These combinations usually take
the form of glass at once, the chief characteristic of which is the
vitreous fracture. When such glasses are formed with oxides of
earthy bases also present, they may assume a crystalline or porcel-
laneous character when cooled.
Porcelain, earthenware, and hard brick (such as the Stafford-
shire or Flintshire blue bricks) consist in substance of such com-
pound glasses, diffused throughout their substance uniformly, or
binding together the finely diffused particles of the excess of
earthy oxides which are present, or binding together fragmentary
bits of uniformly diffused silicic acid (sand, ground flint, etc.).
The degree of fusibility or of partial fusibility (agglutination) of
any hard-baked brick depends, then, not only upon the chemical
nature of the constituents of the clay, but upon the proportions
in which these are present.29
Laws of Induration. — The laws, so far as they have been
ascertained, upon which depends the induration or agglutination
by heat of silicic and earthy compounds, with or without other
metallic oxides present, have been elicited from innumerable
experiments made by ceramic chemists upon very varied com-
pounds. The phenomena are complex, and the results obtained
are mostly only empirical. We must refer for these to the works
of Kir wan (Mineralogy), who made very many experiments upon
known combinations of earths when exposed to heat, and other
writers. Silica, alumina, lime, magnesia, are all infusible, per se,
at the highest temperature of the porcelain furnace or brick-kiln.
Silicic acid combined with any one earth is less fusible than
when combined with two or more — a proof that not only the
silicic acid combines with each earth, but that these in its
presence combine with each other. Binary compounds of silicic
acid and of earths, or of earths with earths, are most usually
infusible except at still higher temperatures. Compounds of
silicic acid with alumina are less fusible than with lime, and these
less so than with the alkalies.
With oxides of iron silicic acid forms fusible compounds in
certain proportions. Magnesia present in large proportions with
either of the other earths produces a very difficultly fusible com-
pound. Where the silicic acid constitutes the largest proportion
of the mass it is much more fusible, the bases being two others
combined, with or without alkalies ; but if the silicic be in great
excess (as in Dinas fire-brick, see p. 310), or if one or other
of the earthy bases be in great excess, more especially alumina or
magnesia, the mass is infusible in the kiln.
CH. XIV.] BRICKS AND CLAYS. 315
All difficultly fusible and pulverulent oxides, as when obtained
by precipitation or by levigation, when exposed for some time to
a high temperature, become hard in grain, i.e. indurated more or
less, and frequently compacted. This is true even of some pure
earths, such as alumina and magnesia, and of nearly all the
oxides of the common metals. Compound oxides, when so
exposed to heat, become still more indurated and compact,
though presenting no traces of agglutination or of fusion. Thus
alumina and sesquioxide of iron become compact. This indura-
tion, which is probably rather a change in the state of molecular
aggregation than a chemical combination, but which may be both,
is much concerned in the production of certain qualities of brick •
for example, the fine, soft, scarlet cutting brick — that which was
so much employed for fine facing brick in the reign of William
III. down to George II. — presents no sign of agglutination, its
constituents have merely become partially indurated and com-
pacted by the fire. The same is true of many of the light-
coloured bricks now in use.29
Contraction. — Two sets of forces, then, are or may be in play
in the burning of brick— chemical, and physical or molecular —
and must be held in view by the scientific brickmaker. To the
latter belongs the contraction that takes place in the process of
firing of all porcelain and brick. This is greatest with those
which contain most alumina, and with any given specimen is
great not only in proportion to the elevation of the temperature
to which it is exposed, but with the duration of the time of
exposure. It is least in compounds in which the silicic acid pre-
dominates ; and if these pass partially from the crystalline to the
vitreous state of aggregation in the firing, the specific gravity is
reduced and the increase of volume may more than equal the
contraction. This is said to be the case with Dinas fire-brick,
which, when highly heated in furnaces built of it, is said to
expand.29
Colours. — Were brick constituted of silicic acid and pure clays
only, it would be perfectly white. Bricks, like porcelain, owe
their colour to admixed metallic oxides ; iron in various states of
oxidation, from protoxide to sesquioxide, or true chemical com-
binations of those with each other or with the earths themselves,
and present in the most varied proportions, give the whole range
of colouring to bricks, from the lightest tawny yellow, through
full yellow, and orange, to the rich scarlet of red facing-brick,
almost as bright as red lead. Where the proportion of oxide of
iron present is very large, and it combines with silicic acid to
form silicates of iron in or on the brick, its colour may be dark
purple or nearly black, as is the Staffordshire blue brick ; and
316 GEOLOGY FOR ENGINEERS. [PT. V.
when a small quantity of oxide of manganese is present also, the
colour is still darker and may become quite black.
For light-coloured bricks the clays must be almost free from
iron, and the latter must not be peroxidised, if possible, in the
burning.
For the production of fine red brick, on the contrary, the clays
must be pure, silicic acid not present in excess, oxide of iron
present in abundant proportion, and be fully peroxidised, but
must not be fused into a silicate of peroxide of iron, which is
fatal then both to the texture and colour.
With a given constitution of brick clay the final colour of the
burnt brick depends upon a large number of conditions in the
process of firing, but mainly upon two — viz. what proportion of
air is admitted to the combustion of the fuel in the kiln — that is
to say, whether the brick is finally burnt with an oxidising or a
deoxidising flame ; and whether or not, or in what proportion,
steam or water is present in the brick, or is brought in the state
of vapour in contact with it, when at elevated temperatures.
Upon an exact knowledge of the effects producible by the play
of these conditions (chiefly) upon the brick in burning rests the
power of the brickmaker to vary or maintain with certainty the
good colour of his ware, or to effect any desirable changes of
colour of which his material may be susceptible.
From this very incomplete sketch, says Mr Mallet, it will be
seen that brick-making is one of the chemico-mechanical arts.
Being so, we need scarcely say that the foundation of all accurate
and predictive knowledge of it must be based upon a sound know-
ledge of chemistry, and of the laws of physics, and of heat
especially, which is but a branch of the latter.29
[TABLE.
CH. XIV.
BRICKS AND CLAYS.
317
49
i
Water.
o
OS
o
10 to o
O* "* OO
00
rH
8
r^ <o o
I— 1
•M
Titanic Acid.
.
OS . N
O
Cl 0
&
Protoxide of
OS
^
i
Manganese.
' o
o
0
a
Peroxide of
^
3
5
o
Iron.
Is
Protoxide of
^ 10W
0
.8
Iron.
•ijl NO
M
0)
1
£_,
Soda. :
;
<<*< •*
lO
i
<^ r o °
o
V
t 0
2
Potash.
CO
;
<N OS
r— (
c5
CO
O O
<N
|
N
Magnesia.
o
3 IS
5
^>.
15
0
CO
*-"* fH O
rH
rH
a
1
*
^N
O>
^^.^
^^u^
a
H
CO
J M 8 CM
.0
PQ
v
Lime.
7*
^«3 0 7^
'S "^
<;
o
^ C^l Z^ O
05 rH
H
CO -4-3
1-1
d
— 1 —
i
cS
00 00 O
•
03
9
Alumina.
T— 1
8
CO
o
rH
00 00 «O
?
8
OS
0 <N «0
00
rS Silica.
OS
oo
O
T1 °° 9*
10 CO 4n
to
iO *•»• «O
50
fH |
J . 1
o
cT "
OH
B
0
so • • •
'o
08
^
rs o
<-! i
o
T3
fe « 0
0
The compositioi
geological periods :—
Eocene —
Light-colour pottery
Cretaceous —
Fuller's earth (Nutfi
Jurassic —
Blue clay (Kimm
Oxford) .
Carboniferous —
Fireclay (Stourbridg
Red tile-clay (Brosel
Silurian —
White saponaceo
(Horderley)
PT. V.
CHAPTER XV.
LIMES, CEMENTS, AND PLASTERS.
To the engineer and builder in Great Britain, where Portland
cement is so largely used, a knowledge of the uses and geological
distribution of the various limes and limestones is of com-
paratively little value; but to those employed on works of
construction in Greater Britain and India, where the cost of
Portland cement is often prohibitive, some acquaintance with
the different limes is of the highest importance. Especially is
this the case where the ordinary lime of the neighbourhood is a
" fat " lime, as by a proper admixture of suitable substances the
engineer may greatly add to its hydraulicity.1
Definition of Cements and Limes. — "Cements," as dis-
tinguished from limes, are materials which are capable of
solidifying when in contact with water without perceptible
change of volume, or notable evolution of heat; "hydraulic
cements and limes " are such as possess the power of " setting "
or solidifying under water. All limes have a tendency to
expand and to fall asunder, or to crumble into powder when
treated with water, and are said to become "slaked." The
purer the lime the more energetic and rapid is this action, while
conversely the greater the quantity of clayey matter combined with
the lime, the less intense, as a rule, is the chemical affinity for water,
and the slower is the act of hydration, and to this extent the
greater is the resemblance of such limes to cements. " Limes,"
therefore, as distinguished from cements, " fall " or crumble when
exposed to the action of water.30
Intermediate Limes. — Certain impure limes, resembling in
their composition the constitution of cements, have been appro-
priately named "intermediate limes," or such as occupy a position
intermediate between the true limes, which undergo disruption
when exposed to the action of water, and the cements which do
not, apparently, become changed when so treated.
It may be assumed that limes of every different degree of
energy, from pure oxide of calcium down to true calcareous
318
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 319
cements, exist in nature ; thus there is an enormous range of
varieties of action to be studied, and any attempt to classify all
limes under two or three sub-heads must be futile and un-
trustworthy.30
LIMES.
Combination of Lime with Water. — The chemical affinity of
lime for water is one of the most powerful with which we are
acquainted, and "quicklime" (calcium oxide), or lime recently
calcined, when exposed to the air, speedily attracts moisture from
the atmosphere, and combines with such water to form calcium
hydroxide, or slaked lime. This hydroxide may occupy as much
as three times the space previously filled by the quicklime, and
therefore the amount of slaked lime produced from a given bulk
of quicklime appears in certain cases to be very considerable.
The water which combines with the lime in the act of hydra-
tion is truly solidified, and the hydrate formed is, when the exact
proportion of water necessary for this purpose has been employed,
an absolutely dry powder. On adding a further quantity of
water, the bulk of this powder is much reduced, and it may be
tempered into an extremely rich and unctuous paste. If this
paste is permitted to dry, it shrinks and forms a porous mass of
no great hardness.30
Quicklime. — Quicklime, caustic lime, or the oxide of calcium,
one of the earthy metals, does not exist in nature, nor is metallic
calcium itself anywhere found in an uncombined form. We
obtain quicklime, the chemical symbol for which is CaO, by
calcining or heating to redness a carbonate of lime, CaC03, and
by this means expelling the carbonic acid gas or carbon dioxide,
C02, with which the lime is combined, and which can be driven
off in the gaseous form at a cherry-red heat (about 440° Centigrade).
Lime combined with carbonic acid is found in a great variety
of rocks in all parts of the world, and in every different degree
of purity (see Mortar Limestones, below).
In a pure carbonate of lime 44 parts by weight of carbon
dioxide or carbonic acid are combined with 56 parts by weight of
calcium oxide. In the oxide itself 40 parts by weight of metallic
calcium, Ca, are combined with 16 parts of oxygen gas, 0. This
oxide cannot be decomposed by heat.
Calcination. — Generally speaking, the limestone or chalk,
when placed in the kiln, contains a certain percentage of
moisture which has also to be expelled, and thus the lime-
burner can rarely, when the stone is thoroughly well burned
and all the carbon dioxide is expelled, obtain more than half its
weight of quicklime from a given weight of stone dealt with in
320 GEOLOGY FOR ENGINEERS. [pT. V.
the kiln, though in theory the yield should be 56 per cent, of
lime.30
Slaked Lime. — When lime becomes " slaked " it is found that
56 parts by weight of quicklime combine with 18 parts by
weight of water, H20, to form 74 parts of calcium hydroxide,
Ca(OH)2. Great heat is evolved in this process, and the action
is expedited by the use of boiling water. Certain "poor limes,"
which will scarcely slake or fall to powder when cold water is
employed, will crumble into dust readily if the water is at the
boiling point.30
Lime slowly recombines with Carbonic Acid. — When exposed
to the air, pure caustic lime is converted very slowly and without
notable increase of temperature into a rather coarse powder. It
is not, under these circumstances, wholly converted into a
carbonate of lime, even after the lapse of many years, but, by
the simultaneous absorption of moisture and carbonic acid, it is
resolved into a double compound having the formula, according
to Fuchs, of CaC03 + Ca(OH)2, or consisting of equal equivalents of
the carbonate and the hydrate of lime. The carbonate thus
produced would seem to result from the decomposition of the
first-formed hydrate, for when moisture is wholly excluded no
combination between the lime and the dry carbonic acid gas
takes place. In order to expel the water of hydration, the slaked
lime must again be heated to dull redness.
The action of carbonic acid mainly superficial. — Lime made
from pure carbonate of lime, when slaked and used for mortar,
likewise gradually recombines with the carbonic acid gas present
in the atmosphere and becomes indurated, but this action is
mainly in the superficial layers of the mortar, as the gas
penetrates very slowly. In fact, years must elapse before the
recarbonisation of the lime is thoroughly accomplished, and in
the case of thick walls the internal layers of mortar never
become completely hard. It is necessary to distinguish between
the so-called "set" of the mortar, which is merely due to the
absorption of the superabundant water, and the actual induration
by means of the carbonic acid gas which is a process of years, or
of ages in the case of pure limes.30
Classification of Limes. — Some writers have attempted to
classify the different varieties of lime in accordance with the
quantity of slaked lime produced, or with the speed with which
they were observed to combine with water. For instance,
limes are frequently classed as fat or rich limes if they readily
become slaked and furnish a large volume of powder, and poor
if they are impure and become slaked slowly, yielding relatively
but little dust ; 30 or, when falling rapidly to quicklime, they are
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 321
rich ; when falling only after eight or ten minutes, they are poor ;
when they require fifteen or twenty minutes, they are medium ;
when requiring an hour or more, they are regarded as hydraulic ;
and when requiring, it may be, several days to break up, they
are highly or energetically hydraulic.^1
It is, however, now known that this slaking action depends upon
numerous conditions which have to be specially studied for each
class of limes, and that any general deductions founded on the
act of hydration alone are likely to be inaccurate and misleading.30
The old classification into fat, poor, medium, hydraulic, and
eminently hydraulic limes is still met with in many engineering
books, and as a rough guide is of considerable value if due
caution is observed.1
HYDRAULIC LIMES.
The Influence of Clayey Matters. — Absolutely pure lime-
stones are only met with in exceptional cases, as nearly all
limestone rocks, and the greater part of the Chalk formation,
contain varying percentages of clayey matters (silicates of
alumina), iron, alkalies, etc., and it is upon the proportion of
these ingredients present that the behaviour of the calcined
lime principally depends. It is, in fact, owing to the presence
of certain of these clayey matters that limes pass over by gradual
stages into the form of cements; that is to say, that these
substances so far influence the slaking action that they may even
bring about the ultimate setting of the mixture without change
of volume — the characteristic property (as already stated) of
cements.30
Artificial Admixture of Clayey Matters. — It is not necessary,
however, that the limestone should have been the source from
which these clayey matters were derived ; they may be conveyed
to the calcined lime by admixture with it at the time when it is
treated with water, or they may be ground up along with the
lump lime before it is slaked. It is this fact which needs
careful consideration when we have to deal with the influence of
heat on mixtures of lime and clay, and the nature of the changes
effected in the kiln. The silica compounds are of a very complex
character, and may be produced, as we shall see, both by heat
and in the humid way. All that is necessary for the due action
of these clayey matters is that they should themselves have been
roasted or calcined either artificially or by volcanic heat.30
Pozzuolana, Trass, etc. — Certain of these substances which
are added to pure limes to bring about this action are called
pozzuolanas or trass. These are clayey or siliceous matters of
21
322 GEOLOGY FOB ENGINEERS. [PT. V.
volcanic origin, but roasted shales, brick dust, and burnt clay or
ballast, all of them, more or less, possess this influence on the
pure limes, and have the power of imparting to them the
attributes of cements.
The volcanic ash found in the island of Santorin, and known
as Santorin earth, is typical of many kinds of scoriae which have
been used successfully with fat or pure limes to impart to them
hydraulic properties. The proportion of silicate of alumina in
this substance is relatively high, and there is much less iron than
in the case of trass and pozzuolana.30
Influence of Heat on the Silicates. — When limes, such as are
combined with varying percentages of silicates, are burnt in the
ordinary way in the kiln, the carbonic acid gas is first expelled
from them, as in the ca.se of the pure limestones, and the clayey
matters assist in its expulsion, owing partly to the affinity of the
silicic acid for the lime, and partly to the fact that the free and
combined water in the clay is driven off, and the steam produced
in this way facilitates the expulsion of the carbonic acid. There
is thus a double change to be effected in the kiln, and the
expulsion of the water from the hyd rated silicate or alumina in
the clay may go on side by side with the dispersal of the carbonic
acid.
These clayey limestones are thus burnt more readily than the
pure limestones ; they also require less fuel and less time.30
LIMESTONES.
Subdivisions. — The minerals which contain the carbonate of
lime and which are designated under the generic name of
" limestones " or " calcareous stones " are of very various natures.
They are mostly composed of carbonate of lime, of magnesia,
of oxide of iron, of manganese, of silica, and of alumina, combined
in variable proportions ; and they are also found with a
mechanical admixture of clay (either bituminous or not), of
quartzose sand, and of numerous other substances. The name
of limestone is more especially applied to such of the above
mixtures as contain at least one-half of their weight of carbonate
of lime. Mineralogists distinguish the subdivisions by the names
of " argillaceous, magnesian, sandy, ferruginous, bituminous,
fetid," etc. The subdivisions, again, are often characterised
by varieties of form and contexture which are known specifically
under the names of "lamellar, saccharoid, granular, compact,
oolitic, chalky, pulverulent, pseudomorphic, concreted," etc., etc,
(see Chapter VII., Section II.).
This nomenclature is important, for every description of lime-
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 323
stone yields a lime of different quality, distinct in colour and
weight, in its avidity for water, and especially in the degree of
hardness it is capable of assuming when made into mortar. But
the physical and mechanical nature of a stone are far from being
certain guides as to the quality of the lime it can yield. A
chemical analysis of a hard sample also frequently gives different
results from those obtained in practice. Experience alone should
be the final guide of the engineer or of the builder.31
Chemical Nature of Stones furnishing Different Sorts of
Lime. — A chemical examination of the stones which furnish the
different limes of the old classification shows that : —
1. The pure calcareous rocks, or such as contain only from
1 to 6 per cent, of silica, alumina, magnesia, iron, etc., either
separately or in combination, gives rich limes upon being burnt.
2. The limestones containing insoluble silica in the state of
sand, magnesia, the oxides of iron and of manganese, in various
respective proportions, but limited to between 15 to 30 per cent,
of the whole mass, yield poor limes.
3. The limestones containing silica in combination with
alumina (common clay), magnesia, and traces of the oxides of
iron and of manganese, in various respective proportions, but
within the limits of from 8 to 12 per cent, of the whole mass,
yield moderately hydraulic limes.
4. When the above ingredients are present in the propor-
tion of from 15 to 18 per cent., but the silica in its soluble
form always predominating, the limestones yield a hydraulic
lime.
5. When the limestones contain more than 20 and up to 30
per cent, of the above ingredients, but with the soluble silica in
the proportion of at least one-half of them, the limestones yield
eminently hydraulic limes.31
CALCINATION.
Kilns and Fuel. — The limestones, after being quarried and
broken into moderate-sized pieces, are calcined, either in
temporary or in continual kilns — that is, in open kilns which
are blown out till the calcined charge has been removed, or in
draw-kilns, where the removal and charging proceed continuously.
To avoid carriage, it is desirable to have the kilns as central as
possible to the face of the quarries; and the longer the stone has
been exposed to the air, the less fuel will it require to drive off
the inherent moisture or quarry-water. The fuel employed in
calcination is ordinary pit coal (1 ton to 4 or 5 tons of limestone),
and in remote districts peat and brushwood ; but for some sorts
324 GEOLOGY FOR ENGINEERS. [PT. V.
of limestone impure or shaly coals (while also much cheaper) are
better adapted than the pure coals, as burning the stone more
slowly and equally, as well as keeping it open and preventing
slagging and sintering. More kiln-dust may be produced by the
use of these slaty coals, but fewer cores and slags will be found
among the lime.
When properly burnt — that is, when not slagged or covered
with a siliceous glaze by too sudden ignition — the limestone
loses its carbonic acid, and is converted into caustic- or quick-
lime.11
Admixture with Ashes. — For many purposes for which lime
is used commercially, it is very important that it should be as
pure as possible, and free from the ash or clinker arising from
the fuel. It is, perhaps, less essential now than was formerly
the case that the lime used by the builder should be kept apart
from the ash of the fuel, as in nearly all important works it is
customary to prepare the mortar in a mill, which would crush
up these substances along with the lump lime and incorporate
them in the mortar. For use of the plasterer, the lime is slaked
and run through a sieve, by means of which all the impurities
and underburnt particles are eliminated. A much better-looking
lime no doubt results from the use of kilns in which all contact
with the fuel is avoided ; and although the cost of doing this
adds to the expense of burning, it is certainly worth while to
endeavour, if possible, to keep out the ash and clinker.30
Results of Calcination. — Those limes which are obtained from
the stones containing much silica in the composition of the clay,
swell in setting, and are likely to dislocate the masonry executed
with them. Those, on the contrary, in which the alumina is in
excess, are likely to shrink and crack. The magnesian limestones,
or dolomites, appear to be the least exposed to these incon-
veniences, and to retain without alteration their original bulk.
The limes obtained from the Oxford Clay generally swell • those
from the Chalk Marl contract.
Limestones which contain many/om7s produce a lime exposed
to the risk of slaking at various and uncertain periods. Whether
it arises from the fact that the decomposition of the animal
matter had previously affected the nature of the limestone in
contact with it, or from that of the different action of the
calcination upon the shells, we mostly find that the fossiliferous
limestones contain black spots which do not slake at the same
time as the rest of the lime, or which retain their avidity for
water to a later period ; and in either case they swell and dis-
integrate the mass around them,31
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 325
TESTING LIMES AND LIMESTONES.
Berthier's Mode of Analysis. — To ascertain whether a stone
be, or be not, fit to be burnt for the purpose of obtaining a
hydraulic lime, the following mode of analysis is sufficient for all
practical purposes : —
The stone should be powdered, and passed through a silk sieve ;
10 grammes of this dust are to be put into a capsule, and by
degrees muriatic acid is to be poured upon it, stirring it up
continually with a glass or wooden rod ; when the effervescence
ceases, no more acid is to be added. The solution is then to be
evaporated by a gentle heat until it is reduced to the state of a
paste; it is then to be mixed with half a litre of water, and
filtered ; the clay will remain upon the filter. This substance is
to be dried and weighed, the desiccation being made as perfect as
possible. Lime water is then to be added to the remaining
solution as long as any precipitation takes place from it. This
precipitate must be collected as quickly as possible upon a filter ;
it is then desiccated and weighed. It is magnesia, often combined
with iron and manganese.31
The condition of the silica present in impure limestones has
an important influence on their value when employed for the
manufacture of hydraulic lime. Any silica existing in the
uncombined state as quartz-sand is unacted upon by the lime
at the comparatively low temperature of the kiln, and con-
sequently after calcination it does not separate as a gelatinous
bulky mass, as is usual with the silica in samples of hydraulic
lime, when treated with hydrochloric acid. Further, it is unacted
upon by a boiling solution of sodium carbonate, and it is not in
a condition to enter into combination ; it is present, in fact,
simply as inert matter. In order to confer hydraulic properties
upon the lime much of the silica must, before burning, occur in
combination, preferably with alumina.
Many of the beds of impure limestone in the Carboniferous
deposits contain free silica as sand in considerable quantity.
Lime prepared from stone of this description is easily recognised
by its friable granular appearance, while, on the other hand, that
which is burnt from stone in which the silica exists in combina-
tion with other substances, as is the case in the beds of the
Lias formation and in some of the Carboniferous deposits, has a
dense, close, even structure, the lumps of quicklime ringing
when struck together.30
When treated with muriatic acid, a limestone that leaves
about 10 per cent, of insoluble matter forms, according to
M. Lipowitz (Manufacture of Cements), a tolerably hydraulic lime ;
326 GEOLOGY FOR ENGINEERS. [FT. V.
but when leaving from 20 to 30 per cent., such a lime will not
slake after burning without first being powdered, after which
process it often produces the best hydraulic mortar. After
calcination and slaking, such limestones as the blue lias require
careful screening to remove unburnt cores, not more than 1| sand
to 1 of lime, and are often improved in hydraulicity by the
addition of a small percentage of pounded surface-clinkers.11
CEMENTS.
The energy of a cement depends upon the rapidity with which
the lime and the silica, or the lime and the alumina, combine in
the presence of water to form stable compounds, or with which
the ready-formed silicates and aluminates become hydrated when
water is added. We have thus the quick-setting cements of the
Roman cement type, which become indurated mainly by hydration
in a few minutes, and the dense cements resembling Portland,
which depend for their induration on a rearrangement of the
silicates, and which may take as many hours to set as the former
substance does minutes.
It should be here noted that when we speak of the setting of
cements we imply the act of induration and not the mere
absorption of the water, which is most characteristic of the
imperfect setting action of a lime mortar.30
Influence of Calcination.— The calcination of these varieties of
cements plays a very important part in their subsequent
behaviour, when tempered with wa'ter. Thus it is possible from
the same clay-limestone to prepare (a) an hydraulic lime ; (b) a
quick-setting cement ; and (c) a cement resembling Portland
cement in character.
At a low temperature in the kiln the mixtures of lime and clay
have not mutually reacted the one on the other, and we obtain
a material in which the energy due to the hydration of the lime
overcomes the tendency of the silicic acid to enter into combina-
tion with this lime, under the agency of water.
When the second stage in the calcination is reached the silicic
acid is liberated or rendered capable of attacking the lime,
yielding a cement which sets with comparative rapidity. While,
lastly, under still more intense firing, the stage of calcination is
approached when silicates and aluminates are formed in the kiln
and when the material acts like a Portland cement, and when the
iron, moreover, which had during the first and second degrees of
calcination remained in the condition of a peroxide, passes into
that of a protoxide (as is always the case in perfectly prepared
Portland cement). This change in the oxide of iron is only
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 327
effected at very high temperatures, and furnishes a certain
indication of the production of a dense, slow-setting cement.
If, in the case of this clayey limestone, the clay had been less
in quantity, we should have obtained a hydraulic lime which
would slake with difficulty, and which would be liable to the evil
effects of "after-slaking." If the proportion of the bases
contained in the clay, relatively to the amount of silicic acid
present, had been greater, the mass would have probably become
vitrified or partially fused before the temperature necessary for
the final stage of calcination was reached.30
Roman Cement. — A peculiar class of the argillaceous limestones
yields on calcination a species of lime capable of setting under
water with considerable rapidity, of acquiring a great degree of
hardness within a very short space of time, and of being employed
without the admixture of any foreign substance. The first
discoverer of this kind of cement was Mr Parker, of London, who
in the year 1796 took out a patent for the manufacture of what
he called Roman cement, from the septaria nodules of the London
Clay 'formation, found in the Island of Sheppey. His process
consisted in calcining the stone, previously broken into small
fragments, to a point equal to the commencement of vitrification,
and then reducing it to powder by some mechanical operation.31
Subsequently a similar material was found at Harwich and in
Yorkshire, also on the coast of France and in Burgundy,1 and
doubtless it is to be met with in all the marl beds intercalated
between the principal stages of the limestone formations, and
very frequently in the Tertiary clays, in the form of detached
nodules of a dark-coloured, argillaceous limestone traversed by
veins filled with calcareous spar. The colour is sometimes blue,
especially when the nodules are obtained from the Lias ; sometimes
brown, or a deep red, in the Tertiary formations, owing to the
presence of the oxide of iron in very considerable quantities.
The mineralogical composition of the stones from which the
cement is made differs very much ; but the characteristic type
may be said to consist of above 30 and below 60 per cent, of clay
and other extraneous matter in combination with the carbonate of
lime. The Sheppey stone usually contains 55 parts of lime, 38 of
clay, and 7 of iron ; the Yorkshire stone contains 34 parts of clay,
62 of carbonate of lime, and 4 per cent, of iron ; the Harwich
stone contains 47 parts of clay, 49 of carbonate of lime, and 3 of
oxide of iron.
The cement stones are burnt in conical kilns with running fires,
and, in England at least, with coke or coal. The mode of burning
requires a considerable degree of attention, for experience has
demonstrated that Parker was mistaken in supposing that a
GEOLOGY FOR ENGINEERS. [PT. v.
commencement of vitrification was necessary. On the contrary,
the practice of manufacturers at the present day is rather to
under-burn the cement, with the object of economising the
expense of grinding. This material differs in this respect also
from the ordinary limes, that the precise point of calcination does
not appear to affect its qualities.
Before being burnt, the stone is of a fine close grain, of a
peculiar pasty appearance; the surfaces of fracture are rather
greasy to the touch, and somewhat warmer than the surface of
the stone. Examined with the microscope, it exhibits many
sparkling points, which may be either crystals of carbonate of
lime or of some of the other constituents. It sticks easily to the
tongue ; it does not strike fire ; its dust, when scraped with the
point of a knife, is a greyish white for the most part, especially
when derived from the Blue Lias formation. It effervesces with
nitrous acid, and gives off nitrous acid gas. During calcination
the cement stone loses about one-third of its weight, and the
colour becomes of a brown tinge, differing with the stones from
which the cement is obtained. When burnt it becomes soft to
the touch, and leaves upon the fingers a very fine dust ; and it
sticks very decidedly to the tongue.31
Magnesium Cements of America. — These are either rock
cements composed of bisilicates of lime and cement, or trisilicates
of lime, magnesia, and alumina. The bisilicates are, as a rule, of
the Portland cement type, and are frequently calcined at a white
heat; the trisilicates are fired at a lower temperature, and are
more of the nature of Roman cement.30
Portland Cement, Selenitic Cement, and Cements formed from
Sewage Sludge are artificial cements with regard to which the
reader is referred to special text-books such as Calcareous
Cement, by Redgrave and Spackman, etc.
PLASTERS.
Plaster of Paris, so largely, employed in France both for
external and internal work, but with us chiefly for interior
mouldings and ornamentation, is derived from common gypsum
or sulphate of lime. Gypsum occurs in several formations, but
in Europe it is found mainly in the Trias and Tertiary, its
presence in beds of great purity in the Wealden being a recent
discovery of the sub-Wealden borings. In Britain available
supplies can be obtained from Chellaston in Derbyshire, Syston
in Leicestershire, Tutbury in Staffordshire, Droitwich in
Worcestershire, Cardiff in Glamorganshire, and at Kirkby-Thore
in Westmoreland, the beds being of various colours, texture, and
CH. XV.] LIMES, CEMENTS, AND PLASTERS. 329
purity. Being baked in ovens to discharge its water of crystallisa-
tion, it falls into a soft white powder (the plaster of Paris of
commerce) ; and this powder, when worked into a paste with
water, though plastic and pliable for a while, soon sets hard with
considerable strength and solidity. When mixed with glue
instead of water, plaster of Paris becomes stucco.
Keene's and Parian Cements. — If, instead of being used with
water, plaster of Paris, in fine powder, is thrown into a vessel
containing a saturated solution of alum, borax, or sulphate of
potash, and after soaking for some time is taken out, rebaked,
once more reduced to powder, and then moistened with a solution
of alum, a hard plaster is obtained that takes a high polish.
This plaster is called Keene's cement if made with alum ; Parian
with borax ; and Martin's with pearl ash.13
GEOLOGICAL DISTRIBUTION.
General Laws. — A knowledge of the laws which appear to
regulate the geological distribution of the rocks which supply
hydraulic and other limes may prevent many useless researches
and save perhaps some injudicious outlay of capital.
