Outlines of Geology ... Second edition revised, etc

Introduction

TO THE SCIENCE FOR JUNIOR STUDENTS AND GENERAL READERS. By JAMES GEIKIE, LL.D., F.R.S. MURCHISON PROFESSOR OF GEOLOGY AND MINERALOGY IN THE UNIVERSITY OF EDINBURGH ; FORMERLY OF H.M. GEOLOGICAL SURVEY ; AUTHOR OF 'THE GREAT ICE AGE,' 'PREHISTORIC EUROPE,' ETC. THIRD EDITION— REVISED WITH FOUR HUNDRED ILLUSTRATIONS LONDON: EDWARD STANFORD 26 & 27 COCKSPUR STREET, CHARING CROSS, S.W. 1896 All rights reserved

Preface

. Having as a teacher felt the want of a moderately sized text book of Geology, I commenced the preparation of this manual some years ago, but other and more pressing work has delayed its completion until now. In a book intended for junior students, and those readers who desire to obtain only a general view of the scope and bearing of the science, originality need not be expected — at all events it will hardly be met with in these pages. Nor has it been thought necessary to give refer ences to original sources of information, since those who wish to go deeper into the various branches of the subject will doubtless consult the larger English, French, and German text-books, which contain copious references of the kind required. I have confined myself to giving an outline of the leading phenomena with which geologists have to deal, and these have not been discussed with equal fulness or brevity.

Knowing what parts of my subject the beginner finds it hardest to understand, I have treated these in greater detail than I have thought it necessary to do with such portions as are more readily comprehended. Difficult questions, such as those relating to the condition of the earth's interior, about which opinions are likely to differ for years to come, are merely touched upon. They cannot, indeed, be adequately discussed in any general text-book of the science, and it is not until the student has got beyond his manual, and commenced investigation on his own account that the more speculative branches of geology can be profitably considered by him. I may also

PREFACE.

. VI state here that I have taken for granted on the part of my readers an elementary knowledge of Physical Geography. That science is now so generally taught in schools that it seemed needless to burden my pages with a bald statement of facts which every moderately educated youth already knows. The space thus saved has enabled me to devote more atten tion to the subject of Physical Geology than could other wise have been given within the limits I had set for myself. Historical Geology has necessarily been treated in a sketchy manner ; all that I have attempted being to give the reader a kind of bird's-eye view of the fields already traversed by the many busy devotees of the science. Of course the mere reading of a text-book will never make one a geologist. The most such a book can do is to aid and direct the student, and to incite him to use his own eyes as an observer of nature.

If in this little treatise I have in any measure succeeded in doing so I shall be more than content with the result of my self-imposed task. The figures of fossils and several other illustrations, which last are distinguished by the letter R, have been borrowed from the well-known Physical Geology and Geography of Great Britain by my friend Sir A. C. Ramsay. For the use of these I am under many obligations. CONTENTS.

Chapter I. Introductory

Definition of Geology — The present as an interpreter of the past — Operations of Nature have always been the same in kind but not necessarily in degree — The two great divisions of geological agencies Pages 1-3

Chapter II. Geological Action Op The Atmosphere

Wind, destructive action of; reproductive action of — Sand-dunes, formation of — Distribution and mode of occurrence of — Source of materials of — Dust showers — Blood rain — Preternatural rains — Fulminary Tubes or Fulgurites . . . . . . . .4-11

Chapter III. Geological Action Op Terrestrial Waters

Rain, chemical action of — Weathering of rocks — Rain, mechanical action of — Underground water — Springs — Wells — Chemical action of underground water — Stalactites and stalagmites — Mineral springs — Origin of swallow-holes, sinks, pipes, etc. — Mechanical action of underground water — Origin of underground galleries and caves — Deposits in caves, etc. — Local subsidences, rock-falls, and landslips . . . 12-27

Chapter IV. Geological Action Of Terrestrial Waters

Continued. Brooks and rivers, chemical action of ; mechanical action of — Attrition of stones by running water — Excavation of river-channels — Forms of river-ravines — Cascades and waterfalls — Narrow gorge and wide valley — Process of valley-formation — Transport of sediment — Deposition and accumulation of sediment in bed of river, in lakes, in the sea — Bars opposite mouths of rivers — Deltas .... 28-42 CONTENTS.