It is known, to quote nearly the words of M. Parandier, that
every stratified geological formation comprehends a series of beds,
whose deposition corresponds with the various periods of existence
of the marine basin in which they were formed, which marine
basin must have had its hydrographical limits, its affluents, etc.
In the first periods, immediately after the cataclysms and the
great erosions (which, in disturbing the status quo of the preceding
geological epoch, had given rise to the new order of things), the
sedimentary deposits must principally have owed their origin to
the matters held in suspension in the liquid. They must have
taken the form, for the most part, and throughout the whole
extent of the basin, of agglomerated rocks, sandstones, clays, etc.,
except in the isolated points of the affluents, in the great
depressions of the bottom, and in the very deep waters, where
the materials brought down by the currents could not arrive,
and where the beds took a degree of compactness different from
that which is to be found on the borders of the basin. By
degrees the matters held in chemical suspension in the waters,
and which were in the beginning mingled with those in
mechanical suspension thus brought down, began to deposit, in
greater relative proportions, as soon as the geological condition
of the basin had resumed a normal state. At times re-
currences of the great agitations of the strata were reproduced
330 GEOLOGY FOR ENGINEERS. [pi. V.
in the same geological epoch, but always during a shorter period,
and, with less intensity, with the same phenomena.
Thus, in the lower divisions of the secondary strata, we find
the marls, the siliceous sands and clays, the calcareous marls, the
ferruginous strata; then the limestones with all the different
varieties of texture and composition; and lastly, we find the
magnesian limestones. The contact of certain formations either
contemporaneous with, or posterior to, the formation of the
different strata often modifies these last. The presence of certain
ingredients, and the secular action of the exterior agents, also
often produce very remarkable modifications or alterations, and
even some molecular transformations, which are very curious,
changing even the chemical and physical properties of the rocks.
But these phenomena have their particular laws, and their definite
epochs of appearance, and we can calculate with a tolerable
degree of certainty upon the extent of their action.31
Probable Position of Different Materials.— It is easily to be
conceived, from what is stated above, that we should be able to
predicate within certain limits the points at which the rocks are
likely to contain the elements the most favourable to the attain-
ment of the object in view in such researches as the one before
us. The materials likely to furnish us the sands and clays fit to
be converted into artificial pozzuolanas are generally to be met
with at the bottom of the sedimentary formations. The lime-
stones likely to yield hydraulic limes occur amongst the marly or
argillaceous beds, or at the points where these last pass into the
purer calcareous rocks, and which are marked by the intercala-
tion of strata of limestones and clays. The upper members of
all the series may be regarded as being too free from argillaceous
matter to furnish anything but rich limes.
Amongst the secondary formations we find, for instance, that
the Lower Chalk marl passes into the clays of the Gault, or the
Upper Greensand, and that it yields a lime which is often
eminently hydraulic. In the Greensand there are few solid
calcareous rocks ; there are few also in the lower members of the
Cretaceous formations below the Greensand. Hydraulic limes are
to be obtained from the beds of limestone intercalated between
the marls of the Kimmeridge Clay ; in the Oxford Clay, at the
passage between the upper and lower calcareous groups of this
division of the sedimentary rocks ; and in the Liassic series.31
Lias Lime. — In England, where the "rule of thumb" prevails
so extensively, it is the general practice to receive the blue lias
lime as a good and a satisfactory hydraulic lime in all cases, and
without any regard to the positions in the series that the beds
of that formation may occupy. It is, however, necessary to
CH. XV. J LIMES, CEMENTS, AND PLASTERS. 331
remark that every bed of the blue lias limestone contains a
different proportion of the silicate of alumina, in combination
with the carbonate of lime, and that therefore the powers of
setting under water must be very different in the limes obtained
from them. Even at the base of the Liassic series, the differences
that occur are as great as between about 8 per cent, of the
silicate of alumina and 90 per cent, of carbonate of lime, and
64 per cent, of the former ingredient to 34 per cent, of the latter.
The first of these would yield only a moderately hydraulic lime ;
the latter would yield, on the contrary, a most energetic cement,
if burnt and ground. The peculiar properties of the blue lias
lime have been established upon the results that have followed
the conversion of the middle beds of the series, which contain
from 16 to 20 per cent, of the silicate of alumina. It would be,
of course, easy to distinguish the best qualities of blue lias lime,
as in fact it is easy to predicate the nature of any description of
that material. Thus the lumps of burnt limestone should be
rather large, and they should present on all sides a conchoidal
fracture ; the lime should swell but little in slaking, and it
should not give out much heat, nor yield to the effect of the
water before about two to five minutes. A lirne of this descrip-
tion requires to be slaked before being mixed with the sand for
use in a building ; but as some builders have a fancy for the
employment of lime " hot," as they call it, it is safer to employ
the blue lias lime after being ground. The best descriptions of
blue lias lime are obtained from Warwickshire, Leicestershire,
Dorsetshire, the neighbourhood of Bath, Aberdare, Rugby, etc.;
but they are all of them of very variable composition, and
they require to be used with great precaution ; at least until
the precise nature of the beds has been ascertained.31
British Limestones. — The limestones, which lie at the founda-
tion of all limes, mortars, and cements, are abundantly diffused
through the stratified formations, there being scarcely a system
which does not present one or more horizons of calcareous
deposits. Indeed, every system, from the oldest to the most
recent, has its limestones : the Metamorphic, its crystalline
marbles; the Silurian, its coralline and shelly beds; the Old
Red, its cornstones; the Devonian, its coralline and shelly
marbles; the Carboniferous, its coralline, encrinal, shelly, and
fresh-water beds ; the Permian, its dolomites ; the Trias, its
muschelkalks and gypsums ; the Jurassic, its oolites ; the
Wealden, its shelly beds; the Cretaceous, its chalks; the
Tertiary, its gypseous and nummulitic strata ; and the Post-
Tertiary, its lacustrine marls.
In Britain the most of these are abundantly developed ; and
332 GEOLOGY FOR ENGINEERS. [PT. V. CH. XV.
for its area few countries can boast of such a varied and available
supply. As mixed rocks they vary, of course, in composition,
some being almost pure carbonates, some dolomitic or magnesian,
and others sulphates or gypsums ; while these varieties may
again be more or less siliceous, argillaceous, ferruginous, or
bituminous.
Whatever the varieties, or in whatever formations they may
occur, the most of these limestones come to the surface in
long stretches of outcrop, and are consequently quarried in open
workings ; hence the numerous openings, great and small, on the
chalks, oolites, magnesian limestones, and mountain limestones of
England, and the mountain limestones of Ireland. England and
Ireland are magnificently supplied with limestones ; Scotland but
scantily so, and hence the more frequent recourse to mining of it
in that country, as well as to its importation from the north of
England and Antrim.11
The Lias of England, which stretches across the country from
Whitby on the north-east to Lyme Regis in the south-west, is our
main repository of water-setting limestones (blue lias) ; but
available beds also occur among the Carboniferous limestones of
Flintshire (Heublas), Northumberland, Lanarkshire (Arden,
Hurlett), coast of Fife (Blebo, etc.), and in the Lothians at
Dunbar, Cousland, and other places. Such beds may be dis-
tinguished in the field by their tougher and earthier texture
— never being so crystalline as mortar limestones — by their not
effervescing so violently under acids, and by their weathering more
slowly into a deeper brown surface.
Some of the argillo-calcareous ironstones known as " curl " or
"cone in cone," containing about 10 per cent, of iron, are also
used (Coalbrook Dale) in the manufacture of hydraulic cements ;
and the septaria from the Lower Lias and London Clay are
well known to cement-makers for their strong and energetic
hydraulicity.11
CHAPTER XVI.
EOADS AND CANALS.
Section I. — Road-making.
SELECTION OP ROUTE.
Value of Geological Knowledge. — Where a new route has to
be chosen, the engineer, from his geological knowledge of the
district, may often show great skill in avoiding expensive cuttings ;
in making cuttings which, though expensive, may more than
repay themselves by the utilisation of the excavated rocks ; and
in keeping clear of peaty and marshy hollows for his enbank-
ments, which are never stable till the soft boggy sludge is
squeezed out, as it were, by three or four times the amount of
carried material that would be required on a firmer bottom. In
choosing a new route, shortness, easy gradients, and the require-
ments of the district are, no doubt, prime considerations ; but in
some instances it may be worth while to deviate from the selected
track in order to come in closer proximity to quarries, clay-pits, and
coal-fields,— the increased traffic arising from which may become
a source of income for the permanent maintenance of the highway.11
Determination of Route. — The first step is to ascertain the
position of the watercourse and watershed lines of the district to
be passed through. The general direction having been selected,
the river-crossings must be examined and decided upon, and the
points determined at which the watersheds are to be crossed. The
approaches to the bridges must be carefully set out, and the ascents
to and descents from the watershed contoured, where they are to be
in side-cutting, from the summits downwards so as to ascertain the
points at which the hills are to be entered. Trial-lines should then
be run between the points thus fixed, and the country carefully
examined on each side of these trial-lines before the route is finally
decided on. The actual survey can then be proceeded with.32
Laying out New Roads. — Reconnaissance. — The general series
of operations preliminary to the formation of a new line of com-
munication are the examination or reconnaissance of the country
333
334 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
between the points to be connected, taking note of the physical
features of the country, its geological formation and sources
from which materials for construction may be obtained, and the
probable requirements of the district to be passed through. In
this work the engineer will be greatly aided by obtaining the
best and most reliable maps of the district. Flying-levels are
generally taken concurrently, in order to ascertain the elevations
of detached points, such as passes across ridges, and valleys,
also points where structures of magnitude may be required.
General Principles in the Field. — In laying out a line for a new
road, the following data should be carefully noted and recorded
in the field-book : —
Examine the inclination of the strata, their nature and con-
dition as to dryness.
Have the surface of road exposed as much as possible to the
action of the air and sun's rays.
Cross valleys and passes at right angles.
Examine beds of rivers at proposed crossings, and up and
down stream, with a view to secure stable foundations for
bridges, culvert, etc.
Examine sources, accessibility, and distances of the supply of
material for the erection of structural works, and for stones
suitable for the road-covering.
Ascertain accurately the level of all existing lines of com-
munication, such as railways, roads, canals, and of rivers and
streams.33
ROAD CONSTRUCTION.
Road-cuttings. — Having selected a route, the engineer has
next to inquire what excavations, what embankment, and what
bridges will be necessary to render the road of easy traction as to
gradients. In the matter of excavation it requires some skill,
according as the cutting may be through tough boulder clay-
through an admixture of drift sands and clays, which are apt to
slip by the percolation of water — through greenstones and basalts,
which, though expensive to remove, may be utilised as road-
material — or through sandstones and limestones which may be
applied to the erection of bridges and retaining walls. Some
acquaintance with the structure of rocks will also be of use to the
engineer, in so far as these may be jointed or full of " backs and
cutters" like some limestones; columnar or subcolumiiar, like
basalts and greenstones ; tabular, as granites ; or in alternate
hard and soft strata, as sandstones and shales. Every formation
has its own lie and structure, and excavating in accordance with
these is always the cheapest and most expeditious method. Where
SECT. I.]
ROADS AND CANALS.
335
the material is of uniform character, little care is needed either
as regards retaining walls or slope of excavation ; but where the
material is of unequal durability, as alternations of sands and clays,
of sandstones, shales, and clays, the weathering of the softer beds
is sure to ensue, and should be protected by facing up immediately
after excavation. From want of this precaution — and especially
in railway cuttings — much of the expense has often been entailed,
and that not till obstructions and accidents have happened through
slips and falls — such contingencies of themselves costing ten times
the amount of any walling-up that might have been at first adopted.
Some care is also necessary when excavations pass through strata
at high angles, so as to prevent slips from the rising side ; and
when water-bearing beds occur, free egress must be made for the
outflow, which otherwise would, in process of time, bring down
the strongest retaining wall. Where cuttings pass through rocks
suitable for building or for roads, a free face should be kept, if
possible, for future quarrying — the situation being so available, not
only for the working, but for the removal of the quarried material.11
Side-slopes. — The forming of the side-slopes requires consider-
able attention, so as to ensure stability and prevent slipping.
The resistance to slip arises partly from the friction between the
grains composing the soil and partly from their mutual adhesion.
Friction is, however, the only force which can be relied upon for
permanent stability, as the adhesion of the earth is destroyed by
the action of air and moisture, this being especially the case
during alternate frost and thaw. The nature of the soil, its con-
dition as to internal moisture and the atmospheric influence,
therefore, combine in fixing the inclination of the side-slopes.
The angle of repose, or, as it is generally termed, the natural slope
at which different kinds of earth, by friction alone, will remain
permanently stable, is shown in the following table given by
Professor Rankine in his Civil Engineering : —
Earth.
Angle of
Repose.
Coefficient of
Friction.
Customary
Designation of
Natural Slope.
Dry sand, clay, and
/ from 37°
075
1-33 to 1
mixed earth
t to 21"
0'38
2-63
Damp clay
45°
1-00
1
Wet clay
/ from 17°
\ to 14°
0-31
0-25
3-23
4
Shingle and gravel .
/ from 48°
1 to 35°
1-11
070
0-9
1-43
/ from 45°
1-00
1
Peat .
\ to 14°
0-25
4
336 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
The slopes most frequently adopted for earthwork are 3 to 2
and 2 to 1, corresponding to the angles of repose 33J° and 26|°
nearly.83
With regard to the slope necessary to be given to the side of an
embankment or cutting, this should always be greater than the
inclination which the earth naturally assumes, and which varies
according to the nature of the soil, as will be observed from the
following details given by Sir H. Parnell : — " In the London and
plastic Clay formation it will not be safe to make the slopes of
embankments or cuttings that exceed 4 feet high with a steeper
slope than 3 feet horizontal for 1 foot perpendicular. In cuttings
in chalk or chalk marl the slopes will stand at 1 to 1. In
sandstone, if it be hard, solid, and uniform, the slopes will stand at
a J to 1, or nearly perpendicular.
" If a sandstone stratum alternate with one of clay or marl, it is
difficult to say at what inclination the slopes will stand ; this will,
in fact, depend upon the inclination of the strata. If the line of
the road is parallel to the line of the bearing of the strata, in such
cases large masses of the stone become detached, and slip down
over the smooth and glassy surface of the subjacent bed. There
are many instances of slips in sandstone and marl strata under
such circumstances as those now described, and here the slopes
are as much as 4 to 1. If the road is across such strata, or
at right angles to the line of bearing, then the slopes may be
made 1£ to 1 ; but if the strata lie horizontal, even though there
should be thin layers of marl between the beds of stone, the
slopes will stand at a J to 1. But it will be necessary, if the beds
of marl exceed 12 inches in thickness, to face them with stone."
If any beds of gravel or sand are found intermixed with clay,
drains should be cut along the top and even in the sides of the
cuttings; for if this precaution be not taken, the water, which
will find its way into the gravel, will, by its hydrostatic pressure,
force the body of clay down before it, and slips will take place
even when the inclinations are as much as 4 to 1 ; and when this
occurs it is extremely difficult to re-establish them.
In limestone strata, if they be solid, slopes will stand at a J to
1 ; but in most cases limestone is found mixed with clay beds,
and in such cases the slopes should be 1J or 2 to 1. In the
primitive strata such as granite, slate, or gneiss, slopes will stand
at a J to I.34
In excavations through solid rock, which does not disintegrate
on exposure to the atmosphere, the sides might be made perpen-
dicular ; but as this would exclude, in a great degree, the action
of the sun and air, which is essential to keeping the road-surface
dry and in good order, it is necessary to make the side-slopes with
SECT. I.] ROADS AND CANALS. 337
an inclination varying from 1 in 1 to 2 in 1, or even more, accord-
ing to the locality, the inclination of the slope on the south side
in northern latitudes being made less steep in order that the
road-surface may be more exposed to the sun's rays.
The slaty rocks generally decompose rapidly on the surface,
when exposed to moisture and the action of frost. The side-
slopes in rocks of this character may be cut into steps, and then
be covered by a layer of vegetable mould sown with grass seed, or
else the earth may be sodded in the usual way.
The stratified soils and rocks, in which the strata have a dip or
inclination to the horizon, are liable to slips, or to give way, by
one stratum becoming detached and sliding on another ; which is
caused either from the action of frost or from the pressure of
water, which insinuates itself between the strata. The worst
soils of this character are those formed of alternate strata of clay
and sand, particularly if the clay is of a nature to become semi-
fluid when mixed with water. The best preventives that can be
resorted to . in these cases are to adopt a system of thorough
drainage, to prevent the surface-water of the ground from running
down the side-slopes, and to cut off all springs which run towards
the roadway from the side-slopes.35
Methods of Drainage. — Great pains should be taken to
thoroughly intercept, from the rising ground, any flow or filtering
of water towards the road bed. This is readily accomplished by
forming catch-water ditches or drains on the uphill side of the
cutting a few feet back from the crest of the slope. These, if
possible, should be carried to the most convenient watercourses ;
but where this is impossible, or too expensive, the water may be
conveyed down the slope in a pipe 18 inches below the surface, to
the side channel. These side channels or drains should be con-
structed at the foot of the slope in cuttings.33
Where slips occur from the action of springs, it frequently
becomes a very difficult task to secure the side-slopes. If the
sources can be easily reached by excavating into the side-slopes,
drains formed of layers of fascines, or brushwood, may be placed
to give an outlet to the water, and prevent its action upon the
side-slopes. The fascines may be covered on top with good sods
laid with the grass side beneath, and the excavation made for the
drain filled with good earth well rammed. Drains formed of
broken stone, covered in like manner on top with a layer of sod to
prevent the drain from becoming choked with earth, may be used
under the same circumstances as fascine drains. Where the
sources are not isolated and the whole mass of the soil forming
the side-slopes appears saturated, the drainage may be effected by
excavating trenches a few feet wide at intervals to the depth of
22
338 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
some feet into the side-slopes, and filling them with broken stone,
or else a general drain of broken stone may be made throughout
the whole extent of the side-slope by excavating into it. When
this is deemed necessary, it will be well to arrange the drain like
an inclined retaining wall, with buttresses at intervals projecting
into the earth further than the general mass of the drain. The
front face of the drain should, in this case, also be covered with a
layer of sods with the grass side beneath, and upon this a layer
of good earth should be compactly laid to form the face of the
side-slopes. The drain need only be carried high enough above
the foot of the side-slope to tap all the sources ; and it should be
sunk sufficiently below the roadway surface to give it secure
footing.
The drainage has been effected, in some cases, by sinking wells
or shafts at some distance behind the side-slopes, from the top
surface to the level of the bottom of the excavation, and leading
the water which collects in them, by pipes, into drains at the foot
of the side-slopes. In others, a narrow trench has been excavated,
parallel to the axis of the road, from the top surface to a sufficient
depth to tap all the sources which flow towards the side-slope, and
a drain formed either by filling the trench wholly with broken
stone, or else by arranging an open conduit at the bottom to
receive the water collected, over which a layer of brushwood is
laid, the remainder of the trench being filled with broken
stone.35
Subsoil Drainage. — Soils of a siliceous and calcareous nature
and rocks generally do not present any great difficulty, as their
porous nature assists in securing a dry and solid foundation. The
side drains in cuttings and the open ditches in the level portions
of a road will, as a rule, be sufficient for this purpose, even where
the roadway is of a great width.
It is the argillaceous and allied soils which require careful treat-
ment, as, being of a retentive nature, they become very unstable
when in contact with water and the action of frost. The drainage
of such soils may be effected by forming transverse or cross drains
in the form of the letter V — the apex away from the direction of
flow — with 2-inch or 3-inch salt-glazed pipes laid about 15 to 18
inches below the formation level, and properly connected to the
side drains.33
MOUNTAIN ROADS.
Crossing Watersheds. — (1) When the route lies across the
valleys, as in the case of a road parallel to a coast-line passing
over the spars of a coast-range. Two modes of treatment are
SECT. I.] ROADS AND CANALS. 339
possible. Either the crests of the hills may be cut down and the
valleys filled up to the extent required to obtain a suitable
gradient on the most direct line, or the road may be contoured on
the hillsides so as to obtain a surface-line of greater length with
easier gradients. The second course is in many cases preferable,
especially where economy is important ; and it may be laid down
as a general rule that the expense of deep cuttings should
only be incurred where the total rise can be reduced by so
doing.
Where sloping ground occurs it is better to follow the contour
lines with long stretches of easy gradients, and to flatten the
curves, where necessary, by cutting off the spurs, than to set out
straight road-lines. These involve either a number of additional
culverts, or the breaking up of the road into a succession of
short, alternating gradients, than which nothing can be more
objectionable.
(2) When the route follows the line of a principal valley cross-
ing the main watershed at its head. The first thing to be done
is to ascertain the lowest point of the range to be crossed, and the
next step is to ascertain its actual altitude and the distance from
the foot of the ascent to the summit of the pass. From these
data the gradient can be calculated approximately ; and it is to
be borne in mind that the actual gradient must be steeper than
the calculated gradient, in order to allow for passing through the
most favourable ground.32
Mountain Passes. — These generally come under one of three
classes : —
(1) A simple saddle connecting the heads of two valleys.
(2) A saddle connecting the head of one valley with the side of
another.
(3) A valley between steep hills, leading from a point near the
head of one valley to a corresponding point in another.
The first two cases are generally very simple in treatment, the
only question generally being whether the summit should be cut
down or passed over by surface gradients. The latter plan
should be adopted where practicable, as it is difficult to keep the
slopes of cuttings in repair at high elevations, to say nothing of
the risk of a road being blocked by snow-drifts in the cuttings.
The third case, however, often requires a great deal of careful
study. The ends of the upper valley forming the pass are often
blocked by moraines, enclosing peat swamps and deep pools of
water, sometimes of sufficient extent to be dignified by the name
of lakes. It will be a matter for consideration whether the
morasses should be drained or skirted, and whether the moraines
should be cut down or passed over. As a general rule, it is
340
GEOLOGY FOB ENGINEERS. [PT. V. CH. XVI.
desirable, at high elevations, to avoid as far as possible both
embankments and cuttings and to adopt surface gradients, when-
ever practicable, although involving a somewhat circuitous
route.32
Line of Descent. — In selecting the line of descent the following
directions should be observed : —
(1) Take the sunny side of the valley if the ground will
permit.
(2) Carefully examine the stratification of the rocks to be cut
through, and avoid, if possible, all strata overhanging the line of
FIG. 89. — Road-cuttings in mountain pass.
road. Thus in fig. 89 the side cutting at a would not be safe
without the protection of a retaining wall ; whilst that at b
would be perfectly secure without any artificial protection.
(3) Run a trial gradient through the work, and find where it
intersects difficult ground. Then lay out the line at these
points so as to obtain the most advantageous levels for the
execution of the work, and readjust the gradient as may be
required. For instance, in fig. 90, if the level of the road is fixed
at a, the floor of the cutting will be in the solid with tight side
cutting ; if at c, the available width would be considerably
reduced, and the amount of cutting increased ; whilst if the level
were at b, a retaining wall would be necessary.
It sometimes happens that advantage can be taken of the
SECT. II.]
ROADS AND CANALS.
341
natural stratification to economise work in a long side cutting.
This was done by Mr E. Dobson in the case
of a road over Evan's Pass, at Port Lyttelton,
New Zealand. The descent of the pass was on
the side of a long volcanic spur, formed by a
succession of lava streams, dipping at an angle
of 1 in 12, the lower part of each lava stream
being hard volcanic rock, whilst the upper portion
was soft and easily worked. The line was
originally set out with a gradient of 1 in 17,
which would have entailed a series of cuttings
through the hard rock, and retaining walls in
front of the softer portions. By altering the
gradient, however, to that of the lava streams, a
solid floor was obtained throughout, the retaining
walls were dispensed with, and the excavation
was made chiefly in soft material. The altera- „ on R d
tion effected considerable saving in time and first cu'tting in
cost, as well as in the cost of maintenance.32 mountain pass.
Section II. — Road Materials.
INFLUENCE OF WEATHER.
Careful observation and study of roads under different condi-
tions of weather give colour to the opinion that the last word has
not been said nor the last thing done with earth, stones, and
water.
Classes of Roads. — Apart from all other classifications of road,
we may conveniently regard them as being wholly, or mainly, in
one or other of two classes, " wearing roads " and " weathering
roads," which, if well made, are wear-resisting and weather-
resisting respectively. The former — for instance, roads with
heavy traffic— would wear out faster under their traffic in ideal
perfect weather than they would in the actual weather without
traffic ; the latter — for instance, most moorland roads — would
wear out faster in the actual weather without traffic than they
would under their traffic in ideal perfect weather. " Wearing
roads " have naturally been the more studied. In dealing with
" weathering roads " there is less knowledge of what are
economical and efficient methods of construction and mainten-
ance and what are the best materials to use. But in the
aggregate such roads are of considerable importance.36
Water. — The variable amount of water which there is at any
given time in the crust of a road, in the air above it, and in the
342 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
subsoil under it, as well as the influence of surrounding objects,
make this part of the subject of much intricacy. With just the
right amount of water an earth road affords excellent going,
while without water we are driven to the use of squared blocks,
or some quite different construction, even then usually depending
upon water to give the subsoil some consistency.
Heretofore little has been done, either in choice of materials or
in treatment of environment, by way of combating the adverse
conditions of drought; but, realising that a water-logged road
more easily gives way under traffic than a dry one, the road-
maker has mainly directed his efforts to getting rid of superfluous
water by drainage, and to providing a more or less waterproof
surface. The object of the latter is to save the binding material
from the scouring effects of water trickling through it, to prevent
denudation of the subsoil under the crust, and generally to keep
both road-crust and subsoil drier than they would otherwise be.
But a further study of water, as an ingredient of the road-crust,
will well repay the trouble, particularly on the best and the
poorest roads. On the former, having a crust of road metal and
binder of such materials that it will hold up traffic even when
water-logged, it is allowable to aim at slow drying rather than at
quick drainage, so that the road may recover better under traffic.
Again, on roads of a modest class, a little extra mud, when every
part is muddy, may well be borne if it imply such conditions as
will prevent the road from breaking up in drought, keep it on
the whole firmer and less dusty during the period of greatest use,
and render it less liable to injury from heavy rains. Cycles are
now much used on by-roads, and many such roads, in the south
of England particularly, become worse very quickly, from the
cyclists' point of view, after a few days of dry hot weather. Much
stone, too, is kicked out and ground up.
One advantage of a nearly waterproof surface is that it retards
evaporation from the subsoil. In dry weather anglers search for
worms under hard-beaten ground, and not in loose earth. One
has to think, too, not only of what happens when soil gets dry,
but of what happens to a dry powdery soil or to a clay soil when
wet comes. In the latter case, if the cracks formed have been
beaten in, the clay will swell when it becomes wet and ooze out
on to the surface, the wet weather getting the blame.
While the injury done to the metalled part of a good main
road by an ordinary dry spell is comparatively small, there are
many roads of a secondary character, and many by-roads, to
which drought does much harm, accentuated, it may be, by undue
camber, excessive drainage, or unreasonable exposure. On an
easily denuded soil drought does a good deal of injury to the
SECT. II.] ROADS AND CANALS. 343
edges of some roads, though the bulk of such injury is usually
ascribed to the wet weather, which shows it up, just as rain is
blamed for turning into mud dust which ought to have been pre-
viously removed or never allowed to form. Geological position
influences the water conditions of a road very much. Recent wet
seasons, for example, have shown up very well how a road gains
by being cut off from the neighbouring land in porous strata
liable to become water-logged, the fall in water-level after rain
being much more rapid in the narrow road-strip than in the wide
strip of land served by the same ditch.
On a clay soil the treatment is radically different, and the
ditches need not be deep so long as they are big enough to carry
off the water which, during and after heavy rains, runs along and
off the surface. A stiff clay gets damp very slowly even in rain,
and dries very slowly by evaporation through the surface. The
stiffer a clay is the less use it is to drain it, the logical system of
under drains being (since 1 inch of clay can stop water) drains of
1 foot width, 12 inch apart, centres. This is the principle of the
Telford pavement — the logical outcome of placing drains close
enough to drain any particular soil. Under it the clay, though
becoming quite damp in time during rains, will remain quite
capable of sustaining the road, and only give way in proportion
as it is interleaved and mixed with water by surface action.36
MATERIALS FOR " WEARING " ROADS.
Local Circumstances. — The material for roads will necessarily
depend on local circumstances, although, where there is a very
rapid wear, the best materials, however costly, will be the
cheapest. The chief quality for a good road-stuff is hardness
combined with toughness, and a texture sufficiently uneven to
ensure a rough surface under wear. There are certain stones,
such as Penmaenmawr, which are exceedingly hard and of fine
grain, and have a high value in some cases ; but as they neces-
sarily wear smooth, they are ill-adapted for cities exposed to
alternations of wet and dry, cold and heat. Granites are for this
much superior, though less durable, as, owing to their composi-
tion, which includes two sets of crystals of different hardness
(quartz and felspar), they always have a tendency to retain a
rough surface, giving foothold for horses. Those basalts which
do not readily decompose are equal, and sometimes even superior
in value to granite. It may be said, in a general way, that all
stones of uniform texture, composed of one ingredient, are unfit
for roads over which the traffic is very large. Thus limestones of
all kinds would be inadmissible on this ground, even if they were
344 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
not too soft and too readily worn into dust and mud. Flints,
which from their hardness would seem valuable, are also inadvis-
able for want of some cause of roughness.13
Suitable Road Metal. — The hard igneous and metamorphic
rocks are chiefly used, the principal being granites, syenites,
diorites, basalts, dolerites, diabases, quartzites, mica schists, as
well as limestones, ragstones, sandstones, flints, and gravel.
Granites. — Those which are compact and fine-grained, and
composed of muscovite and orthoclase, may be taken as reliable
material for road metal. The quartz and mica are practically un-
altered chemically ; the felspar, however, especially when oligoclase
is present, decomposes rapidly into clayey mud on being subjected
to the disintegrating influence of air and water.
Syenite. — The durability of syenite is greater when quartz and
hornblende predominate ; felspar and mica are weak and of a
perishable nature if present to any great extent, and disintegrate
rapidly, especially when the crystals are large and have a dull
appearance. In most cases syenite forms an excellent stone for
road-metalling purposes.
Diorites. — These rocks form a large proportion of the material
used for road repairs in Scotland, and when the component
minerals are fine-grained and compact they make very durable
and satisfactory road metal. On the other hand, those which
contain soda and lime felspar, combined with a coarse texture,
quickly decompose and form clay, which creates on the roads
during wet weather large quantities of mud.
Basalt. — Most varieties are eminently suitable as material for
macadam ; they are generally hard and durable, combined with
the other qualities upon which depends the value of road-
metalling.
Quartzites and greywackes are the best among metamorphic
rocks ; many of these have a sufficient admixture of felspar, which
tempers the natural brittleness of the quartz.
Mica schist is much used as a road material in the Highlands
of Scotland and in some parts of Wales. Owing to its foliation
it is not a suitable stone for repairing roads for heavy traffic. It
binds well, however, and makes a very good surface for light
weather, is very muddy in wet weather, but dries quickly.
Carboniferous or mountain limestones are much used for
macadam and are very suitable for light traffic, but create
much mud and dust and are quite broken up by frost and
thaw.
Ragstone is greatly used in Kent for road material.
Sandstones are much used for bottoming roads, but are too soft
for metalling.33
SECT. II.] ROADS AND CANALS. 345
MATERIALS FOR WEATHER-RESISTING ROADS.
Besides igneous rock, there are many of the tougher and more
durable of other stones which are suitable for any except the
heaviest traffic, others which have special advantages on certain
soils, and some which are valuable on weather-resisting roads.36
Limestone. — Among such stones limestone is the most im-
portant. The dust formed on a limestone road is seldom of a
very irritating kind, the stories do not cut rubber tyres, and
though the glare on white limestone is sometimes rather trying,
the gain is, on the whole, with a road which absorbs less heat
than others, while the lessened radiation diminishes frost. Lime-
stone wears evenly and smoothly, and yields a cementitious
detritus. It is therefore a good weather-resisting material, and
on " weather-resisting " roads a fairly soft stone may be quite
suitable for light traffic. There is less shifting of material on a
limestone road than on most other kinds of similar cost ; and
shifted material does, as a rule, less damage to the road-crust.
Siliceous limestones have the advantage of producing a less
slimy mud than purer or than marly stones, and several useful
stones for road purposes lie on the border between sandstones and
limestones, the presence of carbonate of lime in considerable
quantity in the sandstone having a good effect upon the binding
and toughness of the broken stone. Gritstones are usually better
than sandstones proper, but are apt to yield a more irritating
dust.36
Flints are largely used for by-roads in their districts, and are
exported a good deal. Though some flints are tougher than
others, they are generally too brittle for main roads, and produce
irritating dust. They break "unkindly,'' and the fractures are
sharp and bad for cycle tyres. Unbroken small flints are often
suitable for the "shoulders" of a road of a modest class. Water
does not rest upon the surface of such a strip, which forms a
means of draining the carriage-way without a scour. There is
a kind of interlocking between flints of irregular shapes which
enables them to sustain traffic to some extent with little disturb-
ance. For the carriage-way proper they should be broken small
and well consolidated with a binder, such as a little clay or marl.
For a very cheap road flints may be used to top a loamy gravel,
shoulders being made of the larger pebbles raked out. For roads
of quite an important class, flints have some uses, such as giving
side support, filling spaces where vehicles occasionally pass, and
bottoming or partly filling the cuts to drains.36
Gravel. — The materials commonly known as "gravel" vary
from a mass of pebbles by themselves to what is little more than
346 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
loamy, marly, or sandy matter, with a few pebbles or fragments
distributed through them. Pebble beds often yield very good
material for by-roads, and the objection to their use on more
important roads is often based rather on their shape than their
material. Large quartz pebbles are often broken up for road
metal. Successful roads have been made with gravels containing
50 per cent, of pebbles, 30 per cent, of sand, and 20 per cent, of
clay ; and generally gravels with a proportion of ferruginous clay
will bind well together, and, with a top layer of hard stone, or of
the larger pebbles well broken, make roads of a fair wear-
resistance. With sufficient cohesion in the materials, and the
prevention of excessive dryness, combined with the right kind of
drainage, gravel roads may be fairly weather-resisting.36
It will, however, be easily understood that for country roads
any hard material, that does not soon work up into mud or grind
into dust, and that has the advantage of requiring no expensive
carriage, will be selected. It is well to remember, in such cases,
that sandstone is better than limestone, and hard limestone is
better than slate ; while basalts and granites are exceedingly good
or exceedingly bad, according to the proportion of alkaline earths
(especially soda) which they contain.13
BINDING MATERIAL.
The choice of a binding material is frequently affected by the
kind of stone employed. When a stone wears to a slimy mud,
sand or grit, or chips of siliceous rock are better binders than a
clayey material ; when, however, sand or grit is the chief or sole
detritus, clay is a better binder than sand.36
On main roads clay should never do more than fill the
interstices between stones, or stones and chips which are jammed
tightly together. On a road which is regularly swept and some-
times watered, more importance may be attached to the binder
as a packer, and less to its direct effect upon the traffic. Some-
times more than one kind of binder may be used ; for instance, a
cheap local material, with cheap stone in the lower layer, and a
little of just the right material for the stone used as a wearing
layer.
It is cheaper and easier to make a road with a good deal of
clayey or marly material than to make it of solid stone with
binder crushed in at the surface ; and as the road made with clay
consolidates, the superfluous clay may gradually be removed.
But such a method is not suitable for main roads with much
traffic, on which the camber must be more accurately adjusted,
the pieces of stone must be jammed tightly together, and the
SECT. II.] ROADS AND CANALS. 347
whole crust so compacted that the area of subsoil on which a
wheel rests is as large as possible. Given a certain kind of stone
as most suitable or economical on any particular road, the
engineer can let his choice of binder be influenced by the way he
intends to treat the road.36
On by-roads, return to a normal state after stress of weather
must be more automatic, and the detritus such that, after drizzly
weather, it is capable of being rolled down into a good surface
again by the traffic as it dries, heavy rains being relied upon to
remove the detritus from time to time before it becomes very fine
dust or mud. Usually, on a by-road, conditions of cost and want
of time for attention reduce the problem of maintenance to the
mitigation of, say, one predominant evil in winter and one in
summer. Where water can easily be got, and roads are much
exposed to the sun, it would sometimes pay to give gravelly
by-roads a good soaking, rather than let them come to pieces in a
drought.36
PAVING MATERIALS.
Asphalt (cf. p. 72) is constantly met with in connection with
mud volcanoes (see Chapter II., p. 29). It may sometimes be
absent (as when Humboldt visited the caves of Turbaco, near
Carthagena, in New Granada, in 1801) and yet be found plenti-
fully fifty years later. In other districts the discharge of asphalt
or petroleum is permanent.6
Asphalt is essentially a product of the partial oxidation of
petroleum after the loss of the more volatile constituents, and as
such occurs where oil springs rise to the surface. Among the
more striking localities are the pitch lake of Trinidad, the " gum
beds " of the Canadian oil regions, and the Dead Sea.
Rock asphalt is a granular limestone or dolomite, containing
from 7 to 15 per cent, of bituminous substances soluble in
bisulphide of carbon, found at various places in the Rhone Valley
and the Jura, notably at Seyssel, near Lyons, and Val de Travers,
in Neufchatel. It is of a light or snuff-brown colour, and when
heated to about 140° C. decrepitates and falls into powder.
This, when perfectly free from water and pressed together by
heated cast-iron stampers, reconsolidates, producing a material
scarcely differing in cohesion from the original rock, which is
known as compressed asphalt, and is used for street paving.
Limestones of this character are tolerably abundant in parts of
Germany, Sicily, Savoy, Egypt, and Syria, but the quality best
adapted for paving purposes is mostly obtained from Val de
Travers.14
The Val de Travers works are situated in the Upper Urgovian
348 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
limestone beds of the Swiss Jura. The source of the asphalt is
not by sublimation from any underlying beds, for it is confined to
a special zone, in which the limestone is impregnated with it. Its
origin is attributable to the decomposition of the innumerable
Requienice, Radiolites, and other fossils which abound in that
rock.17
Tar-macadam, which has been hitherto chiefly used for paving,
is now being used on roads. The chief points to be attended to
in preparing it are the cleanliness of the materials and drying the
broken stone, which must not, however, be subjected to sufficient
heat to impair the cohesion of the particles of stone.1
SELECTION OF MATERIALS.
Requisites in a Road Stone. — In selecting a material for the
purpose of road-making, the essential characteristics requisite
may be summarised as follows : —
(1) Hardness, or "that disposition of a solid which renders it
difficult to displace its parts among themselves."
(2) Toughness, or "that quality by which it will endure light
but rapid blows without breaking."
(3) Weather resistance, or " non-liability to be affected by the
weather."
(4) Binding properties. — The latter quality is of no value in
stones for paved streets, and where steam-rolling is practised it is
not of so much moment as when the metalling is consolidated by
wheel traffic.
These qualifications are by no means always found together, and
the selection of a suitable stone for road-metalling under any
circumstances is a somewhat difficult problem. Apart from the
practical experience which one may possess of judging the quality
of any particular rock, more particularly when opening a new
quarry, the only rational and satisfactory means of determining
the point is to make an experimental trial upon a section of road-
way for a sufficient length of time, and over which a known
amount of traffic passes. The results thus obtained can be
compared with those shown on any other section of road similarly
situated, and which has been maintained for years with a material
of recognised quality.
Specific gravity. — Among stones of the same kind that which has
the greatest specific gravity is invariably the strongest, but great
difference may exist in the durability of stones of the same kind
and presenting little difference in appearance.33
Physical Tests. — These are frequently made use of and may
be of great assistance, but the results obtained have only a
SECT. II.] ROADS AND CANALS. 349
relative value, and in many instances the conclusions arrived at
are extremely delusive.
(1) The abrasion test. — This is carried out by placing samples
of broken stone in a revolving cylinder, sometimes along with a
number of small castings. The test is carried out by both the
dry and the wet process ; in the latter case the stones are weighed
first dry, water is added in the cylinders, and after the test the
stones are dried and then weighed, the loss being recorded. The
accuracy of this test depends on the exact similarity of the
samples.
(2) The drop test. — This is carried out by subjecting the
specimen of stone to repeated blows from a falling weight ; 15 Ibs.
falling 10 inches has been used.
(3) The absorption test. — The stone is weighed before and after
immersion in water for twenty-four hours. The stones suitable
for road -metalling absorb, as a rule, the least quantity of
water.
(4) The weathering test. — To determine the ability of the stone,
after absorbing water, to withstand the effects of the disintegrating
action of frost. It may be carried out artificially by Brand's test
(see Chapter XIII., p. 288), but to carry it out properly the stone
should be immersed in water for twenty-four hours and then
exposed to the actual action of frost.
(5) The crushing test. — To determine by means of a hydraulic
press the resistance of carefully dressed cubes of stone to crushing.
This test is a very misleading one so far as road metal is
concerned.33
Durability of Road Stones. — This quality depends partly upon
resistance to mechanical abrasion and partly upon its power to
withstand chemical decomposition. The resistance to abrasion
depends mainly on the composition of the component minerals
and the manner in which they are aggregated together into a
compact mass, while the texture of the rocks also enters largely
into the question. The disintegrating effect, on certain stones, of
the chemical influences met with on the surface of a road, is
brought about by the decomposition of certain of the component
elements causing the formation of a powdery clay. This is
particularly the case with felspar, especially when the soda or
lime varieties exist to any great extent. Biotite, or black ferro-
magnesian mica, is of a very weak and perishable nature, and
affects adversely rocks of which it is a component element.
The efficiency of igneous rocks generally depends on their
compact, granular texture. Those in which the grains are so
small that they are barely visible, forming a continuous mass,
cemented together in a siliceous paste, more or less compact, and
350 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
having a high silica percentage combined with a sufficient quantity
of ferromagnesian constituents which tend to produce toughness,
may be looked upon as satisfactory, being generally of a hard and
durable nature.
Many minerals, when exposed to the action of air and water, are
liable to alteration as distinct from chemical disintegration, their
durability depending on whether the power of cohesion is
destroyed or otherwise. Rocks of a crystalline texture may be
adversely affected by the decomposition of one of the component
minerals ; cohesion is maintained simply by the crystals being
interlaced with each other or wedged together in mosaic fashion,
as in the case of granites.
With some other rocks a more intimate union is observed ; the
crystalline grains are compactly set in a siliceous paste or matrix,
which by itself is sufficiently durable to hold together and with-
stand chemical action, even should the crystals themselves be
adversely affected and become decomposed.
Undoubtedly the best materials for producing the most durable
metalling are basalts, diorites, and syenites ; those having a fine
texture and composed of minerals which remain unaltered when
exposed to chemical influences, give the best results.
Many rocks of the igneous series, however, show great variation,
even when they are of an apparently hard and durable nature,
decomposing and producing much dust in summer and a large
quantity of greasy mud in winter. A trial should be made of
the wearing qualities of each kind of stone of a doubtful char-
acter. This is the only reasonable test which can be depended
upon.33
Regarding the relative strength and durability of various road
materials, Mr Thomas Codrington says : — " It is a difficult matter
to determine. No test but actual wear in the road can be fully
relied on, and though it is easy to see that one stone wears twice
or three times as long as another, it is almost impossible to take
into account all the circumstances under which they are exposed
to wear. The nature of the traffic has a considerable effect on the
relative wear, as well as on the actual wear of different materials,
and the moisture or dryness of the road has often a great effect
on the wear of the same material."
The engineers of the French Fonts et Chaussees have
endeavoured to arrive at a comparative numerical value of the
qualities of the materials used on the national roads, and
coefficients of quality are given for the various materials used
in each department.
The following list has been compiled from a return for
1876:—
SECT. III.]
ROADS AND CANALS.
351
COEFFICIENTS OF QUALITY OF ROAD MATERIALS.
Granitic gravel . . .23
Quartz gravel . . .21
Trap ..... 20
Quartz . . ..10
Basalt . . . .12
Porphyry . . . .10
Quartzite . . . .11
Devonian schist . . .16
Schist .... 4
Sandstone . . . .12
Granite .... 6
Syenite . . . .12
Gneiss . . . . 9
Siliceous pebbles and gravel 8
Silica .... 8
Chalk flints .... . . 7
Siliceous limestone . . 6
Compact limestone . .14
Magnesian limestone . .12
Carboniferous limestone . 9
Oolitic limestone . . . 5
Lias limestone . . . 5
Jurassic limestone . . 5
Limestone . . .- '••'.' 5
Mean of all France . . 10-
to 25 (in one instance 4'8)
to 20
to 20 (in one instance 5)
to 18
to 12
to 16
to 20 (generally 10 to 12)
to 12
to 19 (in one instance 6)
to 16
to 11-6
to 18 (generally about 10 to 12)
to 12
to 10
to 8
to 12
63
The life of road-stone may be increased by seasoning; stone,
like wood, when first removed from its natural formation, is green
and unseasoned ; it is therefore desirable that a stock of stone
should be quarried and broken the year previous to use, so that
it may be exposed to the air for some time before being laid on
the roads.34
Section III.— Canal-making.
In laying down and arranging the general line of a canal,
many points have to be considered in addition to those which
apply to them in common with roads and railways. One of the
most desirable points to be attained is a perfectly level surface
throughout its whole extent. It is, however, very seldom that
the country is so favourable as to allow this to be effected. In
most cases it becomes necessary occasionally to alter the level of
the surface of the canal, the water being retained at the higher
352 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVI.
level by gates so placed that the pressure of water against them
keeps them closed. It is, however, impossible to prevent a small
amount of leakage at the gates, and therefore it becomes
necessary to have the means of supplying the upper portion of
the canal with water, to compensate for that which thus escapes,
as well as that which is necessary to pass vessels from the higher
to the lower level. In addition to these two causes of loss, a
further waste is occasioned by the evaporation from its surface,
and the absorption of the water by the ground through which it
flows (cf. Chapter XII., Section I.).
It is, therefore, an object of considerable importance in the
arrangement of a canal, to obtain some natural feeder (as it is
termed) for the supply of the water thus lost, which object is
usually attained by diverting some of the smaller natural rivers
or streams, and leading as much of their waters as may be required
to supply the highest (technically called the summit) level of the
canal, for that being properly supplied, the lower levels will be
fed by the water which escapes from the upper.
Before forming a canal, the strata through which it will pass
should be carefully examined, more especially with reference to
its powers of retaining water, that is, of not absorbing it. Many
soils, such as clean sand, or gravel, would carry off' the water so
rapidly as soon to drain the canal, and therefore, such strata
should, if possible, be avoided. Where, however, it is impossible
to do so, the canal may be made watertight by lining its sides
and bottom with puddled clay, which consists of good clay,
thoroughly well-beaten up with water, or tempered, and then
mixed with a certain proportion of gravel, sand, or chalk. Pure
clay by itself would not answer, because if at any time the water
in the canal sunk below its ordinary level, the upper part of the
puddle, becoming dry, would crack ; and when the water again
rose it would escape through these cracks, which by its action
would be gradually enlarged until the puddle was rendered
useless.37
Leakage. — The importance of geological knowledge in canal-
making was long ago recognised, and was applied by Mr W.
Smith, in 1811, in a very successful manner. About that time
many canals were being cut in the west of England, and these,
crossing the oolitic hills, were found to be particularly liable to
accidents of leakage, being cut through open-jointed, and some-
times cavernous rocks, alternating with water-tight clays. In
the passage across the former rocks, and more especially when
the summit-level of the canal occurs in them, the water escapes
almost as fast as it enters, and all the skill of the engineer in
puddling and making an artificial bed is sometimes exerted in
SECT. III.] ROADS AND CANALS. 353
vain, and cannot prevent great and ruinous loss. But the
existence of open joints and caverns is by no means the only, nor
indeed is it the greatest source of injury, for innumerable small
faults or slides traverse the country and confuse the natural
direction of the springs, rendering them short in their courses
and uncertain and temporary in their flow, weakening by their
irregular pressure every defence that may be opposed to them,
and causing leaks which let through a portion of the water
contained in that level of the canal.13
The general remedy for all these evils was understood by
Mr Smith and proposed by him for adoption. It is " the entire
interception of all the springs which rise from a level above the
canal, and pass below it through natural fissures and cavities.
This is a process requiring great skill and extensive experience ;
some of the springs, for instance, which it is most important to
intercept come not to the surface at all in the ground above the
canal, but flowing naturally below the surface through shaken
or faulty ground, or along masses of displaced rock which extend
in long ribs from the brows down into the vale, emerge or
attempt to emerge in the banks of the canal ; there no ordinary
surface-draining will reach, and none but a draining-engineer,
well versed in the knowledge of strata, can successfully cope with
such mysterious enemies. But Mr Smith, confident in his great
experience, not only proposed, by a general system of sub-
terraneous excavation, to intercept all these springs and destroy
their power to injure the canal, but further to regulate and
equalise their discharge so as to render them a positive benefit.
This he would have accomplished by penning up the water in
particular natural areas, or pounds, which really exist between
lines of fault in most districts, or between certain ridges of clay
('horses') which interrupt the continuity of the rock, and divide
the subterranean water-fields into limited districts, separately
manageable for the advantage of man by the skilful adaptation
of science." 13 *
* Phillip's Life of William Smith, p. 69,
23
[PT. V.
CHAPTER XVII.
RIVERS.
THE work done by running water has been briefly considered in
Chapter L, Section III., but, to enable the engineer to fully
understand the geological action of rivers, a certain amount of
knowledge of hydraulic laws is essential.
In fact, the geologist must be acquainted with the principles
of hydraulics to enable him to trace the action of rivers, whilst
the hydraulic engineer must have more than a mere smatter-
ing of geological knowledge if he is to be successful in his
undertakings.
MOTION OF WATER IN RIVERS.
Motion of Water. — In flowing water the whole volume does
not move forward in one mass, as is the case with a solid body,
but every individual particle is in motion. As the volume moves
forward, these particles roll round one another in orbits varying
in dimensions according to the section of the stream. The
diameter of the orbit is governed by the distance from the surface
of the water to the bottom of the channel and the distance
between the sides. In shallow streams the particles are con-
tinually circulating in a number of small orbits, rolling round
and amongst one another in all directions, according as they are
diverted by contact with the sides and the bottom. In deeper
streams the orbits are larger, and the disturbing agents fewer in
proportion. Thus with the same velocity the disturbance to the
free flow of the particles decreases as the depth and width of the
stream increases, and the diameter of the orbits consequently
becomes greater. ' In other words, the further the centre of the
stream is from the retarding medium, the less is the effect of
this disturbing rotary motion. This is the cause why a
deep stream has a less eroding effect than a shallow one, and
why, as the hydraulic mean depth is increased, the velocity also
increases.
354
CH. XVII.] RIVERS. 355
The existence of the deep pools which are found in the beds of
rivers, the curved motion which a stream assumes, and its power
to transport material of heavier specific gravity than itself, are
due to this upward and rotary action of the particles of water.
A large volume of water once in motion maintains its flow with
a very slight surface inclination.40
Retarding Force. — If, owing to the action of gravity, water
continued to flow in a river with no resistance, it would be
subject to a constantly accelerating force, but as its motion over
any given length is uniform, there must be also a retarding force.
This retarding force is due to the friction of the particles of the
water against the sides and bottoms, to the adhesion of the
particles of the fluid, to variations in the head and irregularities
in the form of the channel causing disturbance to the motion
and a loss of living force from the particles being reflected in
currents contrary to the general direction of motion, and to
turbidity of the water.40
Velocity. — As rivers increase in size the proportion of the
retarding to the accelerating force continually diminishes, and
they therefore require a less rate of inclination to produce the
same velocity.
Where the flow of water in a channel is uniform, the same
quantity of water will be discharged at the lower end of any
given length as enters at the upper end ; consequently, the same
quantity of water must pass each transverse section per second,
the velocity of the current increasing where the area is diminished
and decreasing where it is enlarged.
The velocity of a stream is not uniform throughout the whole
section. The contact of the particles with the sides and bottom
of the channel retards the velocity of the water immediately
adjacent, and as the particles are reflected they transmit this
retardation to the more distant particles, the particles nearest
the rubbing surface being most affected, and each in succession
being less influenced, and the retardation decreasing towards
the part most distant from the bottom and the sides being at a
maximum at the former point and a minimum at the latter.
The point of maximum velocity is found to be on a vertical line
through the deepest part of the channel and a little below the
surface.
There exists a point where the velocity of the filaments of the
water is at a mean of the whole depth. This point varies with
the depth and other conditions of the river.
Generally, the mean velocity may be taken at 85 per cent,
of the maximum, and its position at the centre, or in deep rivers,
at 0'45 of the depth measured from the surface.
356 GEOLOGY FOR ENGINEERS. [PT. V.
The point of maximum velocity is generally a little below the
surface on the vertical line passing through the deepest part of
the river, the water on the immediate surface being retarded by
the friction with the atmosphere.
The minimum velocity is at the bottom, and its proportion to
the maximum velocity will be affected to a large extent by the
quantity of sediment that is being carried and the depth of the
stream.
Generally, it may be taken that the bottom velocity varies
from about 75 per cent, of the surface velocity for rivers of
depths of about 5 feet, to 50 per cent, for three times this depth,
and 66 per cent, for large rivers.
In these proportions for maximum velocity no account has
been taken of the action of the wind. Gales have a considerable
influence in retarding or increasing the surface, and proportion-
ately the whole velocity. However, observations have shown
that the effect of wind on a river (exclusive of tidal causes) does
not reach beyond mid-depth.40
Contour. — The contour of rivers in their natural condition is
never found to be regular, either horizontally or vertically. The
course of the river, whether tidal or fresh, consists of a series of
curves, and a straight reach of any length is very exceptional.
The bed also consists of a series of pools and shallows, which
maintain their shape and position without change, although the
conditions of the flowing water are continually varying, at one
time running with great depth and velocity, and carrying along
large quantities of solid material, and at other times running
with low velocity and at less depth. Temporary alterations may
occasionally occur, and a river may change its course ; but where
the course remains unaltered the contour of the bed will be found
to remain materially unaltered. Without an investigation of the
cause of this, it would seem natural that the heavy materia
carried by the water in suspension would be deposited in the
pools, and that they would become filled up, and the bed raised
throughout, in the same manner as occurs at the mouth of large
tideless rivers. After the contour of a river has once been deter-
mined, an equilibrium is set up between the erosive action of the
water and the resistance of the material of which the bed is com-
posed, and, this equilibrium being once established, the pools are
maintained by the rotary action of the flowing water.40
Rotary Motion of Particles. — It has been already shown that
the particles of water never move forward in a mass, but that
each particle is deflected from its course by the difference of level
of the surface and the irregularities of the bed. The tendency
of the particles is to move in a curved or rotary path, in which
CH. XVII.] RIVERS. 357
the whole mass of the water participates. This rotary motion,
acting on the sides of the channel, tends to scour away such
portions of the soil as are not sufficiently tenacious to resist the
action, and gradually a hollow is scooped out. This accomplished,
the curved motion of the particles is increased, the filaments of
water are driven out of the straight path and reflected on to the
opposite bank, and so a series of curves is set up. In a pool the
particles of water, being reflected vertically, horizontally, and
longitudinally, are whirled round in every direction, setting up a
centrifugal or screwing motion, but always moving onwards as
fresh particles of water arrive. This action is increased by the
particles having to descend over the edge of the pool at a sharp
angle and then striking the bottom and being reflected upwards.
Particles of solid material in suspension in the water are thus
kept in continual motion. As they descend into a pool they are
thrown upwards and rolled round, until finally they are caught
by the upper current and carried forward.40
Dynamic Action. — In flowing water, in addition to the static
force which at the same depths presses against the sides and
bottom of the channel equally in all directions, there is also a
dynamic force depending on the velocity. If the direction of a
stream be changed, the particles of water are impelled against
the side of the channel, which presents an obstacle to the original
line of direction by this dynamic action. The force thus brought
into play is absorbed chiefly either in cutting and carrying away
the material of which the bank is composed, or, when a state of
equilibrium has been reached and the bank is sufficiently
tenacious to withstand the impact, in heaping up the water and
creating a greater head. In all curves there is, therefore, a radial
dynamic action from the convex towards and on to the concave
side, causing currents in that direction, which tend to deepen the
channel both horizontally and vertically ; or else to increase the
velocity and raise the surface of the water on the concave side,
and to shoal and decrease it on the convex side.
A channel which has once attained a state of equilibrium is
prevented from being further eroded at the curved portions
owing to the varying action of the particles of water as they pass
round the curve. When water which is moving along a straight
channel comes to a part that is curved, the particles of water
which are nearest to the concave side are the first to come in
contact with the curved side of the channel, and are thus the
first to be deflected from their course. The particles next to
these, being later, will collide with those previously deflected, and
a similar action will take place as each parallel series arrives.
The consequence is that the full force of the water, instead of
358 GEOLOGY FOR ENGINEERS. [PT. V.
acting directly on to the hollow side of the bank and eroding it,
will be gradually cushioned by that part of the stream which has
already impinged on it. Even in a sandy estuary, if a deep
trough be once scoured out, the reaction of the tidal currents
flowing up and down and impinging against the sides and bottom
will create an eddying or boring action which maintains the
trough at its greatest depth and prevents deposit. It is due to
this action that the deep pools are maintained, such as the
Sloyne in the Mersey, Lune Deeps in the Irish Sea, Lynn Well in
the Wash, and the steep mounds of sand with deeps on each side
which exist as bars at the mouths of some tidal rivers.40 See
Bars at the Mouth of Rivers, p. 365.
THE TRANSPORTING POWER OF WATER.
Transport of Material. — All rivers during land floods are
charged with a large quantity of alluvial matter which is carried
away in suspension, and their turbid condition then testifies to
the work that is being done in the transport of material. This
detritus is the result of the disintegrating effect of frost and rains,
which break up and loosen the soil sufficiently to allow of its
being washed by the rain into the river (see Chapter I., Sections
I. and IV.). On reaching the channel of the stream it becomes
thoroughly mixed with the water, and is carried along in suspen-
sion. When this material reaches a tidal estuary, it is transported
over the sands and deposited near the banks during the time of
slack tide, where, owing to the shallow depth, there is little or no
scour, causing salt marshes to accrete ; or else it is carried out by
the ebb current and deposited in the sea.40
Erosion. — Flowing water frequently passes along the bed over
which it is flowing without exercising the erosive effect due to the
velocity at which it is running. A very slight cause may change
part of this velocity into erosive energy. A slight obstruction
placed in the bed of a sandy channel will cause erosion, and the
scouring of a pool where previously the water had passed over
without any effect. The deep pools always to be found at concave
bends are instances of the development of this power.
At certain velocities water has an eroding as well as a trans-
porting power. Under normal conditions the sectional area of a
river is sufficient to allow of a velocity slow enough to prevent
erosion, and the natural bed of the river remains in a state of
stability. If, however, the velocity is sufficiently increased, or
any agency comes into play that disturbs the material composing
the bed or banks, the transporting power of the water then carries
away the soil, and the sectional area becomes enlarged. In the
CH. XVII.] RIVBRS. 359
same way detritus brought down at one time and deposited in a
channel may be transported away in floods when the velocity is
sufficient to erode and stir it up. Thus, also, tidal currents may
flow over sands without disturbing or removing them, but if these
sands are broken up by wind or wave action, the sand may be
transported by the tidal current into the rivers. Shingle beaches
are only found where there is a considerable rise of tide and
sufficient wave force is generated to erode the cliffs.
If a stream is loaded to its full carrying capacity, it will not
take a greater burden, but flows against the banks and over its
bed without eroding them. If, however, it is not over-burdened,
and the velocity is sufficient to erode, it will pick up material
from the soil over which it passes.40
The quantity of material carried in suspension varies very
considerably. In some rivers upwards of 2 per cent, in weight of
the total volume of water passing along the channel consists of
solid matter.
Taking the specific gravity of water as 1, the relative weight of
coarse river-sand is 1-88; fine sand, 1'52; clay, 1*90; alluvial
matter from 1'92 to 272. A cubic foot of water weighs 62'5 Ibs. ;
of coarse sand, 117'5 Ibs.; fine sand, 95 Ibs.; clay, 118-75 Ibs.;
alluvial matter, 120 to 170 Ibs. ; silt, 103 Ibs.40
Motion of Particles of Matter in Suspension. — The matter to
be transported, being much heavier than the water, will pass from
a state of suspension to that of deposit when the water in which
it is contained ceases to be in motion. A solid particle, being of
greater density than the water, is continually tending to sink, the
time occupied being proportionate to its size and specific gravity.
The particles of water in running streams have, however, a
considerable upward motion which is sufficient to counteract the
downward tendency of the solid particles. Thus particles of
considerable size may remain in suspension for long distances,
while the finer particles may be altogether prevented from
sinking. The motion of water in running streams is never
uniform, and the relative position of the suspended particles is
constantly being changed. The direction of the particles is
altered by the varying form of the bottom and sides, by impedi-
ments met with on its course, and by the varying velocity of the
whole mass due to the friction of the sides and bottom, and of
the individual particles of water. Continual eddies and whirl-
pools are constantly being generated, by which a rotary motion
is given to the water. The particles of matter in suspension are
carried forward by the velocity of the current, and thrown
upwards by the eddies, and thus kept from sinking to the bottom.
The bed of a river is rarely regular, but consists of a series of
360 GEOLOGY FOR ENGINEERS. [PT. V.
pools and shoals, which have the effect of continually altering
the direction of the particles of water. Even where the bed
approaches to a level surface it frequently contains a series of
ridges, composed of the deposit in transit. These ridges have
almost invariably a gentle slope on the upper side, with a more
vertical inclination on the down-stream side. Even where the
material is sand, the down- stream side often presents an almost
vertical face, over which the moving particles are rolled. These
ridges are constantly altering their form, due to the changing
size of the particles rolled along, a single pebble often altering
the whole shape of the moving detritus.40
Effect of Alteration in Dimensions of Channel.— If the velocity
of the stream be checked by a widening of the channel, the
motion of the water becomes less disturbed, and a portion of the
matter in suspension is deposited, the quantity depending on the
variation in the velocity of the current. This deposit reduces
the area of the channel, and tends to restore the normal velocity.
A slight retardation of the current, however, does not necessarily
produce a deposit. Increase in depth does not cause deposit in
the way that increase of width does. The particles of water in
the latter case, descending on one side of the deep and rising on
the other, cause a rotary or centrifugal motion in the hollow ; the
particles of matter brought into the depression are rolled round
and directed upwards, and ultimately carried off by the film of
water moving above the surface of the pit.40
Proportion of Deposit carried. — When the water is highly
charged with deposit, the greater amount will be found at the
bottom and the least at the surface. When it is undercharged,
the distribution is more general, the amount at any point being
determined by the greater or less disturbance of the particles due
to eddies and whirlpools. In the Rhone delta, where the water
was very highly charged, the proportion was found to be as 100
at the surface to 188 at the bottom. In the Mississippi, in its
ordinary condition, the proportion was only 147 to 188. In a
sandy estuary, where the water was much undercharged, the
author has found the proportion to vary as 8 to 14 and 12 to 28.
The power of water to transport solid matter depends on the
velocity — modified by the depth — which governs the transporting
power, in two ways : one certain, when, the quantity of water
being constant, the amount of material carried will vary directly
as the velocity, and as affected by the time that gravity has to
act on the particles while travelling a given distance ; the other
uncertain, and due to the increase of eddies and whirling motions
set up by the increased momentum of the stream. With regard
to the first, if a given quantity of water carries a given quantity
OH. XVII.] RIVERS. 361
of material in suspension, it is obvious that by increasing the pace
throughout the whole of the channel the quantity of material
carried must also be increased. It is, however, impossible to lay
down any rule for the second factor, as it must depend on the
contour of the channel and the means for setting up the whirling
or rotary motion that keeps the particles in suspension.