Chapter V. Geological Action Op Terrestrial

WATERS— CO?ih')M(e<Z. Lakes, fresh-water and saline — Fresh-water lakes — Obstruction lakes — Crater lakes — Sink lakes — Earth-movement lakes — Glacial lakes — Lakes as receptacles of sediment — Salt lakes, deposits formed in — Erosion along margins of lakes — Existing lakes are all of recent geological age Pages 43-47

Chapter VI. Geological Action Of Ice

Frost — Ice of lakes and rivers ; anchor-ice ; surface-ice — Hail — Snow — Glaciers ; transport of rock-debris on surface of ; within and 'underneath ; giants' kettles — Sediment in glacial rivers — Moraine profonde — Erosion of glacier's bed — Glacial lakes — Ice in the sea ; icebergs ; ice-foot of Arctic regions ; floe-ice ...... 48-65

Chapter VII

geological action of ice — continued. Former great extension of ice-action — Ice-action in the Alps and in Arctic and Antarctic regions contrasted — Evidence of former glacial action in temperate latitudes — Character of stony clays or ground-moraines of low grounds of Switzerland and Italy — Ground-moraines in other parts of Europe— Period of widespread glaciation— Origin of glacial lakes— Lake of Geneva— Fiord-basins and deflection-basins . 66-78

Chapter VIII. Geological Action Op The Sea

Marine erosion— Formation of cliffs, rock-terraces, and beaches— Forms of cliff and beach in homogeneous rock-masses ; in rocks of variable composition—Influence of rock-structure upon the form of cliffs— Transport and accumulation of sediment— Littoral or shore-accumulations — Infralittoral and oceanic accumulations — Abysmal or pela<ne accumulations •■■... 79^90

Chapter IX. Geological Action Op Plants And Animals

Destructive action— Protective or conservative action— Reproductive action- Vegetable soils and peat-Mangrove-swamps-NulliDores- Orgamc oozes— Coral formations; theories of their oiio-in— Shell marl -Shell-sand-Shell-breccia-Bone-breccia-Guano 9T -99 CONTENTS. IX

Chapter X. Classification Of The Products Of Epigene Or Surface-action

Mechanically-formed, chemically-formed, and organically-derived products — Comparison of certain rocks with products of recent superficial or epigene action — Petrology of bedded or stratified rocks — Systems of classification — Geological classification of rocks produced by epigene action Pages 100-119

Chapter XI. Volcanoes And Volcanic Products

Subterranean, plutonic, or hypogene action — Limits of our knowledge — Volcanoes — Phenomena of volcanic action — Volcanic products : lava, fragmentary ejecta, gases and vapours, hot and mineral springs — Geysers — Structure of volcanic cones ; cones of non-volcanic materials, tuif-and-cinder cones, lava-cones, composite cones — Petrological character of lava ........ 120-140

Chapter XII. Mineralogical Composition Op Vitreous And Crystalline Igneous Rocks

Rock - forming minerals — Essential minerals — Accessory minerals — Secondary minerals and decomposition products — Short notes on the more common minerals occurring in vitreous and crystalline igneous rocks 141-149

Chapter XIII. Petrological Character Of Crystalline

(INCLUDING VITREOUS) AND FRAGMENTAL IGNEOUS ROCKS. Descriptive rock-terms — Orthoclase rocks — Plagioclase rocks — Nepheline and Leucite rocks — Olivine rocks — Identity in composition and structure of modern lavas and many old crystalline igneous rocks — Essential difference between modern lavas and a certain class of old crystalline igneous rocks — Fragmental igneous rocks . 150-162

Chapter XIV. Movements Of The Earth's Crust

Earthquakes, general phenomena of ; influence of rock - structure in modifying extent, direction, and force of ; theory of ; effect upon the sea of ; geological effects of ; origin of — Secular movements of the crust — Evidence of elevation — Evidence of subsidence . 163-170 X CONTENTS.