The weight of sand and pebbles, when immersed in water, being
only about half their weight in air, these materials are more
easily transported by currents of moderate velocity. Sand or
pebbles lying on the bottom of a river present an obstacle to the
free motion of the particles of water and check their momentum.
They are therefore acted on by the dynamic force of the flowing
current in addition to the transporting power due to the velocity
alone. It is to this cause that pebbles and shingle are moved
along a beach by tidal currents of small velocity, and when aided
by the disturbance caused by waves, stones of very considerable
size are brought from deep water and left stranded on the shore.
The momentum contained in the deep water of the sea, due to
the tides aided by the current acting on heavy bodies in a partial
state of notation, carries these along and lands them in a position
from which the returning wave has not power to move them.
It has been shown that the particles of water of which a running
stream consists are continually rolling round one another in
circular orbits, and that the size of these circles depends on the
depth of the stream. The deeper and wider the stream the less
the rotary motion is impeded. The smaller the diameter of the
orbits described by the particles the more disturbed is the
condition of the water and of the particles of solid materials
which it contains, and therefore the greater the ability of the
water to retain these in suspension, and the more the energy
expended in rubbing and eroding the sides and bottom of the
channel. The larger also the diameter of the circle through
which the particles move the more easily they will glide over the
surface, and the shallower the water the more direct, frequent,
and effective will be their impulse. The greater agitation in
which shallow water is kept increases its capacity to hold matter
in suspension and to erode its bed. The strength of the stream
is absorbed proportionally in this action, and the velocity accord-
ingly diminished. This is no doubt the cause why shallow streams
frequently erode the soil of their beds and banks, while deep
water passes on over the same kind of soil without exercising the
same effect.40
The material transported by rivers consists either of alluvial
matter, clay, sand, or shingle. The first two, owing to the
fineness of the particles, are easily transported in a state of
362 GEOLOGY FOR ENGINEERS. [FT. V.
suspension. When sand is disturbed, a certain portion, consisting
of the very finest particles, is carried away in suspension, but all
particles sufficiently large to be visibly angular, as also shingle,
require a greater velocity of the current to move them, and their
transport is effected by being rolled along the bottom. Although
clay will not yield to such a velocity as generally prevails in
navigable rivers, if it be disintegrated the particles easily mix
with the water and are carried away. Mr Wheeler has found,
as the result of observation and experiment, that the most
effective results may be obtained by mechanical disintegration
and mixing from warp or alluvial deposits, then from clay, and
the least effect is obtained from sand.
The quantity carried in suspension at a given velocity is not
wholly in proportion to the specific gravity of the material, but
depends more on the fineness of the particles. Even in still
water it will be found that the relative time occupied in settling
does not vary as the specific gravity of the materials.40
THE PHYSICAL CONDITION OF TIDAL RIVERS.
Rivers may be divided into three parts : —
1. The fresh-water or non-tidal portion.
2. The part within the coast-line confined within limited
boundaries, through which the tide ebbs and flows.
3. The estuary, or the part where the coast-line opens out,
leaving a wide mouth or bay.40
Origin and Description of Rivers. — Rivers in their original
condition were formed by the flow of the water off the land to
the ocean, the development of their present shape and direction
being due to the work of ages (see Chapter I., Section III.). In
this part of the world they probably received their main character-
istics after the breaking up of the Glacial Period (see Chapter IX.,
Section I.), when the torrents due to the melting of vast masses
of glaciers and icebergs, pouring off the land and flowing to the
sea, cut deep channels and conveyed the material eroded in their
course with them.
The vast areas of sand which are to be found in many estuaries
are the result of this process. In the early condition of the river
the gradient and the velocity of the water would be much greater
than they are now. The remains of river terraces in many
valleys testify to the magnitude of the streams which then
poured off the land. Gradually the forces of the erosive action
of the water and the resistance of the soil balanced one another,
and the struggle also between the tidal water and the ebb
torrents resulted in an equilibrium being established between
CH. XVII.] RIVERS. 363
the contending forces, and the regime of the rivers as they now
exist became established.
There are two sources from which the water flowing in a
river is derived, distinguished respectively as tidal and fresh
water.
The tidal water enters at the lower end, and is derived from
the tidal wave of the ocean, which, as its crest passes the mouth
of the river or its estuary, raises the level of the water during a
period of a little over six hours, filling the tidal basin and causing
a run of water up the river ; during a similar period, as the trough
of the tidal wave passes the estuary, the process is reversed. The
supply of tidal water is thus constant, the same quantity passing
out of the estuary on the ebb as entered during the flood.
The tidal motion continues as a wave so long as the depth of
water in the low-water channel is sufficient for its generation, but
is converted into a current as the depth shoals. This supply of
tidal water from the sea has enabled many rivers to be used for
navigation which otherwise would not have had the necessary
depth of water.
Fresh Water. — The water poured in at the upper end of a river
also comes from the sea, but by a different process. This is due
to the evaporation caused by the sun, the vapour formed being
collected into clouds, condensed again, and in the form of rain
falling on the land, and is then collected into the brooks and
rivulets which feed the rivers.
The supply of fresh water, therefore, is limited, variable, and
intermittent. This fresh water only travels in one direction.
Obeying the law of gravity, it ever continues a constantly down-
ward course, except during the time it is headed back by the tide,
until it reaches the lowest point attainable, that is, the trough of
the tidal wave.
In the middle zone of the river, between the purely tidal and
the fresh water, the currents assume the oscillating motion due
to tidal influence. The current alternately flows both ways, being
driven back and raised up during the flood tide, and running
down and its level depressed during the ebb. Under certain
conditions the action due to the tide may be simply a raising of
the level without a reversal of the current.40
Agents of Maintenance. — There are two principal agents
always at work in tidal rivers, one tending to shoal and deteriorate
the channel, the other to maintain and deepen it.
The agencies which tend to shoal the channel are the transport-
ing power of the fresh water, which brings detritus down from
the upper reaches ; the winds and waves, which erode the cliffs
and banks ; and the currents which disturb the sand-beds in the
364 GEOLOGY FOB ENGINEERS. [pT. V.
estuary. The material thus brought into the channel, if left at
rest, rapidly subsides in the lower part and raises its bed.
The continual oscillation of the water due to the tides is the
chief agent which keeps the detritus in motion and prevents its
deposit. The current of the fresh water, always flowing in one
direction, is the chief agent of transport which carries the material
away out of the channel to the sea. Its capacity to transport the
solid matter continues in a diminishing ratio until the termination
of its course. As it approaches the tidal portion of the channel,
the conditions of flow become so altered that the tendency to
deposit is greater than the transporting force.
In a tidal river this solid matter is kept in movement by the
oscillating action of the tides, until it is finally carried out to sea
or deposited on the shores of the estuary, where it settles and
forms the salt marshes to be found on the coast.
In non-tidal rivers, as the current slackens on approaching the
sea, the material settles at its mouth and forms deltas.
The ever-continuous motion of the water in tidal rivers, and the
constant reversal of the direction of flow, therefore, give these
rivers a great advantage over tideless rivers, in which the current
of the stream is always in one direction.40
Regime of Rivers.— Under natural conditions, the forces at
work in a tidal river adjust themselves so as to establish an
equilibrium between the eroding agency of the current and the
tenacity of the soil of which the bed and banks are formed, and
the slope becomes so regulated that the velocity is sufficient for
the transport of the detritus.
When unconfined by banks, the direction also of the low-water
channels through beds of sand and silt is the result of a balance
of forces set up by gales, currents, floods, and other disturbing
causes. A comparison of the charts of a sandy estuary extending
over several years will show that, although at times the course of
the channels may be altered by the prevalence of gales from one
direction, of continued land-floods, or of long periods of dry
weather, giving undue influence either to the tidal or fresh-water
agency, yet there is one course, of a more or less stable character,
to which the low-water channel always reverts under normal
conditions.40
Junction of Rivers with the Sea. — The angle or direction in
which a river joins the sea is affected by the shape of the adjoining
coast, the set of the tide, the direction and force of on-shore gales,
and the travel of littoral drift.
An examination of the charts of the coasts of this country will
show that in the great majority of cases the line of direction of
the main low- water stream where it enters the sea is nearly at
CH. XVII.] RIVERS. 365
right angles to the main set of the tidal stream along the coast,
or inclining rath«r in the direction of the set of the tidal ebb and
flow.40
Source of Detritus in Rivers. — Although there may be excep-
tions, the material which a river has to deal with is supplied from
the interior, and not from the sea. Even where the tide flows
over a vast mass of sands, such as those which lie along the coast
outside the mouth of the river Mersey and the Kibble, or of the
Humber and the Severn, it will be found that the tidal water flows
into those estuaries bright and clear, and free from deposit, except
in stormy weather, and that it only becomes turbid after it has
mixed with the ebb.
Effect of obstructing the Free Flow of the Tide. — Any cause
that obstructs the flow of the tidal water and the free propagation
of the tidal wave is detrimental to the maintenance of a river in
its most effective condition, and leads to the shoaling of the
channel.
The placing of weirs across tidal rivers, contractions of the
channel and irregularities in its form, restricted entrances, and
similar causes, are destructive to the maintenance of a deep-water
channel.40
BARS AT THE MOUTH OF RIVERS.
Description. — A bar across a tidal river (c/. p. 17) may be
described as consisting of one or more banks or ridges extending
across the entrance channel, having deeper water both on the sea-
ward and inner sides, and the crest rising above the general level
of the bottom of the channel adjacent. In non-tidal rivers the bar
consists of a long flat shoal at the mouth of the river, which rises
so far above the general level of the bottom of the river, both at
the outfall and in the channel above the shoals, as to render the
channel useless for that class of navigation for which otherwise it
would be fitted.
Bars are not common to all rivers. At the mouths of most
estuaries with sandy bottoms ridges and depressions similar to
bars are to be found, but in many cases, owing to the great depth
of water over them, they cannot be deemed bars. In other
estuaries where well-defined bars exist, the crests of these do not
rise above the general level of the channel inside, and therefore
do not form impediments to vessels going up or down the
channel.40
Bars composed of Hard Material not affected by the Scour of
the Current. — These bars consist of a shelf or ridge running
across a river-mouth, consisting either of stone, very hard clay,
or occasionally of large boulders, or shingle cemented together
366 GEOLOGY FOR ENGINEERS. [PT. V.
with clay. Such bars can only be removed by dredging. The
effect of the removal may be permanent, or the surrounding
conditions may be such that the hard material may be replaced
by sand, and the bar reappear.40
Bars due to the Deposit of Alluvial Matter. — These are to be
found in tideless rivers, or where the rise and fall of the tide is so
small as practically to render the river non-tidal.
In tidal rivers, the ceaseless action of the tides, by which an
enormous volume of water is poured into and discharged from the
river twice every day, not only serves to keep the alluvial matter
contained in water in suspension, but, by diffusing it throughout
the whole volume of the tidal water brought in on the flood,
carries the greater part of it away on the ebb and deposits it in
the deep water of the ocean. In a non-tidal river the alluvial
matter brought down the channel continuously, and to a very
much increased extent in floods, settles at the mouth of the river,
where the current is checked and the velocity is reduced. In
time large deltas are thus formed, through which the water from
the river finds its way to sea by several shallow channels.
The large accumulations of sand found in most tidal estuaries
vary considerably both in their composition and cause of deposit
from alluvial deltas, and also in the fact that they are in
situations where there is generally a considerable rise of tide.
These sands are not continually accreting and forming deposits,
but maintain their original form and extent in a more or less
stable state so long as the natural conditions under which they
exist remain unaltered. In the more open sea the accumulations
of sand may be drifted along the coast during long-continued
gales and form casual bars at the mouths of the rivers, but this
material will be transported away when the normal conditions are
resumed.40
Bars at the Mouths of Sandy Estuaries. — This form of bar is
the type most frequently met with. They possess features of a
most remarkable character, consisting of one or more ridges or
mounds of material, the particles of which have riot the slightest
coherence, yet stand with a slope much steeper than their natural
angle of repose. Rising in some cases as much as from 40 to 50
feet above the bottom, they maintain their positions across
channels subject to a tidal rise of from 20 to 30 feet, through
which currents run at a rate of from 3 to 4 knots, and the
direction of which is reversed four times every day. Exposed to
the storms and waves of the open sea, they are sometimes partly
dispersed or added to, altering their position and shape, yet
having a normal condition to which they are restored when the
disturbing causes cease.40
CH. XVII.] RIVERS. 367
Formation of Sand-bars. — A tidal bar assumes the form of a
ridge, having deep water on either side. The ridge, being once
formed, aids its own maintenance. Sand is moved in an estuary
in a series of ripples or ridges, having a long slope on the upper
side, or that from which the current is coming, and a steep face
on the down-side. Over this steep face, or tip, the particles of
sand are rolled. In a tidal channel where the current is con-
tinually being reversed, the position of this face varies with the
direction of the tide. At the foot of the ridge a rotary or
screwing motion is set up, which whirls the particles of material
round the bottom of the hollow, continually tending to scour it
deeper. The current moving forward along the bottom is
deflected upwards, and rolls the particles up and over the ridge.40
Channels where Bars are absent. — Bars having been once formed
and subsequently maintained by the action set up by their shape,
if removed by dredging, are not liable to be reformed, unless in
situations where there is a strong littoral drift and the ebb
current is not sufficient to keep this out of the channel. The
conditions most favourable to the absence of bars are those where
the estuary assumes a funnel-shaped form, decreasing in width
and depth from the mouth upwards ; when the momentum of the
tide is not unduly checked ; when there is a free propagation and
long tidal run ; when the ebb current is so directed as to have a
preponderating force over the flood in the removal of material;
and when the outfall channel is continued into deep water.40
Theories as to Cause of Bars. — Mr Wheeler, after discussing
various theories in a paper laid before the Institution of Civil
Engineers, sets forth the following views, which were almost
unanimously accepted, and may be taken as mainly correct : —
The existence of tidal bars is due to the action of the sea, and
not to that of the land water. And the chief factors in their
maintenance are tidal currents and on-shore gales.
For their formation it is necessary that the bed of the estuary
and of the adjacent sea should consist of sand or shingle, and
that the depth of water should be sufficiently shallow to allow of
the action of waves and tidal currents on the bed.
Bars owe their origin and existence to the balance of forces
which was established when the coast-line and estuary assumed
their original form. These are forces which have continued to
operate ever since, and which tend to build up or disperse them.
The balance of forces originally set up, however, still continues.
On coasts where there is a travel of material along the shore, it
is drifted in its course across the opening in the coast-line which
forms the outlet for the river. The flood-tide, setting through
this opening into the estuary, tends to carry the material with it ;
368 GEOLOGY FOR ENGINEERS. [pT. V.
the ebb-tide, on the other hand, tends to carry it back and disperse
it into the deep water of the sea.
Wherever there is any considerable motion of the water where
the bottom of the sea is mobile, the material invariably lies in
ridges, these in some cases being of considerable height. Bars
may therefore exist across the mouths of rivers where there is no
drift along the shore, the sand being thrown up and assuming the
form of a ridge or ridges, and thus forming a bar by the action of
the wind, waves, and the tidal current, and being maintained by
the action which its form sets up.40
RIVER IMPROVEMENT SCHEMES.
For principles of improvement, training, and other particulars,
the reader is referred to Mr Wheeler's book on tidal rivers, from
which the above extracts have been taken.
Geological Formation of Kiver Bed. — It may be remarked,
however, that when the bed of the river consists of silts, sands,
gravels, and other drift material, there is, generally speaking,
little difficulty in deepening by dredging. Not infrequently,
however, these superficial matters overlie and mask dykes and
ledges of rock which cross the channel, and then these require
subaqueous blasting and more expensive methods of removal.
A careful survey of the country will generally reveal where
such obstructions are likely to occur, and the methods of
removal may be suggested by a study of their structure above
ground. In the case of the Wear, for instance, which in its lower
course flows over the magnesian limestone, harder dolomitic ledges
may prove the obstruction to dredging ; in the Tyne it may be
harder strata of Carboniferous sandstone ; in the Tees Triassic
sandstones; and in the Clyde it may be a dyke or dykes of
columnar greenstone which reticulate the rocks in that area.11
• •
LAND RECLAMATION.
River works have frequently the effect of making land in the
sense cf altering the disposition of existing materials rather than
of accumulating additional materials, but "Land-making is no
part of sound river engineering," said Mr D. Stevenson, and the
interests of land-making and navigation are often incompatible.
Embanking and Warping.— Along most of our fens, levels,
carses, and tidal estuaries, there is always a considerable margin
of silt and low-lying land, little if at all above ordinary sea-level,
and consequently liable to be inundated during flood-tides and
storms. To reclaim and protect such lands, and further to
CH. XVII.] RIVERS. 369
increase their growth and elevation, are the objects of sea and
river embankments.
Occasionally wood-and- wattle jetties are thrown out to intercept
the silt ; at other times a strong embankment, with sluices which
intercept the tide, but permit the exit of water when the tide is
back, is constructed ; and not infrequently the sluices are so
arranged as to admit the muddy tide, with its burden of silt, and
then, by closing them, to impound the water till the sediment has
fallen and enriched the land. Warping, as this latter process is
called, to elevate and enrich the surface ; embanking, to protect it ;
and intercepting, to increase its area — are the main objects in
view ; and all require considerable ingenuity and skill on the part
of the engineer.11
[PT. V. CH. XVII.
CHAPTER XVIII.
COAST EROSION.
THE action of the sea and the effects produced by it in denuding
and reconstructing coast-lines have been briefly described in
Chapter I., Section V. Coast erosion is, however, a subject of so
much importance both to landowners and engineers that the
geologist and the hydraulic engineer must again work hand in
hand and give one another mutual assistance.1
Section I. — Coast-lines and their Origin.
In dealing with the subject of coast erosion we must first con-
sider how the existing coast-lines originated.
Outline. — Every part of the earth which rises out of the sea is
distinguished by its own peculiar outline. This outline, in which
the ocean marks a definite level around the land, is the sea-coast.
Its fantastic curves on some shores, and scarcely broken, straight
extent on other lands, are not a matter of accident ; for the
causes which raise islands from the sea also determine the main
directions in which the coasts run. Inlets, bays, channels, and
headlands may have to be explained by discovering the courses of
old rivers, or the work of rain, and the kinds of rocks exposed ;
but the coast-line has been produced slowly at successive ages of
the earth's history, and parts of it have from time to time been
portions of lands of far different outline to those of existing con-
tinents and islands, though the ancient lands are now more or
less destroyed and submerged.6
Influence of Altitude. — Nothing perhaps will help so well to
make intelligible the first and simplest law under which a coast-
line may change as to take a map on which are drawn lines
showing the course taken over the country by contours indicating
levels at ever-increasing heights such as would be marked by the
sea, if the land were submerged to that extent. Then the
successive steps would be traced by which a large mass of land
may become broken into islands, and the reason why the smaller
370
SECT. I.] COAST EROSION. 371
islands are formed would be more or less clear, for the sea
necessarily would cover the low land first. Similarly with the
sea ; lines which mark depths of increasing amount in hundreds
of feet enable us to understand how islands may be enlarged,
united together and into continents, and have the course of their
coast-line changed, by being merely uplifted so that the sea drains
off from regions which it once covered.
Wherever a coast-line remains for some time unchanged in
level, the wearing power of the tides will usually convert what
had previously been a shelving shore into a sea-cliff. If, then,
land is upheaved at intervals, with periods of pause during which
no upheaval takes place, then inland cliffs will be formed which
correspond to these intervals of rest. The position in which cliffs
are produced is often governed by the way in which the layers of
rock forming the country are arranged. This arrangement of the
strata into hard beds and soft beds is accompanied by an inclination
of the deposits technically called "dip" (see Chapter III., Section
II., p. 40). The sea acting upon deposits so inclined abrades
and wears away the exposed edges so as to undermine the rocks
and convert them into precipices on the seashore, which are called
cliffs. But when the deposits shelve down gently into the water,
there are no weak places in the single stratum exposed which
make it easy for the sea to cut a way through the formation.
Since the whole country, even in recent geological times, has been
elevated from out of the ocean, terraces must inevitably have been
produced inland in this way at successive heights, though in
many cases the rounding influence of the action of rain has more
or less modified and obliterated the earlier work of the sea.6
Minor Features. — Besides its direction every shore presents
the minor features of bays, inlets, cliffs, and capes, whose existence
is only intelligible by help of a knowledge of the ways in which
the several geological formations which make up the dry land
have been accumulated, folded, and upheaved so that the edges of
strata are exposed on the shores where land rises out of the sea.6
Headlands. — This dependence of headlands upon geological
formations is well exemplified in Flamborough Head, in the
North and South Foreland, in the promontory of Beachy Head,
and in Culver Cliff and the Needles at the east and west ends of
the Isle of Wight. All these headlands consist of chalk, and
although chalk may be worn away by the sea like any other
formation, when acted upon by the grinding power of the breakers,
it cannot be disintegrated and washed up into easily transported
sediment like the underlying and overlying sands and clays. Hence,
since its removal is largely dependent upon the chemical power of
water to dissolve the limestone and take it up into invisible suspen-
372 GEOLOGY FOR ENGINEERS. [PT. V. OH. XVIII.
sion, the rock is more enduring than the associated deposits which
rest upon it and which it covers. And, being a thick homogeneous
formation, which often has its foreshore defended with a barrier
of flint derived from the waste of the Upper Chalk already
destroyed, it happens that this formation juts out into the sea,
while on each side of it the strata are excavated by tidal attrition
into bays. Of such bays, Sandown Bay and Compton Bay are
familiar examples, due to the removal of the soft underlying strata
below the chalk.
Inlets. — But the sea is often admitted into the land without
any regard to the nature of the strata, simply because they
happen to be bent down into a trough, part of which sinks below
the sea-level. This is the case with the estuary of the Thames
and the Southampton water, both of which owe their existence to
lying in synclinal folds, though partly to the ease with which the
sea could encroach on the loose clayey and sandy formations,
when, owing to a different level of the land, circumstances were
more favourable for its work of excavation. The most important
class of inlets occupies the positions of what were formerly dome-
shaped or anticlinal folds.6
The Shore. — As a district became depressed and the sea
admitted, every portion of the land must in succession have been
a shore, and the shore moved gradually with the depression of the
land to a level which was progressively higher. When we
remember the power which the sea possesses of throwing up
around our coasts in stormy seasons not merely the spoils of life
but masses of rock from great depths, a mechanism becomes
discernible which has brought gravel beds and our pebble beaches
gradually into their present position in times antecedent to the
final shaping of the contours of the coasts. The beach follows
the shore, and it may be that much of the material thus brought
back again had previously been scoured from the present seaward
slopes of the country in an antecedent age, when its level was
higher. These materials are ever reinforced with the hard
fragments worn from the nearest local source, and with pebbles
driven along the shore by waves lashed by the wind.6
Sea-cliffs. — The same agencies which have brought the pebble
beds to our shores have been chiefly concerned in the production
of sea-cliffs. We know the rapid waste of certain parts of the
coast, where noble strips of land have in historic times passed,
often with towns and villages upon them, back into the sediments
of which they were originally composed, and have been swept out
over the flow of the German Ocean. But all our coasts happily do
not crumble away like those of Yorkshire, and though the changes
which take place from year to year prove that the existing aspect
SECT. II.] COAST EROSION. 373
of many cliffs is of very recent origin, yet their geological structure
often makes it probable, even when proof is wanting, that they
too have come down to us from an immeasurably distant past.
Some coasts are especially favourable to the formation of cliffs,
because the rocks are hard and not easily worn away, while
the land which they form rises to a fair height from the sea.
Seaside towns generally occur where gaps appear between cliffs,
though there are many exceptions. The gap furnishes a ready
means of reaching the sea, and often owes its existence to a bed of
clay which had been exposed down to a low level on that coast,
and eaten back by the sea into a bay. This bay is usually a point
from which the adjacent harder rocks may be undermined, for,
drained of the moisture they contained, owing to the dip of the
strata, their substance contracts and becomes divided by
innumerable cracks and division planes, separating into blocks
which have no support or firm coherence with the mass of the
stratum, when the underlying portion between, tide-marks has
been removed. After falling, these fragments, when hurled back
by the tidal waters, become battering rams for making further
inroads into the sea-wall of rock, and thus the process goes on,
governed by the direction of the wind and the currents which
move the water (see Chapter I., Section V., p. 21).
The height of a cliff is governed chiefly by the height of the
adjacent land. On some parts of the west coast of Scotland, the
height of cliffs is immense ; and, as a rule, among the contorted
and upheaved Primary formations cliffs are higher than among
the newer formations. But the waste is less rapid, and the cliffs
often show in their retreat from the shore, in their upper portions,
evidence of denudation, and different relative positions of land
and water from those which exist now. The Secondary rocks, from
their loose texture, have wasted at a more rapid rate, and the
cliffs are often high, because easily undermined, and so eaten
back that the traces of earlier denudation have become obliterated.
The Tertiary cliffs of the east and south-east of England are
mostly of moderate height, because the level of these deposits
rises so little out of the sea, as may be seen in the Crag formation
at Felixstowe and Aldborough, while on many parts of this coast
of Suffolk cliffs have no existence.6
Section II. — Forces acting on Coast and Sea-bed.
WAVES.
Sea waves are of two kinds, forced and free ; the former exist
only during the continuance of the wind causing them, but the
latter continue to run for some time after the wind has subsided.41
374 GEOLOGY FOR ENGINEERS. [FT. V. CH. XVIII.
Free Waves. — Mr Hunt gives the following very concise defini-
tion of the character of oscillating, or free waves, as being that
generally accepted : —
"Such swells are composed of ridges above and depressions
below the level of repose of the water.
" They impart to a particle of water itself, or to a light floating
object, a circular motion. Such particle describes the circle with
uniform velocity, and in the direction of the motion of the
wave itself.
" The diameter of the circle is equal to the height of the wave
from trough to crest.
" From the circular motion of the particle it follows that, when
above the level of repose, it is moving forward ; when below that
level, moving backwards. In other words, the water composing
the crest of the wave is moving forwards ; the water composing
the trough is moving backwards."
The trough always precedes the crest in point of sequence. In
of Wcw& Motion,
FIG. 91. — Oscillation of particles of water.
this movement or oscillation it must be clearly kept in mind that
there is no alteration in the position of a particle of water relative
to the bottom after the wave has passed by ; it is left in the same
position in which the wave found it, having merely performed a
circular oscillation in a vertical plane.
It is most difficult to rid the mind of the impression of an
actual shoreward movement of the water itself when watching
from a pier or cliff a series of these waves rolling in, and the
remarkable way in which they retain their individuality. The
motion of the water particles corresponds closely to that of a point
in a long rope which is kept stretched out while one end is
oscillated quickly up and down ; a series of waves is seen to
traverse the rope from end to end, but the rope itself is not drawn
to either end.
These waves are called "waves of oscillation" or "free waves,"
but there is another type of wave called a ivave of translation, in
which the water is actually permanently displaced by the wave ;
this type, however, shall be dealt with later on.
The oscillation of the particles of water, due to a wave of the
SECT. II.] COAST EROSION. 375
first type, extends downwards through the water, the particles
revolving in smaller and smaller circles as the distance from the
surface increases, until eventually the movement dies away
(see fig. 91). It is, therefore, only a surface skin of the ocean
which is disturbed by waves, but what the thickness of the layer
is, is still open to dispute. It has, however, been proved that
the oscillation in deep water decreases in amplitude in geometrical
progression as the depth below the surface increases in arithmetical
progression.41
Waves of Translation. — As free waves approach the shore
they become more or less waves of translation, and the orbits of
the water particles are not closed ; the particles travel in orbits
in a vertical plane, but do not quite return to the starting point.
The velocity of the undulation or wave form is relatively rapid
compared with the forward movement of water, which is slow and
rhythmic, so the excess of forward movement over seaward
decreases as we move seaward from the land margin and the depth
increases. As the waves continue to roll into shallower water,
their velocity and wave length are diminished and their height is
increased ; thus the waves are crowded together near the shore.41
Forced Waves, even in deep water, are not true oscillations ;
there is always a slight forward movement of the water as well as
of the wave form, the former being relatively slow compared with
the latter. Such translatory movement of the water tends to
generate a surface drift with the wind. The relative amount of
horizontal and vertical motion of the water due to a wave depends
on the depth of the particle below the surface, and the total
depth of water compared with the wave length. Where the
water is deep, compared with the wave length, the horizontal and
vertical movements are nearly equal, and their amplitude
diminishes in geometrical progression as the depth increases in
arithmetical progression.41
Close to the breaker line the nature of the motion of the
particles is very different, the horizontal motion being nearly as
great on the bottom as on the surface.
Whenever there is any forward movement of water, caused by
waves of translation, there must be a compensatory seaward
current to remove the water brought shorewards by the waves.
This probably exists in the form of an under-tow, the transporting
power of which may be very powerful in shallow water, but
decreases as the depth and distance from land increases.41
Breakers. — When the wave rolling shoreward eventually
plunges or breaks, its action becomes entirely changed.
The action of such waves when breaking on a beach has been
usefully divided into three separate parts or phases :
376 GEOLOGY FOR ENGINEERS. [FT. V. CH. XVIII.
(1) The "plunge," or act of breaking.
(2) The " uprush " of water shoreward after the plunge.
(3) The "backwash," or return seawards of this water.
It is obvious that the plunge must violently stir up the bottom,
and throw fine matter, such as sand, into suspension in the water \
the " uprush " following immediately upon the plunge, therefore,
starts highly charged with suspended matter, assuming sand to
be present, or, if only shingle is present, a violent shoreward
impact is transmitted to the pebbles. The current then carries
this matter up the gradient shore wards ; but on the water reaching
its highest point, the velocity has died away, and there is a con-
sequent deposition of material, which is left behind by the " back-
wash," since it has no such violent start to help it as the " up-
rush," but simply starts from rest. This action of the breaking
wave is one of the most complicated we have to consider.
There is always a tendency for a balance to be attained between
the relative transporting power of the " uprush " and "backwash."
Obviously, considering the waves alone, and assuming them to strike
parallel to the shore, the quantity of material carried by each
determines the question as to whether erosion or accumulation is
going on. The amount of matter carried up by the "uprush"
tends to build up a gradient such that the help given to the
" backwash " by gravity will counterbalance the help given to the
"uprush" by the plunge.41
Percolation, or the sinking away of the water through the
interstices of the shore material, is a very important factor, and
on shingle shores the force of the backwash may be much
diminished. Gravity, however, is the controlling factor, and
shores tend to assume an inclination of repose near H.W.M.
such that the shoreward wash and backwash act with equal
effect.41
Overtaking of One Wave by Another.— It will be noticed that
this analysis of the action of the breaking wave assumes that the
wave, when it breaks, has time to complete its cycle before it is
interfered with by the following wave. This is not always the
case, and if the waves strike the shore at such frequent intervals
that the " backwash " of one is met by the " uprush " of the
following wave, a very peculiar state of affairs is produced. At
first sight, one would say, here is a case where there must be a
great accumulation going on, since the " backwash " is met in
this way by the "uprush," and its scouring action presumably
destroyed; but no, this is a most deceptive appearance, and is
not borne out by closer observation, for instead of the checking of
the " back- wash " by the water of the incoming wave, it simply
glides up over the surface of the "backwash," thus completely
SECT. II.] COAST EROSION. 377
reversing our first conclusion ; for here we have an undercurrent
flowing seaward, and on the top of it a landward current. It is
thus obvious that the landward current cannot pick up any
material from the bottom, and some of what it may already have
in suspension will be robbed from it by the down-flowing under-
current. This overtaking of one wave by another is very common,
and results sometimes from a crowding of the waves on to each
other by an on-shore wind. Of course, it depends also to a great
extent upon the gradient of the shore, or any cause which is
capable of increasing the frequency of the waves, so that the
intervals between them are less than the time taken for a wave to
go through its complete cycle of " plunge," " uprush," and " back-
wash." The surf seen during on-shore gales is a further develop-
ment of the same thing, all system being destroyed and the whole
sea covered by a mass of broken foaming water.41
Direction of Waves. — In the above consideration it has been
assumed that the waves strike the shore approximately at right-
angles to the shore-line, the waves themselves being parallel to
the shore-line.