Chapter XV. The Structure Op Rock-masses

Stratification — Beds and laminæ — Contemporaneous erosion — Thinning-out of beds — Grouping of strata — Relation between extent and composition of beds — Lateral changes in beds — False-bedding — Surface-markings — Concretions and concretionary structures — Secretions — Dip, outcrop, and strike — Curvature of strata — Cleavage — Joints — Faults — Unconformity and overlap .... Pages 171-202

Chapter XVI. Ore Deposits

Ores diffused or disseminated through rocks — Bedded ores — Ores occurring in fissures, etc. — Ores as constituents of rocks — Metals occurring in rock-forming minerals — Varieties of bedded ores — Placers — Lodes — Gash-veins, pipe-veins, chambers, and pockets — Flats — Impregnations — Stockworks — Theories of the origin of ore deposits . 203-209

Chapter XVII. Eruptive Rocks

: THEIR MODE OF OCCURRENCE. Volcanic and plutonic rocks — Contemporaneous eruptive rocks — General phenomena of lava-flows and bedded tufts, etc. — Fissure eruptions ; general character of their products — Intrusive eruptive rocks — Necks — Intrusive sheets — Dykes and veins — Bosses ; their segregation or contemporaneous veins ....... 210-224

Chapter XVIII. Crystalline Schistoze Rocks

Metamorphism — Foliation — Common minerals entering into composition of crystalline schistoze rocks— Petrology of schistoze rocks— Mode of occurrence and origin of these rocks — Local or contact metamorphism — Regional metamorphism — Archæan schists . . . 225-234

Chapter XIX. Origin Op Some Prominent Rock-structures And Surface Features

Structure and origin of true mountain-chains — Origin of curvature of strata— Reversed faults and cleavage— Oceanic basins and continental ridges— Association of volcanoes with mountain-chains— Condition of CONTENTS. XI the earth's interior — Eegional elevation, normal faults, and fissure eruptions — Composition of the earth's interior — Surface-features clue to denudation Pages 235-246

Chapter XX

PALÆONTOLOGICAL GEOLOGY. Fossils ; physical and climatic conditions deduced from ; relative age of deposits determined by — Rocks containing same fossils not necessarily synchronous — Gradual extinction of species — Breaks in succession — Imperfections in the record — Orderly succession of changes —Systems— Historical Geology— Divisions of strata . . 247-257

Chapter XXI. Historical Geology Primary Or

PALÆOZOIC SYSTEMS. Archæan system — General character of the rocks — Cambrian system ; life, physical conditions — Silurian system; life, physical conditions — Devonian and Old Red Sandstone system ; life, physical conditions 258-282

Chapter XXII. Historical Geology Primary Or

PALÆOZOIC SYSTEMS Continued. Carboniferous system ; life, physical conditions — Permian system ; life, physical conditions — General character of Palæozoic faunas and floras — Evolution of continental areas in Palæozoic times . . 283-305

Chapter XXIII. Historical Geology Secondary Or Mesozoic Systems

Triassic system ; life, physical conditions — Jurassic or Oolitic system ; life, physical conditions — Cretaceous system ; life, physical conditions — General character of Mesozoic faunas and floras — Evolution of continental areas in Mesozoic times .... 306-338

Chapter XXIV. Historical Geology Tertiary Or Cainozoic Systems

Eocene system ; life, physical conditions — Oligocene ; life, physical conditions — Miocene ; life, physical conditions — Pliocene ; life, physical conditions — General character of Cainozoic floras and faunas — Evolution of continental areas in Cainozoic times . . 339-367 CONTENTS.

Chapter XXV. Historical Geology Post-tertiary Or Quaternary Systems

Pleistocene or glacial system ; life and physical conditions — Post-glacial and Recent system ; life, physical conditions — Correlation of Archæological and Geological periods — Geographical conditions of Post-tertiary times — Climate of Post-tertiary times . Pages 368-395