Waves rolling in from the open sea tend to approach the shore
parallel to the general coast-line, for the shoreward end of a wave,
on entering shallow water, is more retarded than the seaward end
in deeper water, and the line of the wave is thus swung round.
The angle at which ocean waves strike the shore, therefore, depends
partly on the gradient of the adjoining sea-bottom. The gradient
has also an important influence upon the amount of material
travelling : the flatter the gradient the less material will be moved
per unit area, although in the aggregate more material may be
moved on a flat shore than on a steep one, owing to the greater
surface exposed to wave and current action.
Waves generated near shore may run very obliquely to the
coast-line ; and we sometimes have two or more sets running at
the same time in different directions. In shallow water the
crests of these sets of waves may break where they cross, and
exert a force which is the resultant of that which either would
exert alone ; for the depth of water in which waves break depends
upon the height of their crests.41
Oblique Waves. — In whatever way produced, the action of
oblique waves is very peculiar, and will best be understood by
reference to fig. 92, where a, 6, c, d, e are supposed to be such
waves, the dotted line representing the shore-line.
When these waves break, the " uprush " does not travel straight
up the shore, but at an angle, nor does the " backwash " return
straight down. The velocity of the " uprush " may be considered
as the resultant of two components — one at right angles to, and
378 GEOLOGY FOR ENGINEERS. [FT. V. CH. XVIII.
one along, the shore. Now, when the wash of the wave travels up
the beach, the velocity at right angles to the shore is destroyed
gradually by gravity, but the other component is unaffected,
except by friction, the result being that a particle of sand taken
from any point P is carried up in a curved path to 0, and down
again to X, if not deposited, the final result being a movement of
the particle alongshore from P to X. Hence these oblique waves
cause a travel of material alongshore in the direction towards
which they are inclined, or, in other words, in the direction of the
wind, the individual path of each particle being approximately
parabolic, such as is described by a projectile thrown at an angle
into the air. The return path from 0 to X will, however, be
somewhat steeper than the path from P to 0, owing to the
retarding effect of friction upon the horizontal component of the
motion of the water particles.
If the moving power of the " shore ward- wash " and "back-
FIG. 92.— Action of oblique waves.
wash " are not equal, the resulting movement due to oblique wave
action may be either landward and alongshore, or seaward and
alongshore. The more oblique the impact of breakers is on the
coast-line, the more powerful is the alongshore drift.
We thus see that the direction of wave impact is an important
factor in the movement of material by wave action. This in its
turn is governed by the aspect of the shore, its exposure, and the
direction of the prevalent wind.
The blows of large waves exert great disintegrating force on the
shore material, and this is especially so when the forward motion
of translatory waves is suddenly checked. There is no true wave
stroke at levels lower than the troughs, and the most efficient
impact of the waves is limited to levels between trough and
crest.
In considering the coastal movement of material, it is important
to keep in view the fact that the power of waves to move particles
on the bottom decreases rapidly as the depth of the water and
the distance from the land increases.41
SECT. II.] COAST EROSION. 379
TIDAL ACTION.
This may be considered under two headings : — (1) The effect of
the slow rise and fall of the water-level, and consequent travel of
the water's edge up and down the foreshore ; (2) The effect of
currents and eddies set up, owing to differences of water-level and
the reaction of the land upon the tidal wave.
Slow Else and Fall. — We may dismiss the slow landward and
seaward current as being too slight to have any effect in moving
material unless the very finest suspended matter. There is
another effect which is due to the travel up and down of the
breaking point of the waves, and this is most important, as what-
ever action is going on at the time, due to the breaking waves, is
applied successively to different parts of the foreshore, between
H.W.M. and L.W.M., whether it be erosive or the reverse. If
the tide rose and fell at a uniform rate, the result would be to
plane out a uniform gradient between the breaking points of the
waves at H. and L.W. ; but this is not so, since the rate of
FIG. 93.— Erosion by parallel waves.
rise or fall is very much faster at about half-tide level than at
either H. or L.W. ; hence the planing action is applied longer to
the parts of the foreshore about these points than about mean
sea-level, and whatever erosive or accumulative effect is being
produced by the waves, is most marked just below H.W.M. and
just above L.W.M. The bearing of this consideration upon the
length of groynes is obvious, as it indicates that they should
extend from above H.W.M, to below L.W.M.
When parallel waves are eroding the shore, the above con-
sideration shows that the result will be to cut out a section of
foreshore something like that shown in fig. 93, hollows being
dredged as seen, and corresponding hills or bars produced
seaward of each hollow ; whereas, if the waves were accumulating,
this effect would be reversed, hills taking the place of the
hollows, and vice versa.*1
Tidal Currents. — The chief effect of tidal currents is probably
to transport material already suspended or disturbed by wave
action. Except where concentrated by narrow straits, etc., they
are not usually sufficiently swift to move coarse material of them-
selves. These currents are, however, most efficient in carrying
380 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
away matter suspended by wave action, or eddies due to a very
rough bottom ; and their preponderating effect, in determining
the direction in which fine material eroded from the coast is
transported, is shown very clearly by the great tendency for sand;
pits at the lee of headlands to point in the direction of the flood-
tide and not in the direction of prevailing winds.41
JOINT ACTION OF WAVES AND CURRENTS.
Movement of Material. — The combined action of waves and
currents may cause the movement of material on the sea-bed
when either alone might not be able to do so. If, for example,
the linear oscillation on the bottom, due to wave action, is taking
place while a tidal current is flowing alongshore, this oscillation
must become a zigzag, each oscillation being deflected by the
current ; so that the path of the particles on the bottom results
of
A A A A A.. _>0
V V V V V V V
FIG. 94. — Joint action of waves and currents.
in an alongshore movement, something like that due to the oblique
wave action previously referred to, and as shown in fig. 94. 41
WIND-FORMED CURRENTS.
Effect of Wind.— We have seen that in the case of forced
waves, running in before the wind, there is a forward translatory
movement of water as well as of the wave form. This slow,
rhythmical advance of the water is an important element of the
wind-formed current. The velocity of this translatory movement
of water decreases from the surface downwards. When the wind
commences to blow, the upper layers of water are drifted with
the wind. This forward movement is gradually propagated to
the lower layers, and, if the wind continues, eventually produces
a movement of the whole body of water, if not too deep.
The surface velocity of a current formed in this way is always
less than the velocity of the wind causing it, and seldom exceeds
one mile per hour. In shallow water near shore these currents
are an effective means of transporting material.
When the surface drift moves against an obstacle, such as an
SECT. III.] COAST EROSION. 381
island, or when its free onward passage is in any way partially
obstructed, relief streams are set up, the velocity of which may be
very great.41
Undercurrents. — An example of this effect is seen in the case
of a wind blowing directly on-shore. This causes a surface
current landwards, which is compensated for by a lateral or
undercurrent seawards. It is an observed fact that on-shore
winds denude a shore by removing material seawards ; similarly,
under certain circumstances, an off-shore wind may cause a
surface current seawards, which is compensated for by an under-
current landwards.
Off-shore winds are never so effective in causing currents near
the shore as on-shore winds, owing to the shelter of the land,
since the strength of the current depends to a great extent on the
fetch or distance which the wind blows across open water. The
underdrift landwards will have little transporting power and will
probably extend only a short distance from the land.41
Alongshore Currents. — It is seldom that a winds blows directly
on- or off-shore, and, owing to irregularities of coast-line, it is
always more or less oblique to some part of the coast. Any
obliquity of direction causes the landward current to be partially
deflected, and there is, consequently, an alongshore or littoral
current. It will be observed that this current must assist the
oblique waves in moving material in the direction towards which
the waves are inclined. Such an alongshore current may be
accompanied by an under-tow seawards.
A wind blowing alongshore is most effective in causing an
alongshore travel of the smaller particles of sand, shingle, etc.
With such a wind we therefore get accumulation on the windward
side and erosion on the lee of any obstacle which is capable of
intercepting this drift. Hence the huge accumulation to the
windward of high groynes, jetties, etc., and the almost invariable
scour at the lee.41
Section III. — Coast Erosion and Reclamation.
PHYSICAL CAUSES OP DENUDATION.
Subsidence and Upheaval of the Earth's Crust. — It has been
sometimes asserted that the continuous loss of land on the south
and east coasts of England is partially due to subsidence of the
earth crust. That continuous earth movements are in progress,
producing alternate upheaval and depression, no one possessing
even a slight acquaintance with geological science will deny ; but
there is no evidence to show that either upheaval or depression
382 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
has, during historic times, affected the encroachment or recession
of the sea on the coasts of the United Kingdom to anything more
than an infinitesimal extent, if at all.
Kaised beaches furnish striking proofs of change of level. In
Northern Europe, on the shores of the Scandinavian Peninsula,
with the exception of its southern extremity, round the islands of
Spitzbergen and Novja Zemlja, and on the Siberian coast, there
are numerous examples of such elevation. Similar proofs are met
with on the west coast of South America. In Great Britain the
best-known examples are to be seen on the west coast of Scotland.
Among evidences of geologically recent subsidence are the sub-
merged forests and beds of peat existing in many places.
Historical evidences of actual subsidence are wanting in Great
Britain, but in Scania, the most southerly part of Sweden,
considerable depression of the coast-line has taken place in
comparatively recent times. The west coast of Greenland is
probably sinking, and historic evidences are said to exist of the
subsidence of land in Holland and Belgium.
It may be safely asserted that the whole of the changes in the
coast-line of Great Britain since historic times are due either to
accretion or denudation, and not in any appreciable degree to
movements of the earth crust. If the oldest existing charts of
the North Sea or English Channel are compared with the latest
issued, it will be noticed that, allowing for probable inaccuracies,
there is little or no variation in the levels of the sea bottom in
depths beyond the influence of wave or tidal action.42
Physical Causes of Sea Encroachment. — The encroachment of
the sea on our coasts is therefore due to the erosion of the cliffs and
shore material. Of the detritus derived from such erosion, a
portion is carried alongshore by the combined action of wind,
waves, and tides, remaining in a state of more or less constant
movement until it is finally deposited to swell some accreting
sand or shingle bank, or is driven against some natural or
artificial barrier, where it lies, and is perhaps buried under
subsequent deposits. In the course of this lateral travel the
particles, large and small, forming the detritus are still further
disintegrated. The lighter material is carried off in suspension
by the sea, and ultimately finds a resting-place on the ocean bed
at a level below the influence of wave action or tidal scour. The
remaining portion of the solid materials, derived from the
destruction of the cliff or shore, is immediately transported into
deep water ; the smaller particles, in the form of mud, silt, and
fine sand, being rapidly swept away by the current until finally
deposited on the sea bed as described above. A certain propor-
tion of the larger material, too heavy to be carried in suspension
SECT. III.] COAST EROSION. 383
for any considerable distance, is drawn down the foreshore and
bed of the sea by the under-tow of the waves, and ultimately
makes its way by gravitation into deep water, where it finds a
resting-place.42
Eiver Detritus. — The amount of solid matter thus finding its
way into the ocean is vast, and is increased to a large extent by the
addition of detritus brought down by rivers and streams, and
derived from the land surface. River detritus, on reaching the
sea, shares much the same fate as we have described in the case
of material derived from the coast-line. A part finds a resting-
place in sand-banks and alluvial deposits at the mouths of rivers
and estuaries, the remainder spreading itself over the deep
sea bed.42
Effect of Deposits on the Deep-Sea Bed. — The effect of these
continuous deposits on the sea bed is enormous when we come to
consider the accretion of geological ages, but the deposit over
large areas, as distinct from purely local cases, is too small to be
of appreciable moment in historic times. The late Mr Tylor,
who made a very careful study of the subject, estimated that the
quantity of detritus now distributed over the sea bottom every
year would, at the end of ten thousand years, cause an elevation
of the ocean level to the extent of at least 3 inches.42
Relation of Littoral Drift to Eroded Material.— As to the
relative proportions of the material more or less immediately
carried away to the deep sea, and that other part which we may
call littoral drift, it is impossible to form any exact estimate. In
the case of chalk cliffs and foreshores it is probable that the
immediately removed material amounts to nearly 90 per cent, of
the whole, while alluvial cliffs and those consisting of boulder clay,
or other similar material, may yield 20 to 30 per cent, of solids
not immediately carried away from the foreshore into the deep
sea. Gravel and rock cliffs naturally yield a higher proportion of
heavy and harder particles. On the whole, it is unlikely that
more than 20 per cent, of the solid material falling on to fore-
shores and derived from the decay of the coast line remains above
low- water mark for any length of time. No protection works of
any kind which man is able to provide, however costly, can
prevent the loss of the large remainder. The material which does
stay for a time on the foreshore is subjected to constant attrition
and wastage, and this, too, in turn goes to swell the ever-increasing
volume of the deep-sea deposit.
Of the detritus derived from rivers and estuaries little or
nothing is available for the replenishing of the coast-line generally.
Some of this alluvial material is, however, available for reclama-
tion purposes in certain localities. Such deposits are purely local
384 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
in extent, and naturally do not occur in the immediate localities
affected by coast erosion. It will, therefore, be seen how limited
is the quantity of littoral drift in comparison with the total
amount of erosion.42
Deep-Sea Erosion. — It must not be supposed that the process
of littoral drift and erosion is confined to the foreshore or beach
above low-water mark. Such changes are continuously in progress
below low-water mark, where wave action or tidal scour is capable
of affecting the sea bottom. These agencies and the gravitating
tendency of the particles continue at work until a condition of
equilibrium of the opposing forces is reached.
It has often been asserted that material is derivable from the
deep sea for the replenishing of foreshores, and is in many cases
so derived. This is a fallacy which has no evidence of any sort to
support it, and which no one with even a superficial knowledge of
physical geology would accept for a moment.
We use the term "deep sea" in this connection advisedly.
Fragments of rock, boulders, and pebbles dislodged from the sea
bed in comparatively shallow depths beyond low water are some-
times cast up on the foreshore, but such instances are exceptional,
and cannot be taken as evidence of the supply of any considerable
volume of material from the sea. Undoubtedly material lying on
the sea bed below low water and in shallow depths is, in certain
conditions, driven back on to the foreshore, but this is merely a
temporary phase in the progress of littoral drift. With change of
wind or tide the conditions may be reversed, and the deposit, or
other material to take its place, will be returned to the sea.42
PROTECTIVE WORKS.
Impossibility of Entire Prevention of Erosion. — It is physically
impossible to stop erosion over any considerable length of coast-
line. The evil may be mitigated, no doubt, but its prevention is
an absolute impossibility, except over limited areas. Let us
imagine it practicable to isolate a considerable length of coast-line,
such as, for instance, the Holderness coast of Yorkshire between
Flamborough Head and Spurn Point, a distance of over 40 miles,
by the construction of barriers at each end, which would prevent
the travel of material past them in either direction. We will
assume the lateral transportation of material along the coast-line
to have ceased. Denudation would still go on. At each high
tide a little of the soft, argillaceous material forming the cliff
would be moved or dissolved at its toe, and a fall of cliff on to the
foreshore would ensue. Rain, frost, wind, and sun aid in the
gradual disintegration. The bulk of the material newly pre-
SECT. III.] COAST EROSION. 385
cipitated on to the beach is at once carried into the deep sea, and
in course of time the remainder is so ground up into fine particles
that it too is swept away or gravitates into deep water. Thus
the erosion goes on. The construction of a wall protecting the
toe of the cliff will not cure the evil, although it may hinder it
for a short time. The wall will prevent the access of the waves
to the cliff, and will retain material dislodged by other natural
agencies, but it cannot prevent the constant grinding together of
the particles on the foreshore under the action of the waves and
wind. The result is the gradual disappearance of the foreshore
itself by gravitation towards the deeper sea, and by removal in
suspension, and ultimately the collapse of the wall.42
Effect of Protective Works on Adjoining Coast-Line. — It is
therefore clear that in order to increase the extent of any fore-
shore, or to maintain it even in its existing condition, the natural
and incessant losses must be made good by the accretion or trap-
ping of material derived from other parts of the coast. How is this
to be done1? The answer is : locally, by the construction of groynes
or other works similar in effect ; but such accretion must in every
case be accomplished to the detriment of neighbouring foreshores.
The direction of the prevailing littoral drift is governed by the
direction of the flood-tide and the shape of the coast. On a
straight line of coast this direction coincides with the main set of
the flood-tide. The direction of drifts is varied from time to time
by the wind, but, in the case of England, at any rate, the direction
of the prevailing winds often coincides with the set of the flood-
tide. On the East Coast the general drift is from north to south,
and on the South Coast from west to east.
Let us again consider the case of the Holderness shore. The
littoral drift is from north to south, or from Flamborough Head
towards Spurn Point. The former is a headland of hard chalk,
jutting out into the North Sea, and subject to slight erosion in
comparison with the softer cliff material to the southward. Deep
water comes almost up to the foot of the cliff, and there is little
or no travel of littoral drift past it. Any material drawn from
the cliffs to the north, and reaching the headland, is thrown into
deep water, and becomes lost for the purposes of replenishing the
shore to the south of the head. On the other hand, there is no
possibility of any of the alluvium or other detritus brought down
the Humber being carried to the north round Spurn Point. It
therefore follows that practically all material trapped or inter-
cepted between the two promontories is derived from the local
cliffs. The construction of walls and groynes in front of particular
areas along the coast, as, for instance, Bridlington, Hornsea, and
Withernsea, must of necessity result in the starving of the fore-
25
386 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
shore to the south of every such obstruction. Such works are
undoubtedly effective, if properly designed, in maintaining the
foreshore and cliffs for a considerable period. The walls prevent
the destruction of the cliff, and the groynes intercept littoral
drift, to make good the natural wastage. But ultimately, it may
be after the lapse of many years, these isolated works of protection
must result in the increased wasting of the unprotected cliffs and
foreshore on their leeward * sides, and ultimate outflanking of the
protected areas by the sea. On the other hand, a uniform
system of protective walls and groynes along the entire coast will,
for the time being, prevent the erosion of the cliff face, but by
preventing or largely diminishing the littoral drift, bring about
the depletion of the foreshore, and ultimately the destruction of
both walls and groynes.
In short, the protection of any localities which are of sufficient
value to bear the cost of defence must result in increased depletion
of other areas, the value of which is insufficient to warrant the
construction of expensive works.42
National Aid in Coast Protection. — This aspect of the question
must be taken into consideration in dealing with the argument
for and against national assistance in foreshore protection.
The cost of protecting long stretches of agricultural land is pro-
hibitive, and, in most cases, is far higher than the value of the
land saved from the sea. Assistance given to seaside resorts or
other localities where the coast-line is of considerable value would
not only be, unintentionally, of a preferential nature, but
indirectly aid in the depletion of poorer lands adjoining.42
Effect of Pier Works and other Artificial Projections. — The
construction of solid piers or other similar obstructions at an
angle with the general shore-line, and projecting into the sea, is,
when occurring on a coast-line subjected to erosion, almost
inevitably followed by serious depletion of the foreshore to
leeward. The solid projection, which in many cases is carried
sufficiently far in a seaward direction to reach comparatively deep
water, effectually prevents the passage of littoral drift from its
windward to its leeward side. Travelling material is thus
collected to windward, and only a small portion of the drift, if
any, ever reaches the foreshore on the other side. For the most
part the material which is swept past the seaward or deep-water
end of the obstruction finds its way into the deep sea. Thus the
erosion of the lee-shore is accelerated by the loss of the travelling
* We use the terms ' ' leeward " and ' ' windward " in the sense understood
by engineers engaged in coast- protection works, viz. "windward" — the
direction whence the prevailing littoral drift proceeds; and "leeward" —
the direction towards which such drift takes place.
SECT. III.] COAST EROSION. 387
material, which, under natural conditions, makes good, to a partial
extent, the ravages of the sea. There are many instances of such
stoppage on the English coast. The construction of the Folkestone
Harbour Pier has arrested the travel of the beach from the west-
ward, and led to the accumulation of a large bank on that side
and the denudation of the foreshore to the east of the harbour
and towards Dover. The construction of the harbour works at
Dover has completely stopped the eastward drift at that point,
and the beach in front of the South Foreland and in St Margaret's
Bay has, in consequence, been starved, thus accelerating the
destruction of the cliff, falls of which have been peculiarly frequent
in recent years. At Lowestoft the construction and subsequent
extensions of the Great Eastern Railway Company's harbour pier
and works, which project at right angles to the coast-line at the
sea outlet of Oulton Broad, have resulted in the accumulation of
a huge bank of shingle to the northward and serious encroach-
ments on the town frontage immediately to the south of the
harbour. Another example is seen in the extension of the
Shoreham West Pier, which led to a stoppage of the east-
ward drift of the shingle, and consequent starving of the
Hove and Brighton beaches. Dungeness Point, acting as a
huge natural groyne, and trapping the shingle derived from
the waste of the chalk cliffs between it and Beachy Head,
has led to the denudation of the shore fronting Dymchurch
and also Romney.42
LITTORAL DRIFT.
Effects of the Coast Contour and River Estuaries. — We
have stated that the direction of the prevailing littoral drift is
governed, generally speaking, by the direction of the flood-tide.
The contour of the coast-line and the direction of the wind have,
however, a considerable influence on the drift. On a straight line
of coast the direction of travel coincides with the main set of the
flood-tide. Where a coast-line is broken up by bays and indenta-
tions, no continuous drift can take place, each bay retaining its
own characteristic material, which is prevented from leaving it by
the projecting headlands extending to low water or beyond, and
forming natural groynes. There are numerous examples of these
conditions on the south coast of Devon and Dorset, and in
Northumberland. In cases where a coast-line is broken up by
estuaries or rivers, the results are variable, depending on the con-
tinual struggle which takes place between the opposing forces of
littoral drift and the tidal inflow and outflow of the river, the latter
sometimes aided to a material extent by the addition of large
388 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
volumes of fresh water. If the tidal currents are strong and
deep, the drifting material is unable to cross the mouth of the
river or estuary, and is carried away to the deep sea, or, as
occasionally happens, is diverted, and drifts up the margin of the
estuary, as in the river Wyre, where the shingle drifts as far as
Fleetwood.
If the force of the current is insufficient to overcome the move-
ment of the drift the formation of a bar results, or the stream is
diverted, or even becomes closed. Notable examples of the
diversion of river courses by travelling shingle are seen at
Aldeburgh and Yarmouth on the East Coast, where the outfalls
of the rivers Aide and Yare have been driven miles to the south-
ward, the streams running parallel to the shore, separated from
the sea by intervening shingle banks. A typical example on the
South Coast is at Shoreham, where the outfall of the Adur has
been deflected to the eastward. Such deflections are invariably
found to follow the direction of the littoral drift.42
Effects of Tide and Wind. — The relative effects of tide and
wind on the condition of a foreshore are matters about which there
exists much diversity of opinion, but there is little doubt that the
prevailing drift is primarily and chiefly due to tidal action,
although in heavy weather the direction of drift may be for a
time entirely changed. During strong winds in a direction
contrary to the trend of the tide, the normal travel of drift may
be nullified, and even reversed for a time. The accumulation of
material on a foreshore is primarily due to tidal action in calm
weather. A beach which has been seriously depleted during a
long spell of heavy weather almost invariably makes up again, at
any rate to a partial extent, on the occurrence of calm sea and
cessation of wind. This replenishing is due to the return of a
portion of the material previously drawn down into shallow water
immediately below low-water mark. That part which has been
precipitated into the deep sea is, however, lost so far as the fore-
shore is concerned. Generally speaking, direct on-shore gales
result in the drawing down of the beach material, and its gravita-
tion towards the deep sea. Off-shore winds, on the other hand,
frequently lead to the accumulation of material on a foreshore.42
SEA WALLS AND GROYNES.
Sea Walls. — We do not propose to discuss the design of sea
walls intended for the protection of low-lying land from the sea,
the preservation of cliffs, and for forming promenades at seaside
resorts. The conditions affecting the design of a wall differ so
materially, that every case must be considered on its merits, and
SECT. III.] COAST EROSION. 389
provided for accordingly. Suffice it to say, that sea walls may be
divided roughly into two classes, sloping arid upright, each class
having its advocates among engineers. Generally speaking, walls
having a sloping face are used in Holland and Belgium, whilst
the vertical, or nearly vertical, face is more common in this
country. Undoubtedly the immediate effect of the construction
of a wall is detrimental to the beach in front of it, although it
affords needed protection to the cliff or banks behind ; the smooth
and more or less vertical face of the wall causes the waves to
sweep along the front, scouring the beach in their progress. This
effect is the more marked when the waves strike the wall at an
angle. On the other hand, when the waves move in a direction
perpendicular to the wall-line, the backwash is of serious moment.
Thus the construction of a sea wall on a sand or shingle fore-
shore is in itself calculated to bring about the denudation of the
beach, and the wall may become, before long, the agent of its own
destruction. Whilst the wall will prevent the erosion of the cliffs
in rear, the beach in front of the wall must be protected and
conserved by the construction of groynes.42
Groynes. — Just as a sea wall cannot in itself be regarded as an
efficient protection for a foreshore, so groynes, however effective
they may be in collecting travelling material, will not in all cases
prevent the waves reaching the toe of the cliff or bank and erod-
ing it to a greater or less extent. A combination of the two
forms of protection is generally desirable. Groynes may be
divided into two classes — (1) high and substantially built
structures of timber or other material ; (2) low structures of
inexpensive and light construction placed at short intervals apart.
The conditions of littoral drift and other physical characteristics
of foreshores vary so much that it is absurd to attempt the
application of any one form of structure or system of groynes
to all parts of the coast-line. Low groynes of light construction
are undoubtedly successful on some beaches, as, for instance, that
at Dymchurch. In general, light low groynes are suitable on flat
sandy foreshores which are not exposed to sudden and extensive
changes of level.
Many of the failures attending the application of this form of
groyne have been due to lack of foundation and holding power in
the beach. Such groynes may often be correct in principle, but
will not prove successful unless constructed in a substantial
manner and deeply secured. Again, a type of groyne suitable
for, and efficient on, a long flat sandy beach cannot be expected
to give satisfactory results and resist destruction on a shingle
beach where temporary changes of level of, perhaps, 6 feet
vertically sometimes occur. In certain situations, especially steep
390 GEOLOGY FOR ENGINEERS. [PT. V. CH. XVIII.
beaches subjected to large fluctuations in level, there is a con-
siderable risk of the shingle, both on the windward and leeward
sides of the groyne — but especially on the latter — being tempor-
arily scoured away to a level below that of the lowest plank,
thus allowing the beach to escape under the groyne to leeward,
and often resulting in wrecking the structure. Sheet-piling
driven for a suitable distance into the beach obviates, to a great
extent, the risk of undermining, and enables the groyne to with-
stand the temporary effects of denuding gales and much loss of
material by drift to leeward.
In protecting a particular length of foreshore it almost always
occurs that the leeward groyne of the series, especially if it be a
high and long one, produces a serious scour on its lee side. The
same thing occurs frequently on the lee side of projecting piers
or breakwaters (see above). The construction of spur groynes
placed on the leeward side of the pier or main groyne is a remedy
sometimes found beneficial. Beach and sand are accumulated in-
shore of the spur, the effect of the latter being to cause an eddy
on its shoreward side, favouring the deposition of material, and
driving the flood-tide seaward off the shore-line.
It is frequently overlooked that a considerable amount of erosion
goes on at, and seaward of, low-water mark. The consequent
advantage of carrying groynes below the level of low water is
therefore apparent.42
For further information on the subject of coast erosion the
reader may consult Coast Erosion and Foreshore Protection, by
Owens and Case — a very practical little book ; also The Sea Coast,
by Wheeler, etc.1
CH. XIX.]
CHAPTER XIX.
USES OF MINERALS.
DISTRIBUTION OF VALUABLE MINERALS AND ROCKS.
Coal is of three kinds: 1. Anthracite or stone coal (used only
in furnaces and for steam engines) ; occurs in South Wales,
Linlithgow, Kilkenny, and Pennsylvania. 2. Ordinary coal (mis-
called bituminous), including cannel, jet; Boghead coal is a
bituminous shale, found in Scotland, from which oil is distilled.
3. Lignite or brown coal, abundant in Central Europe, Punjab,
and found also in Devon, Antrim, etc.
Coal, usually associated with rich iron ores, occurs in many
detached areas, usually geological basins, called coal-fields. Those
in Britain yield 135 millions of tons of coal and 7 millions of tons
of iron annually. The chief are : —
1. The Forth and Clyde, including Ayr and Edinburgh; 2.
Newcastle; 3. Whitehaven; 4. Lancashire and Cheshire; 5.
Yorkshire ; 6. North Stafford or " Potteries " ; 7. South Stafford
or "Black Country"; 8. Bristol: 9, Dean Forest; 10. South
Wales (900 sq. miles). The three latter were doubtless once
united, and Ramsay thinks much coal still lies buried under
newer strata in and around the Severn estuary. Coal occurs in
seventeen counties of Ireland, but nowhere in abundance.
Foreign coal-fields occur in Belgium at Liege, etc., North
France, Rhenish Prussia, Silesia, N.W. Spain, India, Borneo and
Formosa, North America. The above coals are of Carboniferous
age, but coal occurs of other periods, e.g. —
Of Keuper age, at Richmond, U.S.A.
Of Oolitic age, at Brora in north of Scotland, south of India,
Labuan, Philippines.
Of Cretaceous age, in Vancouver Island and the Rocky Moun-
tains district from New Mexico to Canada (the vast deposits here
are lignitic, and may partly be of Eocene age).
Of Miocene age, in North Greenland, North Germany, Switzer-
391
392 GEOLOGY FOR ENGINEERS. [PT. V.
land, Bovey in Devon. Most of this is lignite, and still shows
woody structure.
Graphite. — Plumbago or black lead is nearly pure carbon, and
often only an extreme form of anthracite, and so of coal. It
occurs in Cumberland, Cornwall, Spain, Bohemia, Greenland,
Finland, Norway, and especially Canada; usually in slates,
schists, gneiss, granite. The chief supply is from Ceylon.2
Iron is everywhere diffused, and rich deposits occur in most
coal-fields, as also in Laurentian strata in Canada, in Silurian
strata in Mid-Sweden, in Great Oolite sands at Northampton, in
Middle Lias at Cleveland, Yorkshire. Iron pyrites, the chief
source of sulphur, occurs very abundantly, especially in the Coal
Measures. 50,000 tons are raised yearly in Britain, and 500,000
tons are imported from Spain, etc.2
Gold has been detected in almost every country arid every kind
of rock, especially slates and schists abounding in quartz veins.
It occurs both in the veins and in the rock near the veins, and
still more in the detritus — gravel or alluvium — derived from
these. It is usually in a native state in crystals, grains, or
nuggets. Gold occurs in Carmarthen and in Wicklow. It is
worked in Hungary and Transylvania, Piedmont, the valleys of
the Upper Rhine and Danube, Sweden, Russia in the Ural district,
Siberia in the Altai Chain, Thibet, Yunnan in China, Borneo, etc.
But the chief supplies are obtained from Australia, chiefly south-
east part, Tasmania, New Zealand ; and North America, both the
east side from Nova Scotia to Georgia, and the west side from
British Columbia to Mexico ; above all in California ; in South
America, Brazil, Minas Geraes, Chili, Peru, New Granada, and
Nicaragua ; in South Africa the Transvaal and Guinea coast are
important sources.2
Silver occurs in Britain, chiefly in lead ores, as at Alston Moor •
also in Spain, Saxony, Hartz Mountains, Austria, Hungary, Asia
Minor, Siberia, Nevada U.S.A., but most abundantly in Mexico at
Guanascuato, and the Andes of Peru and Bolivia at Pasco, Potosi ;
and Chili at Copiapo.2
Platinum is found only in alluvial deposits, and only in the
Ural region, in Brazil, and in New Granada.2
Mercury (quicksilver) occurs in Spain at Almaden, Austria
at Idria, the Palatinate, Tuscany, Ceylon, China, Japan, California,
Mexico, Peru.2
Tin occurs in Cornwall and Devon (both in lodes and in stream
deposits), and in Saxony, Bohemia, Spain, Bolivia ; but especially
in Tenasserim and Banca Island in South-East Asia.2
Copper occurs native in huge masses, up to 1 ton in weight,
near Lake Superior, and largely in South America and Siberia.
CH. XIX.J USES OF MINERALS 393
Various ores occur in veins in Cornwall, Devon, North Wales,
Saxony, etc.2
OTHER USEFUL MINERALS.
Barytes when of a good colour is ground for mixing with white
lead in the manufacture of paint, and for this purpose when
slightly tinged by brown iron ore it may sometimes be rendered
sufficiently white by treating it with hydrochloric acid. The dark-
brown stalactitic variety from Derbyshire is used as an ornamental
stone, being carved into vases and similar objects. It may also
be used for a production of the chloride, nitrate, and other barium
salts, being first reduced to sulphide of barium by heating with
carbon, after which it is soluble in acids. Generally these salts
are made from witherite, which is soluble without any special
preparation. Nitrate of baryta is used by pyrotechnists in mak-
ing green fire, and also to some extent as a nitre in certain
blasting powders and similar explosive substances.14
Anhydrite. — Some of the compact and siliceous varieties are
used as building-stones. It may be used for making plaster, but
must be calcined before grinding, as, although anhydrous, it will
not set by absorbing water when in the natural condition, except
with extreme slowness. In rock-salt mines its presence is un-
welcome ; being tough and hard, it blunts the boring tools, and
increases the cost and labour of driving.14
Gypsum. — The applications of gypsum are very numerous.
The clear transparent crystals, also called fielenite, are used for
optical purposes in thin plates for producing definite colours in
polarised light. The fibrous silky variety called /Satin Spar is
carved into beads and other ornaments giving chatoyant or " cat's-
eye " reflections. This is principally got in Nottinghamshire and
Derbyshire. The compact, finely granular kind, or Alabaster, is
used for small ornamental sculptures, the principal supply being
from Volterra, near Pisa, where it is found in irregular nodules of
clay. The best kinds are very similar to fine statuary marble in
colour and texture ; other kinds, variegated with blue or black
veins, are cut into vases. The chief use is, however, in the pro-
duction of plaster of Paris, which is made by driving off the water
in the kiln from the rough gypsum or plaster stone, and after-
wards grinding it to a fine powder. Gypsum is also used as a
manure.14
Asbestos. — The only variety of hornblende directly of economic
value is asbestos, which can be carded and spun like flax or cotton,
and is largely used for steam-engine packings, fireproof cloth, and
mill-boards, and as a filtering material for chemical purposes, it
394 GEOLOGY FOR ENGINEERS. [FT. V.
being unchanged by heat or mineral acids. The supply is chiefly
derived from Italy and Canada.14
Mica. — The transparency, flexibility, and toughness of cleavage
plates of mica render them useful as substitutes for glass in
special cases. The purest and best crystals are obtained from
Russia, India, the north-eastern states of America and Canada,
both muscovite and phlogopite being used. Finely ground mica
has also been lately introduced as a lubricating material for
machinery.14
MINERAL PIGMENTS.
Among the most common and abundant of these pigments or
colouring matters are the hydrated oxides of iron, known as
ochres, boles, reddles, and the like. Strictly speaking, ochre is
a hydrated peroxide of iron, consisting of about 80 per cent, of
the hydrate and 20 of water ; but it is very rarely found pure,
being often, in fact, clay coloured yellow by hydrate of iron,
though a fair ochre should not contain less than 15 to 20 per
cent, of the hydrate. Naturally it varies from pale yellow to a
deep orange or brown ; but the manufactured article is usually
toned to any shade by treatment and admixture. It occurs in all
formations, much of that used in Britain being obtained from the
Coal formation, where it appears as a product of decomposition.11
Bole is the term usually applied to friable clayey earths
coloured by the peroxide of iron, and varying from yellow to
yellowish red and reddish brown. The term is rather an indefinite
one, and loosely applied ; but a useful variety may consist of
about 32 per cent, of silica, 20 alumina, 21 iron peroxide, and
17 water. Bole occurs in irregular beds and disseminated masses
in various formations, some of the finest sorts (Sinopian earth)
being procured from Italy and Asia Minor. The better-known
varieties are the Armenian, of a bright red colour ; the Sinopian,
of a deeper red ; the Bohemian, of a yellow red ; the Blois, of a
pale yellow ; the French, of a pale red ; the Lemnian, of a
yellowish red ; and the Silesian, of a similar, but brighter, hue.11
Keddle, which is merely a corruption of red clay, is another of
those argillaceous hydrated peroxides of iron, usually of a deep
red, and, in fact, a decomposed haematite. It occurs abundantly
in England, France, and Germany, and usually in the haematite-
yielding districts of the Carboniferous limestone, as Cumberland,
North Lancashire, Somerset, and Devon.11
Umber is properly a soft earthy combination of the peroxides of
iron and manganese, with minor proportions of silica, alumina,
and water ; the percentages are about 48 iron peroxide,
CH. XIX.] USES OF MINERALS. 395
20 manganese peroxide, 13 silica, 5 alumina, and 14 water. It is
usually found in veins in the crystalline schists, and appears to be
a product of decomposition. Commercially it is obtained from
the island of Cyprus, Anglesea, Isle of Man, Forest of Dean, and
other localities. Much of the umber of the colourman, however,
is merely an ochreous admixture ; and that from Cologne is
said to be only brown lignite finely pulverised.11
Whiting or Spanish White, one of our most common, but
useful, colouring matters, is obtained from the softest and purest
white chalks by grinding and elutriation. It is extensively used
as a whitewash, and occasionally, when carefully and delicately
prepared, as a cheap white paint. A serviceable whitewash for
external walls, and one possessing disinfecting properties, is
obtained by diluting quicklime — the purer and whiter the lime-
stone, the more brilliant the whitewash. Coloured washes and
rubbing bricks for external use have usually a basis of whiting or
clay, the basis being obtained from ochre, reddle, bluestone, or
other cheap material.11
Ultramarine was originally prepared from the lapis - lazuli.
This mineral, which occurs in the old crystalline schists and lime-
stones, is rather rare, and often treatment yields only a small
percentage of the colouring matter ; hence the former high price
of the pigment. The artificial pigment can be made to rival the
natural in beauty and softness at the same time that it admits of
a greater variety of shades and tonings. It is manufactured
principally in Germany and France, and consists of definite
proportions of kaolin or silicate of alumina, calcined sulphate of
soda, calcined soda, sulphur, and pulverised charcoal or pit-coal —
other ingredients, as gypsum, baryta, etc., being added to tone the
colour to special requirements.11
Metallic Pigments. — A great many pigments are obtained
from the metals — lead, zinc, copper, cobalt, chromium, arsenic,
iron, manganese, mercury, etc. ; but as the processes are purely
technological, they belong to chemistry rather than geology.
Table. — The following table shows the mineral and metallic
sources from which the different colours are derived : —
White pigments, from lead, zinc, heavy-spar, or sulphate of
baryta, chalk, and admixtures.
Yellow, from antimony, lead, arsenic, chromium, chalk, and
admixtures.
Orange, from ochre, chromium, lead, chalk, and admixtures.
Broivn, from umber, Terra di Sienna, manganese, and
admixtures.
Red, from ochre, bole, reddle, chrome, mercury, arsenic, lead,
and admixtures
396 GEOLOGY FOB ENGINEERS [FT. V. CH. XIX
Black, from iron, manganese, asphalt, coal-tar, and admixtures.
Blue, from cobalt, copper, iron, lapis-lazuli, potash, soda, and
admixtures.
Purple, from gold and tin, and from admixtures.
Green, from copper, chrome, arsenic, potash, and admixtures.
Intermediate shades, like compound colours, are all obtained by
skilful admixture.11
INDEX.
ABBREVIATIONS in list of minerals,
69.
Aberdovey slates, 300.
Abney's level, 191.
Abrasion test for road stone, 349.
Absorbent power of rocks, 290, 294.
Absorption test for road stone, 349.
Abyssal rocks, 94.
Acadian area, 145, 147, 186.
Acanthodus, 181.
Acicular, 65.
Acid, carbonic, 7, 8, 53, 202.
citric, 224.
definition of, 53.
humic, 134.
hydrochloric, 202, 224.
hydrofluoric, 217.
muriatic, 325.
nitric, 202, 224, 225.
oxalic, 224.
rocks, 96.
sulphuric, 202, 224, 226.
tartaric, 224.
Acidic, 96.
Acids, solubility of minerals in, 57.
use of, in rock testing, 216.
Acrodus, 172, 175.
Actinolite, 70, 206, 231.
Actinozoa, 154, 183.
Adamantine lustre, 67.
Adularia, 70, 77.
Adur, River, 388.
Advantages of lakes, 267, 270.
JEolian action, 4-9.
Aerial deposits, 138, 165.
Afghanistan, 171.
Africa, 7, 152, 168, 173, 175, 179,
181, 188, 392.
African desert, 113.
Agates, 34.
Agencies effecting change on earth's
surface, 4.
Agents of maintenance in tidal rivers,
363.
Agglomerate, 34, 101.
Aggradational processes, in sea, 20.
Agricultural purposes, rocks used for,
295.
Air, 4-7.
Alabama, 144, 145.
Alabaster, 70, 81, 119, 393.
Alaska, 176.
Albite, 70, 77.
Alcyonia, 155.
Aldborough, 170, 373, 388.
Aleutian Islands, 173.
AlgJB, 162.
Algeria, 188.
Algonkian, 187.
Alkaline earths, 53.
Alleghanies, 146, 173, 178, 186.
Alliaceous odour, 57, 232.
Alluvial deposits, 138, 144, 164,248,
366.
fans or cones, 15.
plains, 16.
| Alluvium, 15.
Almaden, 392.
Alongshore currents, 381.
Alps, 5, 9, 88, 107, 168, 170, 172,
173, 175, 176, 178, 180, 275.
Alston Moor, 392.
Altai Mountains, 184, 392.
Altered andesites, 111.
and metamorphic rocks, 45-50, 95,
98, 102-3, 122-8.
rocks, 123-4.
Alternation of beds, 39.
Altitude, influence of, on coast-lines,
370-1.
Alumina, 5, 6, 230.
Aluminium, 54, 233.
Alum shales, 306,
Amber, 24.
397
398
GEOLOGY FOR ENGINEERS.
America, North, table of strata,
144-7.
Ammonia, 224, 226.
Ammonites, 160, 172, 174, 175, 176.
Amorphous minerals, 58.
metamorphic rocks, 103.
Amphibia, 161, 172.
Amphibole, 70, 203.
Amuri, New Zealand, 151.
Amygdaloids, 34, 98, 101.
Analcime, 70, 93.
Analyses of rocks, 210-2, 284-6,
290, 302-4, 317.
Anamesite, 282.
Ancaster, 297, 303.
Andalusite, 70.
Andes, 110. 173, 175.
Andesites, 110, 130, 203, 205, 283.
Angiosperms, 162.
Angle of repose of earth, etc., 335.
Anglesea, 275, 277, 280, 395.
Anhydride, 54.
Anhydrite, 69, 70, 393.
Anhydrous calcium sulphate, 70.
oxides, 69.
silicates, 69.
Animal action, 25.
life as indication of rocks, 194.
Animals, classification of, 153.
Annan, 178.
Annelida, 156, 179, 187
Annulosa, 156.
Anodonta, 158.
Anomodonts, 175.
Anorthic system of crystals, 61.
Anorthite, 70, 80.
Anorthosites, 149.
Anthracite, 121, 391.
Anthrocosia (misprint for Anthra-
cosia), 179.
Anthropozoic, 138, 163.
Anticlinal, 42.
Antimony, 232, 233, 395.
Antrim, 172, 332, 391.
Apatite, 69, 70, 206.
Apennines, 171, 176.
Aplite, 108.
Apophyllite, 71, 93.
Appalachian Mountains, 146.
Apparatus and reagents, blowpipe,
226-9.
Aqua regia, 224, 226.
Aqueous rocks, 37-45, 95, 97, 101-2,
112-22.
Arabia, 113, 173.
Arachnida, 157, 181, 182.
Aragonite, 69, 71, 116.
Archaean, 87, 137, 138, 143, 147,
149, 186, 187, 295.
Archseopteryx, 161.
Architectural geology, 3.
Arctic clay, 138, 166.
Ardara granite, 273.
Ardmillan series, 185.
Arenaceous rocks, 95, 97, 112-4.
Arenicolites, 156.
Arenig, 143, 167, 184, 185.
Argentina, 177, 178.
Argillaceous, 57, 95, 97, 114-6, 133,
204, 206, 290, 299-301.
Ariyalur, 148.
Arkansas, 146.
Arkose, 114.
Armadillo, 161.
Armagh, 178.
Armenia, 394.
Arran, 275, 278.
Arsenic, 230, 232, 395, 396.
Artesian springs and wells, 26, 259.
Articulata, 56, 179, 180.
Artois, 259.
Arvonian, 187.
Aryan group, 148.
Asaphus, 157, 184.
Asbestos, 64, 72, 206, 393.
Ash, volcanic, 28, 34.
Asia, 7, 168, 171, 173, 175, 178, 180,
184, 185, 186, 187, 392.
Asia Minor, 184, 392, 394.
Asmanite, 91.
Asphalt, 69, 72, 347, 396.
Assam, 187.
Assay, 229.
Asteroidea, 156, 185.
Astraea, 154.
Astropoda, 156, 172.
Atherfield clay, 140.
Atlantosaurus, 161.
Atlas Mountains, 170, 173.
Atmosphere, work of, 4-9.
Atrypa, 158.
Augen-gabbro, 128.
-gneiss, 128.
-schist, 128.
Augite, 63, 72.
andesite, 287.
-granite, 107.
-hornblende group, 69, 72-3.
syenite, 108.
Australasia, 7, 164, 165, 168, 171, 173,
175,179,181,184,185,186, 188.
Australia, table of strata, 150.
INDEX.
399
Austria, 168, 392.
Auvergne, 283, 287.
Available rainfall in drainage areas,
268.
Avalanches, 17.
Avicula contorta, 176, 177.
Awamoa, New Zealand, 151.
Awatere, New Zealand, 151.
Axes of minerals, 61.
Aymestry limestone, 143.
Ayr, 282, 391.
BABYLON, 6.
Bacchus Marsh, 150, 179.
Backs, 44.
Backwash, 376.
Bag and belt, 190.
Bagh beds, 148.
Bagneres, 132.
Bagshot beds, 139, 170, 293, 207.
Baked shale, 123.
Bakevellia, 177.
Bala, 143, 184, 185, 300.
Balkans, 172, 176.
Ballantrae, 184.
Ballow, 279.
Baltic, 167.
Banat, 278.
Banca Island, 392.
Banded structure of rocks, 99.
Bangor slates, 300.
Bannister slates, 183.
Bardiglia, 294.
Barium, 54, 230.
Barriers at mouths of rivers, 22, 365.
Bars, tidal, 16, 358, 365-8.
Barton series, 139, 170.
Barytes, 63, 69, 73, 393.
Basaltic andesites, 110.
Basalt rocks, 36, 37, 111, 130, 131,
205, 282, 343, 344.
Base, definition of, 53.
Base level of erosion, 22.
Basement complex, 188.
Basic rocks, 96, 97.
Basin, 43.
Bass, 306.
Basset, 40.
Bastion series, New Zealand, 151.
Bath oolite, 140, 297, 303.
Baton, River, New Zealand, 151.
Bats, 161.
Batt, 306.
Bavaria, 168.
Bay of Biscay, 7.
Beachy Head, 371, 387.
Bear Island, 182.
Beavers, 25, 162.
Bedding, 38, 102.
Beds, or strata, 38.
Belemnites, 160, 172, 174.
Belgium, 170, 172, 180, 182, 183, 185,
382, 389, 391.
Bellerophon, 160, 178, 184.
Bembridge beds, 139, 170, 299.
Bengal, 80, 187.
Ben Nevis, 278.
Benton group, 145.
Berthier's mode of analysis, 325.
Bevelment of crystals, 62.
Bhimas, 149.
Big Horn Mountains, 145.
Bihargebirge, 279.
Bijawars, 149.
Bilin, 310.
Binary compound, 62.
Bind, 306.
Binding material, 346.
Binoxide, 53.
Binstead limestone, 299.
Biotite, 73, 87, 108.
granite, 107.
Birds, 161, 169.
Birkhill shales, 183.
Bismuth, 232, 233.
Bitter spar, 73, 75.
Bitumen, 72, 73, 182.
Bituminous coal, 121, 391.
rocks, 118, 290.
Bivalent, 54.
Blackband ironstone, 73, 84.
Black cotton soil, 24.
Forest, 168.
lead, 73, 80, 392.
mica, 73, 87,
Sea, 170.
soil plains, 150.
Blasting powder, 393.
Blastoids, 155.
Bleached gravels, 133.
Blende, 73, 225, 231.
Blois, 394.
Blowpipe, behaviour of minerals be-
fore, 57.
apparatus, 226.
examination, 226-36.
operations, 229-36.
use of, 228.
Blue-bind, 306.
Bluehearted rocks, 105.
Bluestone, 114, 395.
Bodmin Moor, 275.
400
GEOLOGY FOR ENGINEERS.
Boghead coal, 391.
Bog iron ore, 24, 73, 83.
Bog manganese ore, 73, 86.
Bognor series, 139, 170.
Bogs, 121, 138, 167.
Bohemia, 175, 178, 180, 182, 183,
185, 278, 283, 392, 394.
Bb'hmerwald, 278.
Bokkeveld beds, 152.
Bole, 394, 395.
Bolivia, 392.
Bolsover Moor, 297.
Bombs, 27.
Bone-beds, 120, 143.
-breccia, 120.
Borax bead, 233-4.
Bords, 44.
Borneo, 181, 391, 392.
Boron, 230.
Borotungstate of cadmium, 215.
Borrowdale series, 184.
Boskowitz, 279.
Bosses, 35.
Botallack, 273.
Botryoidal, 65.
Botzen series, 178.
Boulder clay, 138, 144, 166, 167.
Boundary lines, tracing, 193.
Bournes, 250.
Bovey Tracey, 307, 392.
Bowenfels beds, 150.
Brachiopoda, 158, 169, 172, 176, 177,
179, 182, 185.
Brachydiagonal, 60.
Bracklesham series, 139, 170.
Bradford clay, 140.
Branches of geology, 2.
Branchiosaurus, 177.
Brand's process, 288, 291, 349.
Bray Head, 186.
Brazil, 178, 188, 392.
Breakers, 21, 375.
Breccia, 34, 101, 114.
Brecciated limestone, 117.
Brick earth and clays, 9, 138, 308.
Brick- making, science of, 311-6.
Bricks and clays, 305-17.
Bridlington, 385.
Brighton, 387.
Bristol, 391.
Bristol Channel, 165, 180, 182.
British clays, 307.
British Columbia, 176, 392.
British granites and syenites, 275-8.
British limestones, 331-2.
Brittany, 278.
Brixham cave, 164.
Brontotherium, 169.
Bronzite, 72, 73, 231.
Brooks, 12.
Brora, 174, 391.
Browgill shales, 183.
Brown coal, 129, 391.
haematite, 83.
iron ore, 65, 73, 83, 205.
spar, 73, 75.
Brown Willy District, 275.
Brunn, 278.
Bryophyta, 162.
Building, selection of stone for, 290.
stones, 272-304.
use of knowledge of geology for, 1.
Bundelkhand, 187.
Bunter series, 141, 176, 292, 302.
Burgundy, 327.
Burrum beds, 150, 175.
By-roads, 347.
CADER Idris, 184.
Caelenterata (misprint for Coelen-
terata), 154, 183.
Caerfai group, 186.
Cainozoic, 138, 168-71.
Caithness flags, 142, 291, 300.
Calais, 182.
Calamites, 162, 182.
Calcareous grit, 140.
rocks, 95, 97, 116, 247, 290.
tufa, 117.
Calceola, 154, 181.
Calciferous sandstone, 142, 180, 185,
293.
Calcination of limestones, 319, 323-4,
326.
Calcite, 69, 73. 116, 220.
Calcium, 54, 56, 230.
oxide, 319.
Calc schist, 117, 127.
California, 145, 146, 164, 173, 176,
392.
Callipteris, 177.
Calymene, 157, 183, 184.
Cambrian rocks, 138, 143, 185-6,
291, 300, 302, 304.
Canada, 279, 281, 391, 392, 394.
Canadian period, 147.
Canal-making, 2, 351-3.
Cannel coal, 391.
Capacity of drainage area, 270.
of rocks for water, 245-7.
Cape Horn, 173.
Caradoc rocks, 143, 302.
INDEX.
401
Carangeot, 219.
Carbon, 54, 55.
Carbonaceous rocks, 118, 122, 290,
306.
Carbonate of soda, 227, 233.
Carbonates, 69, 83, 209.
Carbonation, 8.
Carboniferous limestone, 120, 142,
180, 296, 298, 303, 344, 351,
394.
period, N. America, 146.
system, 121, 136, 137, 141, 177,
179, 291, 295, 301, 391.
Cardita beaumonti beds, India, 148.
Carinthia, 178, 287.
Carlisle, 174.
Carlow flags, 292.
Carlsbad twinning, 218.
Carmarthen, 392.
Carnivores, 162, 169.
Carolina beds, 144.
Carpathian Mountains, 170, 278.
Carrara marble, 294, 304.
Caryophyllia, 154.
Caspian Sea, 170.
Catchment area, 267.
Catskill period, N. America, 146,
182.
Cattin, River, New Zealand, 151.
Causes of success or failure in wells,
261.
Caustic lime, 319.
Cavan, 183, 185.
Cave deposits, New Zealand, 151.
Cavern deposits, 164.
Caverns, 10.
Caves, ossiferous, 10.
Cawk, 73, 74.
Celestine, 69, 74.
Cellular structure of rocks, 98.
Cements, 318, 326-8.
Central Asia, 6, 131.
Provinces, India, 269.
Centroclinal dip, 43.
Cephalaspis, 161, 182,
Cephalopoda, 160, 176, 177, 179,
181, 183, 185.
Ceratite formations, Salt Range,
India, 148.
Ceratites, 160, 176.
Ceratodus, 175.
Cetaceans, 161, 169.
Cetiosaurus, 174.
Ceylon, 187, 392.
Chabasite, 93.
Chalcedony, 65, 74, 91, 204.
Chalcopyrite, 74.
Chalicotherium, 169.
Chalk, 117, 250, 294, 302, 395.
marl, 117, 204.
system, 120, 140, 248.
Chalybite, 83, 231.
Cham plain period, N. America, 144.
Changes in rocks, 30-2.
of temperature, 5.
within the earth, 26-32.
Channel Islands, 275, 277.
Channels, river, 360, 367.
subterranean, 10.
Chara, 162.
Characteristic fossils, 136.
Charcoal, 227, 232, 395.
Chari series, Indian Empire, 1 48.
Charleston buhrstones, 144.
Charnwood Forest, 187, 275, 276.
Charnockite, Indian Empire, 149.
Chazy epoch, 147, 185.
Chellaston, 328.
Chemical balance, 221.
characters of minerals, 57, 224.
constituents of rocks, 96, 97, 98,
290, 298.
examination of rocks, 209-12.
Chemung period, N. America, 146,
182.
Chert, 91, 119.
Cheshire, 166, 275, 391.
Chiastolite, 69, 70, 74, 304.
Chikkim series, Indian Empire, 148.
Chili, 175, 392.
Chillesford Crag, 138.
China, 7, 180, 184, 186, 392.
clay, 74, 85, 305.
Chisel, 190.
Chitral limestones, Indian Empire,
149.
Chlor-apatite, 71, 74.
Chlorides, 69.
Chlorine, 54, 55.
Chlorite, 69, 74, 89.
schist, 127, 206.
Christiania, 275, 279.
Chromium, 54, 234, 395, 396.
Chrysolite, 63.
Chywoon Morrah, 274.
Cincinnati limestone, 147, 185.
Cipolino, 294.
Cirque, 18.
Citric acid, 224, 226.
Claiborne group, 144.
Clarence series, Australia, 150.
Clastic laminee, 64.
26
402
GEOLOGY FOR ENGINEERS.
Clastic rocks, 101, 112.
Clay, T14, 246, 250, 299, 305-9, 311.
edible, 24.
ironstone, 74, 83, 205.
slate, see Slate.
Clays, colouring of, 307.
origin of, 129.
red, in deep sea, 23.
Cleavage of minerals, 62, 63, 219.
of rocks, 36, 47, 48, 103, 299.
Cleaved structure of rocks, 201.
Cleveland, 174, 392.
Cliffs, 372-373.
Climate, effect of, 13.
Clinkstone, 110.
Clinochlore, 74, 89.
Clinodiagonal, 60.
Clinometer, 191.
Clinton, N. America, 147, 184.
Closed tube, glass, 227, 231.
Clyde beds, 138, 166, 282.
River, 368, 391.
Clymenia, 160, 181, 182.
Coal, 24, 121, 391.
-fields, British, 391.
foreign, 391.
formation, New Zealand, 151.
measures, 137, 141, 179, 271, 291,
292, 302, 307, 309, 392, 394.
Australia, 150.
tar, 396.
Coarse tuffs, 111.
Coarsely fragmented rocks, 101.
Coast contour, effect of, on drift,
387.
erosion, 370-90.
and reclamation, 381-90.
forces, acting on, 373-81.
lines and their origin, 370-3.
effect of works on, 385.
protection, national aid in, 386.
Cobalt, 234, 395, 396,
nitrate, 227, 232-3.
Coccosteus, 181.
Coefficients of quality of road
materials, 350-1.
Ccelenterata, 154, 183.
Colloidal state of minerals, 58.
Collyweston slates, 300.
Cologne, 395.
Colombia, 176.
Colorado, 77, 145, 175.
Colour and lustre of rocks, 104, 132,
202, 307.
of minerals, 62, 67.
Colours of bricks, 315-6.
Colourwash, 395.
Columbia, British, 176, 392.
Columnar structure of minerals, 63,
64.
structure of rocks, 37, 99.
Compact structure of rocks, 98, 201.
Compass, 191.
Compound, 52. 56.
radicle, 52.
Compton Bay, 372.
Conchoidal fracture of minerals, 65.
Concretionary structure of rocks, 102,
117, 201.
Cone-in-cone, 332.
Cones, alluvial, 15.
volcanic, 28.
Conformability of formations, 137.
Conformable strata, 40.
Conglomerate, 34, 102, 114.
Conifers, 162, 174, 176, 177.
Coniston grits, etc., 183, 184, 289.
Connecticut, 107, 145, 177.
Connemara marble, 294.
Consolidation, 31, 32.
Contact goniometer, 219.
Contemporaneous rocks, 34.
Continental Europe, 167-8, 170, 172,
174, 176, 178, 180, 182, 183, 185,
186, 187.
Contorted drift of East Norfolk, 138,
166.
Contortions, 42.
Contour of rivers and river beds, 356.
Contours, 192.
Contraction in bricks, etc., 315.
Copiapo, 392.
Copper, 65, 67, 230, 233, 234, 392,
395, 396.
pyrites, 67, 69, 74.
Coprolites, 71, 74, 120, 121.
Corallian beds, 140, 298.
Coral limestone, 118.
Coralline crag, 138.
Corallines, 39, 162.
Coral reefs, 25.
Corals, 154, 172, 174, 179, 181, 182,
183.
Cornbrash, 140.
Corniferous period, North America,
146, 182.
Cornley sandstones, 186.
Cornstones, 118, 142.
Cornwall, 7, 45, 72, 79, 131, 273,
274, 275-6, 280, 392, 393.
Corsica, 100, 278.
Corsite, 100,
INDEX.
403
Coulee, 34.
Crag and tail, 20.
Craigleith sandstone, 290, 292.
Cretaceous system, 137, 140, 172-4,
293, 298, 391.
Crevasses, 19.
Crinoids, 155, 174, 176, 177, 179,
185.
Crocodiles, 161, 174.
Croghan Kinshela granite, 273.
Cross bedding, 38.
faults, 45.
Crushing of rock-constituents, 213.
test for road stone, 349.
weight of rocks, 290.
Crust movements, 20, 29-30.
Crustacea, 23, 156, 176, 181, 183.
CryDtocrystalline structure of rocks,
98, 100.
Cryptogams, 162, 179.
Crystal angles, 219.
forms, 58-62, 218.
systems, 59.
Crystalline limestone, 117, 124, 203,
304.
marbles, 294.
schists, 281, 395.
structure of rocks, 98, 102, 201.
Ctenoids, 160.
Cubic system of crystals, 59.
Cuddalore sandstones, Indian Empire,
148.
Cuddapah, Indian Empire, 149.
Culm type, 180.
Culver cliff, 371.
Cumberland, 5, 275, 282, 392, 394.
Curl, 332,
Current bedding, 38.
Currents, 14, 379, 380, 381.
Curvature, 42, 198.
Cutch, 175.
Cyanosite, 225.
Cycads, 162, 174, 176.
Cycles, 137.
Cycloid, 160.
Cyclopteris, 162.
Cyprina, 158.
Cypris, 156.
Cyprus, 395.
Cyrena, 158.
Cystideans, 155, 177, 184.
DACITE, 111, 287.
Dakota, 145, 175, 248.
Dalecarlia, 183.
Damourite, 126.
Damuda series, Indian Empire, 148,
179.
Danube, 392.
Daonella, 176.
Darley Dale sandstone, 290.
Dartmoor, 275.
Deccan, 148, 173, 281.
Decomposition of silicates, 129.
Decrepitation, 231.
Deep leads, Australia, 150
sea erosion, 384.
wells, 261.
Deltas, 16, 138.
Denbigh grits and flags, 183, 289.
Denmark, 172.
Dense liquids, 215, 223.
Density, 66.
of large masses, 223.
of rocks, 294.
Dent marbles, 294, 295.
Denudation, 4, 381.
marine, 20.
Deoxidisation, 7, 133.
Deposit carried in rivers, 360-1.
Deposition by glaciers, 19.
by running water, 15-7.
marine, 22.
Deposits, deep-sea, 23.
littoral, 23.
non-littoral, 23.
oceanic, 20.
pelagic, 23.
shallow water, 23.
terrigenous, 23.
Depression of land, 29, 30.
Derbyshire, 328, 393.
Desert sandstone, Australia, 150.
Deserts, 113.
Destructive action of rain, 7-8.
effects of underground water, 10-11.
Determination of minerals, 218-26.
of proportions of minerals in a
rock, 217.
of rocks, 200-6.
of route for new road, 333.
Detritus in rivers, 365.
Deutozoic, 138.
Devitrification of minerals, 58.
of rocks, 99.
Devonian system, 136, 138, 142, 181,
295, 301.
Devonshire, 180, 275, 280, 387, 391,
392, 393.
Dew, 12.
Dharwarian beds, Indian Empire,
149.
404
GEOLOGY FOR ENGINEERS.
Diabase, 111, 129, 281, 344.
Diallage, 65, 72, 75, 109.
Diastrophisin, 20.
Diatoraaceous earths, 144.
Diatoms, 24, 144, 162, 310.
Dicotyledons, 162, 169.
Dictyonema, 154, 186.
Dicynodon Gordonia, 176.
Didymograptus, 154.
Dimensions of river channels, 360.
Dimetian group, 187.
Dimetric system of crystals, 59.
Dinas bricks, 310, 314.
Dinorwig slates, 300,
Dinosauria, 161, 174, 177.
Dinotherium, 169.
Diorite, 109, 129, 131, 203,281, 344.
gneiss, 125.
Dioxide, 53.
Dip, 40, 195-7, 248.
Diphy cereal, 160, 181.
Diplograptus, 154.
Dipnoid, 161, 175, 181.
Dipterus, 181.
Disintegration of rocks, 130.
Dislocation, 44, 45, 199.
Displacement apparatus, 208, 222.
Dolerite, 111, 130, 203, 282, 344.
Dolgelly, 186.
Dolomite, 69, 75, 116, 118, 203, 294,
303, 304.
Dolomitic limestone, 118.
Dome, 42.
Donegal, 187, 273, 275, 277, 278,
281.
Dorsetshire, 176, 331, 387.
Doulting freestone, 303.
Dover, 387.
Down, 183, 185.
Downthrow of fault, 44.
Down ton sandstone, 143.
Drainage areas, 267-70.
of roads, 337-8.
Dravidian group, Indian Empire, 149.
Dresden, 278.
Drift bedding, 38.
Drifts, 138, 144, 165-6, 247.
Drigg in Cumberland, 5.
Druidical remains, 6.
Drumlins, 166.
Drusy, 65, 99.
Dry-weather flow, 243.
Ductility of minerals, 65.
Dudley coal-field, 258.
Dunes, 5, 7, 24.
Dungeness, 387.
Dunkeld, 113.
Durability of road stone, 349.
of rocks, 274, 290, 297, 298.
Durham, 79, 178.
Durness limestone, 186.
Dwyka conglomerate, S. Africa, 152,
179.
Dyad, 54.
Dyas system, 141, 177.
Dykes and veins, 35, 45, 282.
Dymchurch, 387, 389.
Dynamic action, in rivers, 357-8.
Dynamical geology, 3.
Dynamo or regional metamorphism,
47.
EARTH, constituents of, 54-6.
definition of, 53.
pillars, 9.
Earthenware, 314.
Earthquakes, 30.
Earthwork, use of geology for, 1.
Earthy structure of rocks, 201.
Ecca beds, S. Africa, 152, 179.
Echinodermata, 155, 172, 184.
Echinoidea, 156, 174.
Ecuador, 283.
Edentates, 161.
Edible clay, 24.
Edinburgh, 391.
Effervescence of rocks, 202, 225.
Egypt, 113, 171, 181, 188, 281, 347.
Elseolite, 89.
Elasmobranchii, 161, 169, 172, 174,
175, 179, 182.
Elba, 278.
Electrical properties of minerals, 62.
Elements, 52, 54.
Elephant beds, 138, 170.
Elevation and subsidence of land, 29,
30.
Elk Mountains, 145.
Elvan, 108.
Embanking for land reclamation,
368-9.
Encrinal marble, 294.
Encrinites, 155, 177, 179, 182.
Encrinus liliiformis, 155, 176.
Encroachment of sea, 382.
Endogens, 162.
England, 332, 385, 394.
Enon conglomerate, S. Africa, 152.
Enstatite, 72, 75.
Eocene formations, 139, 169, 391.
Eozoic, 138, 186-8.
Eozoon Canadense, 187.
INDEX.
405
Epidote, 69, 75.
Epigene action, 34.
Equipment for outdoor work, 190-1.
Equisetites, 162.
Equisetum, 162, 176, 179.
Eriboll, 186.
Erosion by glaciers, 19.
by running water, 12-4, 358.
by sea, 21, 384.
Erratic blocks, 19, 138, 144, 166,
167.
Eruptions, volcanic, 27-8. «
Erzgebirge, 275, 278.
Eskers, 138, 166.
Essex, 169.
Estheria, 156, 176.
Esthonia, 183, 185.
Estuaries, 362, 368, 387.
Euganean hills, 283.
Eurite, 108.
European granites, 278.
syenites, 278.
Eurypterids, 156, 177, 179, 181.
Eurypterus, 156, 184.
Evaporation, 241-3.
Examination of rocks, see Rocks.
Exogens, 162.
External form of minerals, 218.
Extrusive rocks, 34.
Eye-structure of rocks, 103.
FALMOUTH, 276.
False bedding, 38.
Fans, alluvial, 15.
Faroe Islands, 281, 283.
Fascines, in side slopes, 337.
Fat limes, 318.
Fault-line, 44, 45.
-plane, 44.
-springs, 257-8.
Faults, 44, 199.
Fauna, 137, 153-62.
Favosites, 155, 181.
Feel of rocks, 104, 202.
Felixstowe, 373.
Felsite, 110, 205, 279.
Felsitic matter, 99.
Felspar porphyry, 109, 110.
Felspars, 30, 69, 76-8, 106, 128.
Felspathic composition of rocks, 96,
290.
mud, 305.
Felstones, 109, 200, 279.
Fenestella, 158, 177.
shales, Indian Empire, 149.
Fens, 121.
Ferns, 162, 174, 177, 181, 183.
Ferric oxide, 81.
Ferromagnesian mica, 87.
Ferrosoferric oxide, 81.
Ferrous carbonates, 83.
oxide, 81.
sulphide, 84.
Ferruginous rocks, 122, 290.
Fetid odour, 57.
Ffestiniog slates, 300.
Fibrous structure of minerals, 64.
Fichtelgebirge, 278.
Field-book for road making, 334.
Fife, 332.
Filiform shape of minerals, 65.
Finland, 167, 187, 279, 392.
Fire-bricks, 309.
-clay, 129, 205, 309.
-marble, 294.
-stone, 293, 310.
Firn, 19.
Fishes, 160, 175, 177, 181, 182.
Fissure eruptions, 28.
Flaghill beds, New Zealand, 151.
Flags, 299, 301.
Flagstone, 114, 180, 291, 292.
Flamborough Head, 172, 371, 384,
385.
Flame-colouration, 229, 230.
Flaser-gabbro, 127.
-gneiss, 127.
Fleetwood, 388.
Flexure, 42.
Flint, 65, 91, 119, 204, 344, 345, 351.
Flintshire, 332.
Floating bricks, 310.
Flora, 137, 153, 162.
Florence, 129.
Flow of streams and rivers, 265.
of surface water, 252-3.
of water in rivers, 263.
Fluidal gneissic structure, 99, 125
structure of rocks, 101, 103.
Fluor-apatite, 71.
Fluorescence, 62.
Fluorides, 69.
Fluorine, 54, 55.
Fluor-spar, 63, 69, 79, 220.
Fluviatile deposits, 163.
Fluvioglacial deposits, 166.
Fluviomarine formations, 165.
Folds, 31, 42.
Foliaceous structure of minerals, 64.
Foliated rocks, 103, 124, 201.
Foliation, 47, 49, 103.
Folkestone beds, 293.
406
GEOLOGY FOR ENGINEERS.
Foraminifera, 23, 25, 153, 169, 172.
Forced waves, 375.
Forceps, for blowpipe work, 227.
Forces, internal, 26.
Foreign bodies in clays, 312.
Forelands, the, 371, 387.
Forest bed, 138, 169.
marble, 140.
of Dean, 180, 395.
Forests, 24, 29, 165, 253.
Forfarshire flags, 142.
Formations, geological, 136-7.
Formosa, 391.
Forms of bedding, 38.
Forth, River, 391.
Fossils, 136, 152, 169, 172, 174, 175,
177, 179, 184, 187.
Fouque's method, 214-5.
Foxhills group, N. America, 145.
Foyaite, 108.
Fracture of minerals, 62, 65, 224.
of rocks, 104, 202.
Fragmental rocks, 101, 112, 201.
France, 131, 170, 172, 174, 178, 180,
183, 185, 278, 279, 281, 311, 391,
394, 395.
Frangibility of minerals, 65.
Freestone, 114, 133.
Free waves, 374.
French chalk, 89.
Freshwater portion of tidal river,
362-3.
Frost, 17,
Fucoid beds, 186.
Fucoidal greensands, New Zealand,
151.
Fuller's earth, 129, 140.
Fumaroles, 28.
Fungi, 162.
Fungia, 154.
Fusibility, blowpipe, 230-1, 232.
of bricks, 314.
of rocks, 105.
scale of, 231.
Fusion-place, 229.
Fusulina, 180.
Fusus contrarius, 169.
GABBRO, 109, 203, 281.
Gaj series, Indian Empire, 148.
Gala group, 183.
Galapagos, 283.
Galashiels, 281.
Galena, 67, 69, 79, 220.
Galeosaurus, 175.
Galicia, 278.
Galway, 183, 187, 278, 281.
Gannister, 141, 292, 310.
Ganoid, 160, 181.
Ganoidei, 161, 172, 174, 175.
Garlic odour, 57.
Garnet, 63, 69, 79.
schist, 125.
Gaspe, 182.
Gasteropoda, 160, 169, 176, 177, 184.
Gault, 140, 307, 330.
Geneva, 270, 302.
Geode, 65.
Geological age of granite, 274.
distribution of limestones, 329.
features in reservoirs, 271.
formations, 136-7.
observation, 189-236.
plan, 192.
section, 192, 194.
surveying, 191-4.
Geology, definition of, 1.
practical uses of, 1.
Georgia, 147, 392.
Geotectonic geology, 3.
German Ocean, 372.
Germany, 7, 167, 170, 174, 176, 178,
180, 278, 281, 347, 391, 394,
395.
Geromagny, 279.
Gervillia, 176.
Giallo-antico, 294.
Giant's Causeway, 99, 111.
Girvan district, 184, 185.
Glacial agencies, 17-20.
deposits, 138, 165-8.
period, N. America, 144.
Glaciers, 18, 19, 20.
Glamorgan, 828.
Glassy rocks, 100.
state of minerals, 1.
Glatz, 278.
Glauconite, 23, 69, 80, 298.
Glenkiln shales, 185.
Globigerina, 23. 153.
Globular shape of minerals, 65.
Glossopteris, 174, 177, 178, 179.
beds, New Zealand, 151.
Gneiss, 125, 206, 279, 351.
decomposition of, 130.
Gneissoid rocks, 47.
Gneissose granite, 107.
Godavery alluvium, Indian Empire,
148.
Gold, 233, 392, 396.
Gondwana system, Indian Empire,
148, 175, 177.
INDEX.
407
Goniatites, 160, 181, 182.
Goniopholis, 161.
Gordon river beds, Australia, 150.
Gothland, 149, 183.
Gower, 258.
Gradation, 20.
Grampians, 278.
Granada, 392.
Granite, decomposition of, 130.
jointing of, 37.
gneiss, 125.
porphyry, 108.
Granites, 35, 36, 37, 106, 203, 273-9,
343, 344, 351.
Granitite, 107.
Granitoid rocks, 273-9.
Granular structure of minerals, 65.
of rocks, 98, 201.
Granulite, 103, 108, 127.
Graphic granite, 107, 131.
structure of rocks, 99.
Graphite, 24, 65, 67, 69, 80, 392.
Graptolites, 115, 177, 183, 184,
185.
Gravels, 138, 144, 246, 345, 351.
Great Britain, formations, 138-43,
166-7, 169, 172, 176, 178, 179,
181, 183, 184, 187, 382.
table of strata, 138.
Greece, 280, 281.
Greenland, 173, 283, 382, 391, 392.
Green rocks, 133.
Greensand, 140, 248, 250, 293, 310,
330.
Greenstone, 281.
Grey oxide of manganese, 86.
Greywacke, 114, 180, 344.
Grinshill sandstone, 290.
Grit, 114, 289, 345.
Ground-mass, 99.
Groups of igneous rocks, 96.
Groynes, 388-90.
Gryphsea, 158.
Guadarrama Mountains, 278.
Guano, 71, 80, 121.
Guanascuato, 392.
Guernsey, 131, 277.
Guiana, 188.
Guinea, 392.
Gulf Stream, 22.
Gully, formation of, 13.
Gwaliors, Indian Empire, 149.
Gymnosperms, 162, 179.
Gypseous composition of rocks, 96.
Gypsum, 64, 69, 80, 119, 203, 204,
220 328, 393.
HACKLY surface of minerals, 65.
Hade, 44.
Haematite, 30, 36, 69, 81, 82, 231.
Haimantas series, Indian Empire,
149.
Hamilton period, N. America, 147,
182.
Karaites, 160.
Hammer, 190.
Hampshire basin, 134, 139, 169, 170.
Hanover, 167, 172.
Hard manganese ore, 86.
Hardness, Moh's scale of, 66.
of minerals, 62, 66, 220-1.
rough scale of, 220.
of rocks, 104, 201, 207.
Harlech series, 143, 186.
Hartfell shales, 185.
Hartshill quartzite, 186.
Harwich, 327.
Harz, 275, 278, 392.
Hassock, 293.
Hastings sands, 293.
Hawick, 281.
Hawkesbury series, Australia, 150.
Headlands, 371-2.
Headon beds, 139, 170.
Heat, changes in rocks due to, 31.
internal, 26.
Heavy spar, 64, 73, 81, 395.
Hebrides, 283.
Heddon sandstone, 290.
Heliolites, 155, 181.
Hemicrystalline structure, 98, 100.
Hemihedral, 62.
Hempstead beds, 139, 170.
Hensborough, 276.
Herm, 277.
Heterocercal, 160, 181.
Hexacoralla, 155.
Hexagonal system of crystals, 61.
Himalayas, 168, 170, 173, 175, 176,
184, 187.
Hipparion, 169.
Hippurite limestones, 173.
Hippurites, 158, 172.
Hirnant, 184.
Holderness, 384, 385.
Holland, 5, 167, 382, 389.
Hollybush sandstone, 186.
Holocrystalline rocks, 98, 99.
Holohedral, 62.
Holoptychius, 181.
Holothuria, 156.
Homalonotus, 183.
Homocercal, 160.
408
GEOLOGY FOR ENGINEERS.
Hornblende, 63, 72, 81, 107, 393.
andesite, 110.
basalt, 111.
gabbro, 109.
granite, 107.
rock, 205.
schist, 125, 127.
syenite, 108.
Hornsea, 385.
Hornstone, 91, 204.
Horny structure, 100.
Horse-radish odour, 57.
Hove, 387.
Hsipaw series, Indian Empire, 148.
Hudson River, 147, 185.
Human relics, 163.
Humber, 385.
Humus, 24, 121.
Hungary, 171, 175, 279, 283, 287,
392.
Hunstanton, neocomian sandstone,
302.
Huronian series, 147, 149, 187.
Hyalite, 81, 91.
Hybodus, 161, 175.
Hydrates, 56.
Hydration, 8, 30.
Hydraulic limes and cements, 318,
321-3, 325, 330.
limestone, Kimmeridge, 302.
Hydrocarbons, 69.
Hydrochloric acid, 202, 224, 225.
Hydrofluoric acid, 217.
Hydrogen, 54, 55.
Hydroida, 185.
Hydro-metamorphism, 46.
Hydrous oxides, 69.
silicates, 69.
Hydrozoa, 154, 183.
Hyperbyssal, 95.
Hypersthene, 72, 81, 109.
Hypogene action, 3.
Hythe beds, 293, 298.
ICE, 36.
Icebergs, 18.
Iceland, 29, 283, 287.
Ice-sheets, 17.
Ichthyodorulites, 181.
Ichthyosauria, 161, 172, 174, 175.
Idria, 392.
Igneous rocks, 33-7, 94, 96, 98-101,
106-112, 128-132.
Iguanodon, 161, 172.
Ilfracombe group, 142.
Illinois, 146.
Ilmenau, 107.
Ilmenite, 69, 81, 82.
Imbibition, 244-5.
Impengati beds, S. Africa, 152.
Impounding reservoirs, 267, 271.
Inclination of rocks, 40.
India, 14, 131, 148-9, 168, 171, 173,
175, 178, 185, 186, 269, 281,
318, 391, 394.
Indications of nature of rocks, 193.
Indoor work, 207-36.
Indurated talc, 127.
Induration of clays, 314.
Indus, 265.
Infusorial earth, 310.
Inlets on coast, 372.
Inlier, 41.
Insecta, 157, 182.
Insectivores, 161.
Intensity of lustre, 68.
Interbedded lava-sheet, 34.
Intercepting silt, 369.
Intermediate group of rocks, 96, 97.
limes, 318.
Internal forces, 26.
Interposed strata, 38.
Intrusive rocks, 35.
Inversions, 42.
Invertebrata, 153.
Iowa, 146.
Ireland, 107, 166, 167. 172, 176,
178, 180, 182, 183, 184, 187,
278, 280, 282, 332, 391.
Iron, 54, 56, 69, 81-4, 233, 392, 395,
396.
pyrites, 69, 84, 290, 300, 312,
392.
Ironstones, 122.
Irrawaddy series, Indian Empire,
148.
Irregular grouping of crystals, 62.
Isle of Man, 395.
Sheppey, 327.
Wight, 371.
Isolation of constituents, 212-8.
Italy, 131, 278, 283, 287, 311, 394.
JADE, 73, 85.
Japan, 173, 176, 180, 392.
Jasper, 204.
Java, 287.
Jersey, 131, 277.
Jet coal, 391.
Jetties for intercepting silt, 369.
Joint action of waves and currents,
380.
INDEX.
409
Joints of altered and metamorphic
rocks, 49.
of aqueous rocks, 43.
of igneous rocks, 36, 37.
Jolly's balance, 215, 223.
Jubbulpore series, Indian Empire,
148.
Junction of rivers with the sea, 364.
Jura, 175, 347, 348.
Jurassic system, 137, 140, 174-5,
293, 297.
KAIHIKU beds, New Zealand, 151.
Kakberg, S. Africa, 168.
Kames, 138, 166.
Kansas, 145, 146.
Kaolin, 30, 69, 85, 128-9, 305, 309.
Karoo series, S. Africa, 152, 176, 179.
Kasauli series, Indian Empire, 148.
Katadgis series, Indian Empire, 148.
Katrol series, Indian Empire, 148.
Keeweenawan series, 187.
Kellaway's rock, 140.
Kendal, 281.
Kent, 310.
Kentish rag, 298.
Kenton sandstone, 290.
Kentucky, 146.
Kereru beds, New Zealand, 151.
Kerry, 183.
Keswick, 281.
Ketton limestone, 297, 303.
Keuperbeds, 141, 176, 292, 391.
Kienitz, 278.
Kilkenny, 294, 296, 391.
Killaloe slates, 301.
Kilns and fuel for lime, 323.
Kimberley slates, S. Africa, 152.
Kimmeridge clay, 132, 133, 140, 330.
Kinder Scout, 6.
Kirkby moor flags, 183.
Kirthar series, Indian Empire, 148.
Klein's solution of borotungstate of
cadmium, 215.
Koenigsberg, 5.
Kremnitz, 283.
Kudernatch, 278.
Kurnool series, Indian Empire, 148.
LABRADORITE, 78, 85.
Labuan, 391.
Labyrinthodonts, 161, 175, 177.
Laccolites, 35.
Lacustrine deposits, 163, 164.
Lagoons, 22.
Lake deposits, 182.
Lake District, 184, 185, 277, 280.
Superior, 392.
Lakes, 16, 267, 270.
Lamellar structure of minerals, 64.
Lamellibranchiata, 158, 169, 174,
176, 185.
Lameta series, Indian Empire, 148.
Laminae, 38.
Laminated structure, 102.
Lamorna, 131.
Lamps for blowpipe work, 226.
Lanark, 311, 332.
Lancashire, 165, 166, 275, 310, 391,
394.
Land, elevation and subsidence of,
29, 30.
reclamation, 368-9.
Land's End, 130, 276.
Landslips, 11.
Lapilli, 27, 34.
Lapis-lazuli, 395, 396.
Lapis ollaris, 127.
Laramie series, N. America, 145, 173.
Laterite, 132.
Laurentian, 147, 149, 392.
period, N. America, 187.
Lava, 27, 34, 132, 283.
Laying out new roads, 333-4.
Lead, 230, 232, 233, 392, 395.
Leakage in canals, 352.
Lebanon, 168.
Leicestershire, 187, 275, 276, 328,
331.
Leinster, 273, 274, 278.
Leipsic, 167.
Lemnian, 394.
Lens, 227.
Lenticular, 38.
Lepidolite, 85, 88.
Lepidotosaurus, 177.
Leucite, 69, 85.
Lewisian series, 143, 187.
Lias lime, 330.
Liassic series, 132, 133, 141, 174, 248,
293, 297, 307, 330, 331, 332,
392.
Libyan desert, 173.
Liege, 391.
Life of road stone, 351.
Lightning, effect of, 5.
Lignite, 121, 391, 392.
Lime felspar, 76, 78.
Lime, hydraulic, 297, 318.
kilns, 323.
Limes, 319-21.
and limestones, testing, 325-6.
410
GEOLOGY FOR ENGINEERS.
Limes, cements, and plasters, 318-
32.
Limestone, 116, 205, 287, 293-9,
322-3, 325-6, 343, 345, 351, 395.
Limnaea, 160, 164, 169.
Limoges, 107.
Limonite, 30, 60, 83, 205, 225.
Lincolnshire, 165, 166.
Line of descent for mountain roads,
340-1.
of saturation, 245, 253-4, 255-6.
Lingula, 158.
flags, 143, 186.
Linlithgow, 391.
Lipari Islands, 283.
Liparite, 109.
Lithia mica, 85, 88.
Lithium, 54, 230.
Lithographic limestone, Solenhofen,
302.
Lithoidal rocks, 98, 100.
Lithological character of rocks, 289,
293, 299.
characters of formation, 136.
Lithology, 51.
Lithomarge, 65, 85.
Lithophyse structure, 101.
Lithostrotion, 154.
Littoral deposits, 23.
drift, 383, 387-8.
Liverworts, 162.
Lizard, The, 187.
Lizards, 161, 175, 177.
Llanberis slates, 300.
Llandeilo flags, 143, 184, 185, 300.
Llandovery group, 143, 183.
Llangollen slates, 300.
Loam, 115, 306, 312.
Loess, 7, 138, 144.
Lombardy, 168.
London basin, Eocene, Great Britain,
139, 169.
clay, 133, 139, 307, 327, 332.
Londonderry, 183.
Long clays, 308.
Longmynd group, 143, 187.
Lophiodon, 171.
Lothians, 332.
Lower Helderberg period, N. America,
147, 184.
Silurian system, 184.
Lowestoft, 387.
Lubricating material, 394.
Ludlow beds, 143, 183.
Lumachello, 294.
Lustre of minerals, 67, 68.
Lustre of rocks, 104-5.
Lycopods, 162, 177, 179, 181, 183.
Lydian-stone, 47, 91, 123.
Lyme Regis, 174, 332.
Lynton group, 142.
Lyons, 168, 347.
MACRODIAGONAL, 60.
Madeira, 283.
Madras, 187.
Madrepora, 155.
Madrepore marble, 181.
Magma, 99.
Magnesian limestone, 118, 141, 178,
247, 248, 296, 297, 303, 332,
351.
Magnesite, 85.
Magnesium, 54, 56, 230, 233.
cements of America, 328.
Magnet, in blowpipe work, 227.
Magnetic iron ore, 81.
separation of rock constituents,
214.
Magnetism of rocks, 105.
Magnetite, 30, 67, 69, 81.
Mahadeva series, Indian Empire,
148.
Maitau series, New Zealand, 151.
Makran series, Indian Empire, 148.
Malleability of minerals, 66.
Malmesbury beds, South Africa, 152.
Maltha, 72.
Malvern Hills, 182, 186, 187, 275.
Mammals, 161, 169, 175, 176.
Mammillary shape of minerals, 65.
Manatee, 16.
Manchester waterworks, 271.
Manchhar series, Indian Empire,
148.
Mandelato, 294.
Manganese, 23, 54, 56, 69, 85, 234,
395, 396.
Manganite, 86.
Mansfield dolomite, 297.
sandstone, 178, 290, 292, 303.
Maps, 192-4.
Marathon, 280.
Marbles, 47, 124, 294-5.
Marcasite, 69, 84.
Margapakeka beds, New Zealand,
151.
Marine action, 20-23.
beds, Australia, 150.
denudation, 20, 22.
deposits, 163.
terraces, 138.
INDEX.
411
Marl, 116, 141, 306.
-slate, 306.
Marsipobranchii, 161.
Marsouin, 185.
Marsupials, 161, 169.
Massive minerals, 58.
rocks, 33, 98.
Master joints, 44.
Mastodon, 162, 169.
Mastodonsaurus, 161, 175.
Mataura series, New Zealand, 151,
Material transported by water, 15,
358, 359, 361, 380.
Maymyo limestone. Indian Empire,
149.
Mayo, 185, 278, 282.
May hill sandstone, 143.
Meagre feel of rocks, 202.
touch of minerals, 66.
Mechanical analysis of rocks, 213.
Medina, 147, 184.
series, N". America, 184.
Mediterranean, 170, 172, 173.
Medium limes, 321, 323.
Medway mud, 307.
Megalosaurus, 161, 174.
Megatherium, 161.
Meissen, 278.
Melaphyre, 129.
Melbourne, 243, 283.
Mendip Hills, 180.
Menevian beds, 143, 186.
Mercury, 392, 395.
Mer de glace, 1 9.
Merioneth, 184, 186.
Mersey, River, 165, 358.
Mesozoic, 138, 140, 171-7.
Metamorphic rocks, 45-50.
Metamorphism, 31.
Metal, definition of, 53.
Metallic pigments, 395.
Metalloid, 54.
Metals, occurrence of, 56.
Methods of drainage for roads, 337-8.
Methylene iodide, 216.
Mexico, 29, 391, 392.
Miascite, 108.
Mica, 394.
andesite, 110.
schist, 124, 125, 126, 206, 344.
slate, 124.
syenite, 108.
trap, 281.
Micaceous composition of rocks, 96 ..
haematite, 80, 82.
iron ore, 89.
Micaceous rocks, 290.
Micas and talcs, 64, 65, 69, 86-9,
107, 202, 220.
Michigan, 146.
Microcline, 77, 89.
Microcosmic salt, 227, 235.
Microcrystalline structure of rocks,
98, 100.
Microlestes, 176.
Microphytal earths, 24, 310.
Midford sands, 140.
Miliola, 153.
Millstone grit, 6, 141, 180, 271, 289,
291, 292, 302.
Minas Geraes, 392.
Mineral chemistry, 52-7.
constituents of rocks, 97.
forms, 57-62.
pigments, 394.
vein, 45-6.
Minerals, 51-93.
definition of, 51-2.
distinguishing characters of, 51-2.
distribution of, 391-3.
external form of, 218.
extraction of, 218.
mode of occurrence, 218.
physical characters of, 62-8.
rock- forming, 69-93.
study of, 52-68.
uses of, 391-6.
Minette, 281.
Minor features of coast-lines, 371.
Miocene formations, 139, 169, 391.
Miohippus, 169, 171.
Mississippi, 16, 144, 146, 173, 180,
360.
Missouri, 145, 146.
Moa beds, New Zealand, 151.
Modern era, N". America, 144.
Modified forms of crystals, 62.
Moffat series, 185.
Moh's scale of hardness, 66.
Mohr's displacement apparatus, 207,
222.
Molasse of Switzerland, 171.
Moldavia, 278.
Mole, 161.
Mollusca, 158, 187.
Molluscoida, 157, 187.
Molybdenite, 80.
Molybdenum, 230.
Monad, 54.
Monoclinal, 42.
Monoclinic system of crystals, 60.
Monocotyledons, 162, 169.
412
GEOLOGY FOR ENGINEERS.
Monograptus, 154.
Monometric system of crystals, 59.
Monoxide, 53.
Mont Blanc, 278.
Montmorency, 144.
Monzoni, 279.
Moraines, 19, 138
Moravia, 278, 283.
Moray Firth, 176.
Mortar, 293, 295.
and pestle, 227.
Morven, 172.
Mosasaurus, 161, 172.
Moscow, 167.
Mosses, 162.
Motion of water in rivers, 354.
Mount Arthur series, New Zealand,
151.
Potts series, New Zealand, 151.
Sorrel, 276.
Mountain limestone, 137, 332, 334.
meal, 24, 310.
passes, 339.
roads, 338-41.
Mourne Mountains, 278.
Movements of land, 29-30.
Mud springs, 29.
volcanoes, 29, 347.
Mudstone, 116, 150, 205, 305.
Mull, 172..
Muriatic acid, 325.
Murray River beds, Australia, 150.
Murrumbidgee beds, Australia, 150.
Muschelkalk, 176.
Muscovite, 87, 89.
granite, 107.
Mylonitic structure of rocks, 103,
127.
Myophoria, 176.
Myriapoda, 157.
Mytilus, 158.
NACREOUS lustre, 68.
Nagel flue of Switzerland, 171.
Nairn, 7.
Namaqualand schists, S. Africa, 152.
Nantes, 278.
Naphtha, 72, 89.
Narbada, Indian Empire, 148, 173,
179.
Nari series, Indian Empire, 148.
Natal, 181. *
Native elements, 69.
Natrolite, 93, 231.
Nautilus, 160, 176, 181.
Necks, 35.
Needles, the, 371.
Needwood, 174.
Neobolus bed, Indian Empire, 149.
Neocomian, 172.
beds, 140, 307.
Neolithic, 164.
Neozoic, 138.
Nepal, 168.
Nepheline, 68, 69, 89.
syenite, 110.
trachyte, 110.
Nephrite, 73, 85, 89.
Nerbudda, see Narbada.
Neuropteris, 162.
Nevada, 175, 176, 392.
Nevadite, 109.
Neve, 19.
New Brunswick, 182.
Caledonia, 176.
Jersey, 173.
Red Sandstone, 177, 246, 248.
South Wales, 171, 175, 179, 181.
Zealand, 151, 164, 168, 171, 173,
176, 185, 188, 283, 392.
Newcastle, 391.
beds, Australia, 150.
Newfoundland, 186.
Niagara Period, N. America, 147,
184.
Nicaragua, 392.
Nickel, 233.
Nile, 265.
Nineveh, 6.
Niobrara group, N. America, 145.
Nith, 178.
Nitrate of baryta, 393.
of cobalt, as reagent, 227.
Nitric acid, 202, 224, 225.
Non-glacial deposits, 164.
-littoral deposits, 23.
Norfolk, 166.
Norite, 109, 149.
Normal constituents of brick clay,
313.
Norman's Kill, 185.
North America, 87, 144, 164, 168,
171, 173, 175, 178, 180, 182,
183, 186, 187, 281, 391, 392,
394.
Northampton, 392.
Northumberland, 79, 178, 332, 387.
Norway, 82, 182, 183, 275, 279, 281,
392.
Norwich Crag, 138, 170.
Note-book, 191.
Notosaurus, 175.
INDEX.
413
Nottinghamshire, 178, 393.
Nova Scotia, 180, 392.
Novja Zemlja, 382.
Nullipores, 162.
Nummulites, 153, 169.
Nummulitic beds, New Zealand, 151.
limestone, 171, 299.
Nuneaton, 186.
OAMARA beds, New Zealand, 151.
Oberlausitz, 278.
Oblique system of crystals, 60.
waves, 377-8.
Obsidian, 109, 206.
Ocean currents, 22,
Oceanic deposits, 23.
movements, 22.
Ochre, 83, 89, 394. 395.
Octocoralla, 155.
Odenwald, 278.
Odontopteryx, 169.
Odour of minerals, 57, 224, 232.
Ogygia, 157, 184.
Ohio, 180.
Oil shales, 306.
Old Red Sandstone, 136, 137, 142,
181, 291, 295.
Olenellus, 156, 185, 186.
zone, Indian Empire, 149.
Olenus, 156, 184, 185.
Oligocene formations, 139, 169.
Oligoclase, 78, 89.
Olivine, 69, 89.
-basalt, 111.
-gabbro, 109.
Oneida, 147, 184.
series, N. America, 184.
Onondaga, 147, 184.
Oolite, 117, 287, 293, 294, 297, 303.
and Liassic period, N. America, 145.
series, 140, 174, 248, 307, 310,
391, 392.
Oolitic structure of rocks, 102.
Opal, 65, 90, 91.
Open tube, 227, 232.
Ophite, decomposition of, 131.
Ophitic structure of rocks, 100.
Ophiuroidea, 156.
Orange River Colony, 181.
Orbicular structure of rocks, 100.
Orbitoides limestone, 144.
Orbulina, 23.
Ordinary springs, 253-4.
Ordovician system, 138, 184-5.
Oregon, 145.
Oreti series, New Zealand, 151.
Organic acids, 226.
action, 24-5.
Oriskany period, N. America, 147,
182.
Ornithosauria, 161.
Orohippus, 169.
Orthis, 158.
Orthoceras, 160, 176, 177, 181, 182.
Orthoclase, 77, 90, 218, 231.
•porphyry, 108, 109.
thodiagonal, 60.
Or
Orthophyre, 109.
Osborne beds, 139, 170.
Ossiferous caves, 10, 138, 150.
Ostracodermi, 156, 161, 181, 182.
Ostrea, 158.
Otapiri series, New Zealand, 151.
Otatara stone, New Zealand, 151.
Oulton Broad, 387.
Outcrop, 40, 198, 248.
Outdoor work, 190-206.
Outlier, 41.
Outline of sea coast, 370.
Overfolds, 42.
Overlap, 41, 198.
Overtaking of waves, 376.
Overthrust, 43.
Oxalic acid, 224, 226.
Oxford clay, 133, 140, 174, 330.
Oxidation, 7.
Oxide, definition of, 53.
Oxidising flame, 229, 235.
Oxygen, 55.
PACIFIC ocean, 176.
Paint, 393.
Pakhalis series, Indian Empire, 149.
Palaeolithic, 164.
Palseoniscus, 175, 177.
Palaeontology, 152.
Palaeospondylus, 181.
Palseotherium, 162, 169.
Paleozoic, 138, 141, 177.
Palagonite, 132.
Palatinate, 392.
Palestine, 171.
Palisade area, N. America, 145, 176,
177.
Paludina, 160, 164, 169.
Panchet series, Indian Empire, 148,
177.
Paradoxides, 156, 185.
Paraffin, 24.
Paragonite, 126.
Parameter, 59.
Pareora beds, New Zealand, 151.
414
GEOLOGY FOR ENGINEERS.
Parian cements, 329.
marble, 294.
Pariasaurus, 176.
Paris, 174.
Parker's cement, 327.
Parthenay, 278.
Particles of matter, motion of, 359.
of water, motion of, 356.
Pasco, 392.
Patagonia, 168.
Patcham series, Indian Empire, 148.
Pau, 132.
Paving material, 347-8.
stones, 291, 293.
Pea grit, 117.
Pearl spar, 75, 90.
Peat, 121.
bogs, 138.
mosses, 24.
Pebbly structure of rocks, 102.
Pebidian series, 143, 187.
Pecopteris, 174, 176.
Pegmatic structure of rocks, 99.
Pegmatite, 107.
Pelagic deposits, 23.
Penarth beds, 141.
Penganga series, Indian Empire, 149.
Penmaenmawr, 184, 343.
Pennant grit, 292.
Pennines, 89, 90, 178, 180.
Pennsylvania, 180, 391.
Penrhyn, 131, 300.
Pentacrinus, 155.
Pentamerus, 143, 183, 184.
Perched blocks, 19.
Percolation on shores, 376.
Periclinal dip, 42.
Peridotite, 111, 124.
Periods and systems, 137.
Perlitic structure of rocks, 100.
Perm, 178.
Permian period, N. America, 145.
system, 137, 141, 177, 292, 296.
Peroxide, 53.
Persia, 171, 173, 178.
Persian gulf, 29.
Peru, 29, 175, 176, 392.
Pervious between impervious beds,
254.
on impervious strata, springs, 253.
Petrifaction, 152.
Petrifying springs, 11.
Petrography, 51.
Petroleum, 72, 90, 182, 347.
Petrology, 51.
Phacops, 157, 183
Phanerogams, 162.
Philippines, 391.
Phlogopite, 87, 90.
Phonolite, 36, 110, 205, 287.
Phosphate of lime, 70.
Phosphates, 69.
Phosphatic rocks, 120, 144.
Phosphatite, 120.
Phosphorite, 71, 90, 206.
Phosphorus, 54, 56, 230.
Phyllite, 124.
Phyllopods, 156, 179.
Physical causes of sea encroachment,
382.
characters of minerals, 62, 219-24.
of rocks, 104.
tests for road stones, 348-9.
Picrite, 124.
Piedmont, 168, 392.
Pier works, effect of, 386-7.
Pierre group, N. America, 145.
Pigments, 394-6.
Pillau, near Koenigsberg, 5.
Pilton group, 142.
Pisa, 393.
Pisolite, 117, 298.
Pisolitic structure of rocks, 102.
Pitchstone, 110, 206.
Place bricks, 313.
Placodus, 175.
Placoid, 160.
Plagioclase, 77.
Planorbis, 160, 164, 169.
Plants and trees, 24, 121, 162.
Plasters, 328-9, 393.
Plastic theory of glacier movements,
19.
Plate, River, 306.
Platinum, 227, 392.
Platysomus, 177.
Pleistocene deposits, 138, 164.
Pleochroism/ 62.
Plesiosauria,' 161, 172, 174.
Plication, 31, 42.
Pliocene formations, 138, 169.
Plumbago, 80, 90, 392.
Plunge of rivers, 376.
Plutonic action, 3.
rocks, 94, 106-9.
Pocket lens, 191.
Point Levis beds, N. America, 185.
Polarisation, 62.
Polishing slate, 24, 310.
Polyzoa, 157, 169, 172, 179.
Pondicherry beds, 173.
Poole clay, 307.
INDEX.
415
Pools in rivers, 358.
Poor limes, 320, 323.
Porbandar stone, Indian Empire,
148.
Porcelain, 314.
Porcellanite, 47, 123.
Porosity of rocks, 249.
Porphy rites, 111, 205.
Porphyritic structure of rocks, 99,
273.
Porphyry, 35, 108, 204, 279, 351.
Porpoises, 161.
Port Elizabeth, 243.
Portland beds, 120, 140, 174, 298,
303.
cement, 307, 328.
Post-glacial deposits, 138, 163.
Post-tertiary period, 137.
Potash felspar, 76, 77.
mica, 87, 90.
Potassium, 54, 56, 230, 396.
Pot holes, 13.
Potomac, 173.
Potosi, 392.
Potsdam epoch, 147.
sandstone, 186, 248.
Potstone, 127.
Pozzuolana, 321.
Practical geology, 237.
uses of geology, 1.
Pre-Cambrian period, 137, 143,
186.
Predazzo, 279.
Prehistoric ages. 164.
Prehnite, 93.
Preliminary examination of forms of
minerals, 218.
traverse, 193.
Pressure, 26.
changes in rocks due to, 31.
resistance to, 294.
Prevention of erosion, 384.
Primates, 162, 169.
Primordial period, N. America, 147.
Proboscideans, 162.
Productus, 158, 177, 182.
shales, Indian Empire, 148.
Prome series, Indian Empire, 148.
Proportion of deposit carried, 360.
Protective works on coast, 384-7.
Proterosaurus, 161, 177.
Protogenic granite, 107.
Protozoa, 153, 179, 187.
Protozoic, 134.
Prussia, 172, 391.
Psammitic structure of rocks, 102.
Pseudomorphism, 62.
Psilomelane, 69, 86, 90.
Psilophyton, 181.
Pteraspis, 161, 182.
Pteridophyta, 162.
Pterodactylus, 161, 172, 174.
Pteropoda, 23, 185.
Pterosauria, 161.
Pterygotus, 156, 181.
| Ptychodus, 172.
Puddingstone, 114.
Puddled clay, 352.
Puddler's ore, 82, 90.
Pulverulent, 65.
Purniceous structure of rocks, 98,
101.
Punjab, 175, 391.
Purana group, Indian Empire, 149.
Purbeck beds, 120, 140, 174, 298.
Pututaka beds, New Zealand, 151.
Pyrenees, 107, 131, 173, 275, 278.
Pyrites, 84, 90, 306, 392.
Pyritous composition of rocks, 96.
Pyroclastic sediments, 34, 111,
112.
Pyrolusite, 86, 90, 225.
Pyroxene, 72.
andesites, 110.
diorites, 109.
QUADER sandstone, 172.
Qualities of rocks, 294, 300, 307.
Quantity of water derivable from
wells, 260.
of water in rivers, etc. , 263.
of water in wells, 262.
Quantivalence, 54.
Qua-qua- versa! dip, 42.
Quarry, selection of, 301.
Quartz, 36, 46, 63, 64, 68, 69, 90,
106, 220, 351.
Quartz-andesite, 111.
diorite, 109.
Quartzite, 46, 91, 114, 123, 204, 302,
344, 351.
Quartzose composition of rocks, 96,
290.
sands, origin of, 130.
Quartz porphyry, 108, 279.
Quartz-trachyte, 109.
Quaternary period, 137, 138, 163.
Quebec, 147, 185.
Queen Charlote Group, 173.
Queensland, 173, 175, 179.
Quicklime, 319.
Quicksilver, 392.
416
GEOLOGY FOR ENGINEERS.
RADIATED and divergent structure of
minerals, 64.
Radicles, 54.
compound, 52.
organic, 54.
Radiolaria, 153.
Radiolites, 348.
Ragstone, 298, 344.
Rain, 7, 8, 12, 238-40.
Rainfall and evaporation, 238-43,
267-9.
Rainfall, effect of, on underground
water, 10.
Rain wash, 9.
Raised beaches, 138, 165, 382.
Rajmahal series, Indian Empire, 148,
175.
Ranikot series, Indian Empire, 148.
Rastrites, 154.
Reagents, 227-8.
Recent or post-glacial deposits, 138,
163.
Reconnaissance for new roads, 333.
Red clays, 23.
crag, 133, 138, 170.
Reddle, see Ruddle.
Red ochre, 82, 90.
Red Sea, 280.
Reducing flame, 228.
Reefton beds, New Zealand, 151.
Refraction, 62.
Refractory qualities of clays, 308.
Regelation theory of glacier move-
ments, 19.
Regime of rivers, 363, 364.
Regur, 24.
Remedy for leakage in canals, 353.
Reniform shape of minerals, 65.
Reptilia, 161, 172, 174.
Requisites in a road stone, 348.
Reservoirs, 267, 271.
geological features, 271.
sites, 271.
Resinous lustre, 68.
Retarding force in river, 355.
Reticulated structure of minerals, 64.
Reversed faults, 45.
Rhaetic series. 141, 176.
Rhine, River, 182, 287, 392.
Rhizopoda, 153.
Rhode Island, 146.
Rhombic pyroxene, 72.
system of crystals, 59.
Rhombohedral system of crystals, 61.
Rhone, River, 168, 270, 360.
Rhynchonella, 158, 174.
Rhyncosaurus, 175.
Rhyolite, 109, 283.
Riccarton beds, 183.
"Rich" limes, 320, 323.
Richmond earth, 310.
U.S.A., 144, 391.
Rigidity of minerals, 66.
Rio Janeiro, 131.
Ripidolite, 89, 90.
Rise and fall of sea-water level, 379.
Riverbed, geological formation of, 13,
368.
detritus, 382.
improvement schemes, 368.
plains, 164.
schemes, 264.
terraces, 16, 113, 138, 164, 362.
water, 251.
Rivers, 12, 263-6, 354-69.
flow of, 265.
flow of water in, 263.
motion of water in, 354-8.
quantity of water in, 263.
self purification, 264.
Road construction, 334-8.
cuttings, 334-5.
making, 1, 333-41.
use of knowledge of geology for, 1.
materials, 341-51.
metal, 344.
stone requisites, 348.
Roads and canals, 333-53.
classes of, 341.
laying out, 333-4.
Robschutz, 129.
Roches moutonnees, 1 9.
Rock-bind, 306.
Rock decomposition, 128-34.
definition of, 51, 94.
salt, 69, 90, 119, 203, 393.
specimens, 200.
Rocks, 94-134.
absorbent power of, 290.
analyses of, 290.
changes in, 30-32.
chemical examination, 209-12.
classification of, 94.
composition of, 96-8.
crushing weight of, 290.
determination of, 200-18.
durability of, 290, 297, 298.
extrusive or contemporaneous, 34.
formed by chemical or organic
agencies, 116-22.
fusibility of, 212.
igneous, 33-7, 94-5.
INDEX.
41'
Rocks, inclination of, 40.
indications of nature of, 193.
intrusive or subsequent, 35.
lithological character of, 289, 293,
299.
massive, 33.
mode of origin of, 94-5.
physical characters, 207.
preparation of material for examina-
tion, 209.
qualities of, 294, 300.
relations between igneous, aqueous,
and metamorphic, 49, 50.
specific gravity of, 290, 298.
structural character of, 33-50.
structure of, 98-103, 294.
table of, 203-6.
texture of, 98, 294, 297.
transformation of, 31.
weight of, 290.
Rocky Mountains, 168, 186, 283,
391.
Rodents, 162.
Roman cement, 327.
Romney, 387.
Ross of Mull, 278.
Rosso-antico, 294.
Rotary motion of particles in rivers,
356-7.
Rothliegende, 178.
Rottenstone, 118.
Route, selection of, 333.
Ruddle, 82, 90, 394, 395.
Rugose corals, 154, 177.
Ruminants, 162.
Running water, 12, 17.
deposition by, 15, 17.
erosion by, 12, 14.
source of, 12.
transportation by, 14, 15.
Russia, 82, 172, 178, 180, 182, 186,
278, 392, 394.
SAAR, 178.
Saarbriick, 180.
Sabathu series, Indian Empire, 148.
Sahara, 5, 170, 181.
Saint Austell, 276.
Bees, 178.
Gothard, 77, 126, 278.
Lawrence, 185, 186.
Louis, 168
Margaret's Bay, 387.
Saliferous beds, S. Africa, 152.
composition of rocks, 96.
Salina group, N. America, 147.
Salt, common, 313.
definition of, 53.
range of Punjab, 148, 173, 179,
184, 185, 186.
Salterella grit, 186.
Sand, 112, 130, 246, 248, 250.
-drift, 7.
-dunes, 7, 24.
Sandbars, 367.
Sandhills, 5.
Sandown Bay, 372.
Sandstone, 43, 113, 130, 204, 248,
287, 289-93, 344, 351.
Sandy estuaries, 360, 366.
Sanidine, 77, 90.
Santorin earth, 217, 322.
Sardinia, 186.
Satin-spar, 68, 81, 90, 119, 393.
Saturation and imbibition, 244-5.
line, 245, 253-4, 255-6, 263.
Savoy, 347.
Saxony, 79, 107, 129, 172, 178, 275,
278, 283, 392, 393.
Scale of fusibility, 231.
of hardness, Moh's, 66.
rough, 220.
Scandinavia, 167, 184, 185, 187, 275,
278, 279, 382.
Scania, 183, 382.
Scaphites, 160.
Schemnitz, 283.
Schist, 49, 125, 206, 351, 392.
Schistose gneisses, Indian Empire,
149.
rocks, 103.
decomposition of, 132.
Schistosity, 49.
Schorl, 90, 92, 108.
Schwarzwald, 278.
Scilly Isles, 276.
Scinde, 29, 173.
Scoriaceous structure of rocks, 98,
100, 101.
Scoriae, 27, 322.
Scorpions, 182.
Scotland, 126, 166, 167, 171, 174,
176, 178, 180, 182, 183, 184,
186, 187, 275, 277, 280, 282,
332, 382, 391.
Screes, 9.
Sea, aggradational processes at work
in the, 20.
beaches, 29.
bed, forces acting on, 373-81, 383.
cliffs, 372-3.
coasts, 370.
27
418
GEOLOGY FOR ENGINEERS.
Sea encroachment, 382.
geological work affected by, 20.
level, variation in, 29.
walls, 388.
weeds, 185.
Secondary period, 137, 140, 171-7,
373.
Sectility, 65.
Secular movements, 30.
Sedimentary strata, 132-4.
Selection of materials for roads,
348-51.
of specimens of rocks, 200.
Selenite, 81, 90, 393.
Selenitic cement, 328.
Selenium, 230.
Septa, 154.
Septaria, 102, 327, 332.
Sequoia, 169.
Serpentine, 65, 68, 69, 111, 124,
129, 202, 205, 280.
decomposition of, 131.
Serpents, 161.
Serpula, 156.
Servia, 283.
Sesquioxide, 53.
" Set " of mortar, 320.
Severn, River, 164, 391.
Seyssel, 347.
Shale, 114, 115, 205, 299, 306.
Shallow water deposits, 23.
wells, 260-1.
Shap granite, 273, 277.
Shell marble, 294.
Shetlands, 182.
Shiffnall red sandstone, 302.
Shift of fault, 45.
Shineton shales, 186.
Shiver, 306.
Shoaling of channels, 363.
Shore, position of, 372.
Shoreham, 387, 388.
" Short " clays, 308.
Shrinkage planes, 36.
Shropshire, 166, 183, 185, 186, 187.
Siberia, 164, 168, 176, 184, 382,
392.
Sicilian marbles, 294.
Sicily, 29, 178, 347.
Side slopes of roads, 335-7.
Siderite, 69, 83, 90.
Sienna earth, 83, 90.
marble, 294.
Sierra Morena, 278.
Sikkim, 168.
Silesia, 180, 278, 391, 394.
Silica, 56, 351.
effect of, on lime, 325.
series, 69, 90.
Siliceous composition of rocks, 96.
rocks, 119, 290.
Silicon, 54, 55.
Silky lustre, 68.
Sills, 35.
Silt, intercepting, 369.
Silurian system, 138, 148, 182,291,
295, 300, 392.
Silver, 232, 233, 234, 392.
Simla, 184, 185.
Sind, see Scinde.
Sinks, 10.
Sinopian, 394.
Sirenians, 161.
Sites for impounding reservoirs, 271.
Sivatherium, 169, 171.
Siwaliks, Indian Empire, 148, 171.
Skelgill shales, 183.
Skiddaw, 184, 273, 300, 304.
Skye, 174, 186.
Slaked lime, 318, 320.
Slate, 123, 205, 299, 300, 304, 392.
-polishing, 24.
Slickensides, 45.
Slips in road cuttings and slopes,
335, 337.
Sloyne, River Mersey, 358.
Smaragdite, 73, 92.
Smell of rocks, 105, 202.
Snakes, 161.
Snow, 12, 17.
Snowdon, 184.
Snow-line, 17.
Soda carbonate, 227, 233.
felspar, 76, 77, 78.
Sodium, 54, 56, 230, 396.
chloride, 313.
Soft manganese ore, 86.
Soil, 8, 9, 10, 52, 200.
Solenhofen limestone, 175, 302.
slate, 115.
Solfataras, 29.
Solubility of minerals in acids, 57, 224.
Solva group, 186.
Solvents, action of, 225.
Solway, 176, 248.
Somersetshire, 394.
Sommo Sierra, 278.
Sonstadt's solution, 224.
South Africa, table of sedimentary
strata, 152.
America, 77, 164, 165, 173, 175,
178, 187, 382, 392.
INDEX.
419
Southampton water, 372.
Spain, 72, 131, 170, 172, 180, 187,
278, 391, 392.
Sparta, 280.
Spathic iron ore, 83, 92.
Specific gravity bottle, 222.
gravity of minerals, 62, 66, 221-4.
of rocks, 105, 207, 290, 298, 348.
Specular iron ore, 65, 82, 92.
Sphaerosiderite, 83, 92.
Sphagnum, 162.
Sphene, 69, 92.
Spheroidal structure of rocks, 99.
Spherulitic structure of rocks, 100.
Spirifera, 158, 177, 183.
Spitzbergen, 176, 178, 182, 382.
Splendent lustre, 68.
Spongida, 23, 153, 179, 185, 187.
Spotted shale, 123.
Spring water, 251.
Springs, 253-60.
and wells, 253-62.
artesian, 259.
as a source of supply, 260.
in side slopes of roads, 337.
intermittent, 255.
ordinary, 253-4.
petrifying, 11.
Spur groynes, 390.
Spurn Point, 384, 385.
Stafford, 183, 311, 391.'
Stagonolepis, 161, 175.
Stalactites, 11, 118.
Stalactitic shape of minerals, 65.
Stalagmites, 11, 118.
Steam, cause of earthquakes, 30.
Steatite, 88, 92, 202, 206.
Stegocephala, 174.
Stellated structure of minerals, 64.
Step-faults, 45.
Stereognathus, 161.
Stilbite, 64, 93.
Stinchar group, 185.
Stines, 44.
Stockingford shales, 186.
Stone-bind, 306.
coal, 391.
Stonesfield slate, 140.
Stormberg series, S. Africa, 152, 168.
Strata and their inclination, 195-9.
or beds, 38.
alternation of, 39.
character of, 39.
classification of, 163.
thickness of, 197-8.
Stratification, 37, 195.
Stratified rocks, classification of,
136.
Stratigraphy and palaeontology, 136-
162.
Streak of minerals, 62, 67.
of rocks, 105, 201.
Streams, flow of, 12, 265-6.
Strike, 40, 195, 198.
Stromatopora, 181.
Strophomena, 158.
Strontium, 230.
sulphate, 74.
Structural character of rocks, 13, 33-
50.
characters of rocks, 195-200.
geology, definition of, 3.
Structure of minerals, 62, 63, 64,
65.
of rocks, 98, 201, 294.
Stylonurus, 181.
Styria, 283.
Sub-aerial denudation, 4.
-carboniferous period of N. America,
146.
Sublimate, 232.
Submarine plain, 22.
Submerged forests, 29, 165.
Subsidence and elevation of land,
29.
and upheaval of earth's crust,
381.
Subsoil, 8, 52, 200.
drainage, 338.
Subterranean channels and caverns,
10.
Sudd regions, 265.
Sudetic Alps, 278.
Suffolk, 373.
Sulphate of baryta, see Heavy-spar.
Sulphates, 69.
Sulphides, 69.
Sulphur, 29, 55, 68, 69, 92, 228, 236,
392.
Sulphuric acid, 202, 224, 226.
Summer discharge of rivers, 266.
Sumter period, N. America, 144.
Superposition of geological forma-
tions, 137.
Surface action, 3.
of saturation, 251, 263.
waters, 251-3, 263.
Surrey, 310.
Swallow holes, 10.
Swanage, 172, 174.
Sweden, 82, 167, 283, 310, 382,
392.
420
GEOLOGY FOR ENGINEERS.
Switzerland, 171, 391.
Syenite, decomposition of, 131.
gneiss, 125, 279.
porphyry, 109.
Syenites, 108, 203, 275, 278 344,
351.
Syenitic granite, 107.
Synclinal, 42.
Syphon action causing springs, 255.
Syria, 171, 173,347.
Syringopora, 155.
Systems and periods, 137.
Syston, 328.
TABLE Mountain Sandstone, S.
Africa, 152.
of rocks, 203-6.
of strata, Australia, 150.
Great Britain, 138.
Indian Empire, 148.
New Zealand, 151.
North America, 144.
South Africa, 152.
Tabular structure of minerals, 64.
Taconic Ranges, 147, 186.
Taipo beds, New Zealand, 151.
Talc, 65, 69, 88.
granite, 107.
schist, 127, 206.
slate, 127.
Talcher bed, Indian Empire, 148,
179.
Talus, 9.
Tape measure, 191.
Tar, coal, 396.
macadam, 348.
Tarannon shales, 143, 183.
Tartaric acid, 224, 226.
Tasmania, 168, 171, 392.
Taste of minerals, 57, 224.
Tatra, 278.
Tcheskaia Bay, 167.
Teanan series, New Zealand, 151.
Tectonic geology, 3.
Tees, River, 174, 368.
Teleosaurus, 161, 174.
Teleostei, 161, 169, 172, 174.
Telerpeton, 161, 175.
Temperature, changes of, effect, 5.
Tenacity of minerals, 62, 65, 66.
Tenasserim, 392.
Tendula project, 269.
Teneriffe, 283, 287.
Tennessee, 145, 146.
Terebratula, 158, 174, 176.
Terminations ; ous, ic, ate, ide, 53.
Ternary compound, 52.
Terra-cotta, 311.
Terra di Sienna, 395.
Terrace period, N. America, 144.
Terraces, river, 16.
Terrestrial deposits, 163.
Terrigenous deposits, 23.
Tertiary period, 115, 137, 168-71,
293, 299, 373.
Testing limes and limestones, 325.
Tests for road stone, 348-9.
Tetracoralla, 154.
Tetrad, 54.
Tetragonal system of crystals. 59.
Texas, 173, 178.
Texture of minerals, 63.
of rocks, 98, 294, 297, 298.
Thallophyta, 162.
Thames, 164, 165, 372.
Thanet sands, 139, 170.
Thermal properties of minerals, 62.
Thermo or contact metamorphism,
47, 95.
Thibet, 392.
Thickness of strata, calculating,
197-8.
Thomsonite, 93.
Thoulet's washing apparatus. 214.
Thuringerwald, 275, 278.
Tidal action, 379-80.
bars, 16.
currents, 379.
rivers, physical condition of,
362-5.
Tide, effect of, on drift, 388.
Tides, 20.
Tile clays, 308.
Till, 138, 166.
Tin, 233, 392, 396.
Tin-foil, 228.
Tintagel slates, 301.
Tiree crystalline marble, 304.
Titanate, 69.
Titaniferous iron ore, 82, 92.
Titanite, 92.
Titanium, 234.
Titanosilicate of lime, 97.
Torridonian series, 143, 187.
Tors, 131.
Tortoises, 161.
Touch of minerals, 62, 66.
Tourmaline, 65, 69, 92.
granite, 107.
Trachyceras, 176.
Trachyte, 110, 202, 205, 283.
Trachytic andesite, 111.
INDEX.
421
Tracing boundary lines, 193.
Transformation of rocks, 31.
Transgression, 41.
Translucency of minerals, 62, 66.
Transparency of minerals, 67.
Transportation by glaciers, 19.
by running water, 14-15, 358-
62.
marine, 22.
Transvaal, 181, 392.
Transylvania, 283, 287, 392.
Trap rocks, 281-7, 351.
Trass, 321.
Traverse, preliminary, 193.
Travertine, 11, 117. 118.
Trees, 24, 121.
Tremadoc slates, 143, 186.
Trematosaurus, 175.
Tremolite, 73, 93.
Trent, River, 178.
Trenton period, N. America, 147,
185.
Triad, 54.
Triassic system, 137, 141, 175-7, 248,
292, 307, 313.
Trichinopoly beds, Indian Empire,
148, 173.
Triclinic system of crystals, 61.
Tridymite, 91, 93.
Trigonia, 174.
series, S. Africa, 152
Trilobites, 177, 179, 182, 183, 184
185.
Tripoli, 24.
Trough-faults, 45.
Truncation of crystals, 62.
Tufa or tuff, 28, 34.
Tuffs and ashes, 112.
Tungsten, 234.
Turf, 121.
Turkestan, 176.
Turrilites, 160.
Turtles, 161.
Tuscany, 310, 392.
Twin crystals, 62.
Tyne, River, 368.
Tyrol, 279.
UlNTAH, 145.
Uitenhage formation, S. Africa, 152,
175.
Ullersdorf, 278.
Ultra-basic rocks, 96.
Ultramarine, 395.
Umber, 83, 93, 394-5.
Umia beds, Indian Empire, 148.
Umtafana beds, S. Africa, 152.
Unconformability, 41, 199.
Unctuous feel of rocks, 202.
Undercurrents, 381.
Underground water, 9-12, 243-
251.
Underthrust, 43.
Under-tow, 21.
Undulations. 42.
Ungulates, 162, 169.
Unio, 158.
United States, 248, 279.
Univalent, 54.
Upheaval of earth's crust, 381.
Uprush of waves, 376, 377.
Upthrow of fault, 44.
Urals, 108, 168, 172, 180, 182, 278,
281, 392.
Uranium, 233, 234.
Uriconian series, 143, 187.
Useful minerals, 393.
Uses of minerals, 391-6.
Utah, 146, 175.
Utatfir beds, Indian Empire, 148.
Utica, 147, 185.
VAL de Travers, 347.
Valencia slates, 301.
Vale of Eden, 178.
Valley drifts and gravels, 138.
Valleys, 13, 339.
Vancouver Island, 173, 391.
Van Wyk's Vley, 243.
Vegetable organisms, action of,
24.
Vegetation as indication of rocks,
194.
Veins and dykes, 35, 45.
Velocities of streams and rivers, 14,
355-6,
Venetia, 168.
Venezuela, 173, 188, 283.
Vents, volcanic, 28, 112.
Verde antique, 294.
Vermes, 156.
Verrucano, 178.
Vertebrata, 153, 160, 182.
Vesicular, 98.
Viborg, 279.
Vicksburg, 144,
Victoria, 171, 175, 179, 185, 283,
287.
Vienna, 171.
flysch grits, 172.
Vindhyan system, Indian Empire,
147.
422
GEOLOGY FOR ENGINEERS.
Virginia, 146, 177.
Vitreous lustre, 68.
structure of rocks, 201.
Volcanic action, 20.
agglomerates, 111.
fragmental rocks, 101, 111.
rocks, 109-11.
sands, 111.
Volcanoes, 27-29, 112.
Volterra, 393.
Von Kobell's scale of fusibility, 212,
231.
Vosges, 275, 278, 279.
Vulcanisra, 20.
WACK£, 129.
Wad, 86.
Wahsatch beds, 144, 145.
Wainamatta beds, Australia, 150,
175.
Wairoa series, New Zealand, 151.
Waitotara beds, New Zealand, 151.
Walchia, 177.
Wales, 180, 182, 183, 184, 187, 275,
277, 391, 393.
Walker's balance, 207, 222.
Walking-stick, 191.
Walton-on-the-Naze, 170.
Wanganui series, New Zealand,
151.
Warping for land reclamation,
368-9.
Warsaw, 167.
Warwick, 331.
Wash, the, 358.
Washing of rock constituents, 214.
Watcombe clay, 307.
Water as a solvent, 224.
-bearing strata, 247-9.
changes in rocks due to, 30-31.
influence of, on road materials,
341-2.
interstitial, in rocks, 31.
quality of, 250, 262, 263.
running, 12-17.
deposition by, 15-17.
erosion by, 12-14.
source of, 12.
transportation by, 14-15.
-sheds, crossing, in road-making,
338-9.
slope, 243.
superheated, 26-7.
supply, 1, 238-71.
underground, 9-12, 243-51,
source of, 9.
Wattle, use of, for jetties, 369.
Wave action, 20-22, 373-8, 380.
Waves, direction of, 377.
of translation, 375.
Waxy lustre, 68.
Wealden beds, 140, 172, 328.
Wear, River, 368.
Wearing roads, 341, 343-4.
Weather, influence of, on road
materials, 341.
-resisting roads, 341, 345-6.
Weathering, 7, 128-34, 272, 287.
test for road stone, 349.
Weight of rocks, 290.
Wells, 260-2.
as a source of supply, 262.
quantity of water derivable from,
260.
Wenlock beds, 143, 183.
Westleton sands, 138.
Westmoreland, 183, 273, 275, 282,
328.
Westphalia, 180.
Wexford, 7, 185.
Weybourne Crag, 138.
Whinstone, 282.
Whitby, 174, 332.
Whitchurch, 174.
White Crag, 138, 170.
Whitehaven, 178, 391.
White iron pyrites, 84, 93.
White lead, 393.
Whitewash, 395.
Whiting, 395.
Wicklow, 185, 186, 282, 392.
Wind, effect of, 5.
effect on drift, 388.
effect on waves, 380.
-formed currents, 380.
Windstein, 279.
Witherite, 393.
Withernsea, 385.
Witteberg quartzite, S. Africa, 152.
Wolfram, 231.
Wolverhampton. 258.
Woods and forests, 24.
Woolhope, 183.
Woolwich and Reading beds, 139,
170, 293, 307.
Worcester, 328.
Worm-burrows and tracks, 187.
Wrekin quartzite, 186-
Wyre, River, 387.
YARMOUTH, 388.
Yarralumla, Australia, 150.
INDEX.
423
Yarsbeds, Australia, 150.
Yellow copper ore, 74.
Yenangyaung series, Indian Empire,
148.
Yield of water, 249.
Yoredale group, 142, 180, 291.
sandstone, 302.
Yorkshire, 174, 176, 178, 310, 327,
372, 384, 391.
Yorktown period, N. America, 144.
Yunan, 392.
ZAMBESI, 181.
Zechstein, 178.
Zeolites, 69, 71, 93.
Zewan beds, Indian Empire, 149.
Zinc, 232, 395.
Zincblende, 69, 93.
Zoantharia, 155.
Zoophytes, 154.
Zumberg quartzite, S. Africa, 152.
Zwartkop, S. Africa, 152.
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