The geology of South Australia

Front matter

if

SIA,

SS

Chapter II. Main Physical Divisions

The Lithosphere: Continents, Continental Islands, Oceanie Islands, Interior of the Earth, Meteorites. The Hydrosphere: Sea-floor, Pacific Type of Ocean, Atlantic Type of Ocean, Movements of Sea-water, The Sea and Climate, Evolution of Continents and Oceans. The Atmosphere: Composition of the Atmosphere, Carbon Dioxide, Atmospheric Effects. Australian Meteorology: Air Currents, The Thermal Equator, Condensation of Water Vapour, Mean Atmospheric Pressure, Rainfall, Droughts, Weather [Forecasts . : a:

Chapter III. Minerals And Their Properties

Hlementary Substances. Estimated Percentages of the Elements in the Earth’s Crust. Distinction between Rocks and Minerals. Crystalline Form. Cleavage. Fracture. Seale of Hardness. Specific Gravity. . Lustre and Feel. Colour. Chemical Tests. Characteristic Forms of Minerals and Rocks

Cuaprer IV.

COMMON ROCK-FORMING MINERALS.

Minerals consisting of Silica. Silicates. Hydrous Silicates. Oxides. Carbon Group. Carbonates. Sulphates. Sulphides. Saline Minerals Maneater a re OL voce ry Si Foe ee ies

CHAPTER JV.

GROLOGICAL PROCESSES.

Main Divisions of the Subject: Disintegration, Degradation, Aggradation,. Vuleanism, Diastrophism .. Ba ER. ERTS he

Pages.

1-7

31-39

39-55

Vill. CONTENTS OF DIVISION I.

Part Ii—subaerial Forces That Are Destructive. Chapter VI

DISINTEGRATION (WEATHERING).

Disintegration by Chemical Action: Seen in Limestones, Felspars, Granites, Micas, Hornblende, Basic Igneous Rocks, iragmental Rocks, Effects of Vegetation, Acids other than Carbonic Acid, Differential Weathering. Disintegration by Mechanical Action: Freezing, Solar Heat, Weathering in Polar Regions, Three Characteristic Types of Weathering .. 56-64

Pages.

CHAPTER VII.

DENUDATION-—WORK DONE BY WIND AND RAIN.

Definitions, Wind a Denuding Force. Dreikante. Erosion by Ice Crystals. Denudation by Rain. Rain Sculpture. Gables and Washouts. Soil Creep. Landslips 64-70

CHAPTER VIII.

DENUDATION (continued) —WORK DONE BY GROUND-WATER.

Quarry-water. Springs. Artesian Springs. Hot Springs. Chemical Effects of Underground Water. Cementation. Caves and Underground Passages. Swallow-holes. Mounds and Terraces.) = Medicinal AWaters.. 20 en 0c ety eee 71-78

CHAPTER IX, DENUDATION (continued)—WORK DONE BY RIVERS.

Mineral Substances carried in Solution. Effects of Climate on Mineral Solutions carried by Rivers. Solid Material carried by Rivers. River Erosion. Powers of Transport. Average and Total Results 79-83

CHAPTER X.

DENUDATION (continued)—LIFE HISTORY OF RIVERS.

Young Rivers. River Grade. Influence of Rock-structure on Valley Development. River Capture. Mature Rivers. Old Rivers. Base-level in Rivers. Revived or Rejuvenated Streams. Dead Rivers. Antecedent Rivers. Superimposed Rivers. Betrunked and Engrafted Rivers. Origin of Lakes. Varieties of Lakes. Lakes only a Temporary Feature .. 84-98

CHAPTER XI.

DENUDATION (continued) —WORK DONE BY SNOW AND ICE.

The Snow-line. Neve or Firn. Avalanches. Glaciers. Ice-Cap. Icebergs. Alpine Type of Glaciers. Movements of Glaciers. Crevasses. Icefall. Glaciation. Evidences of Past Glaciations. River and Glacier Action Contrasted .......... 98-110

CONTENTS OF DIVISION I.

CHAPTER XII.

Pages.

MARINE DENUDATION—WORK DONE BY THE SEA.

Wave Action. Effects of Rock-structure on Coastal Denudation. Encroachments by the Sea. Denudation by Ocean Currents. Plains of Marine Denudation. Cycles of Erosion

CHaprer XIII.

LAND FORMS CAUSED BY DENUDATION.

Valley Sculpture. Curve of Erosion. Cols. Water-gaps. Wind-gaps. Valleys caused by Warping. Fiords. Buried Valleys. Plateau Erosion and Dissection. Peneplains. Monadnocks. House-roof Structure. Outliers and Inliers. Conformity and Unconformity. Overlap and Transgression

Part Iii—reconstructive Processes. Chapter XIV

110-120

120-129

SUBAERIAL DEPOSITS OF A CLASTIC OR FRAGMENTAL KIND.

Aeolian (Wind) Deposits: Sand-dunes, Cause of Deserts, Stony Deserts, Gibbers, Sandy Deserts, Dust Storms, Loess, Salinas, Deserts of the Past, Geographical Cycles under Desert Conditions, Residuals. Fluviatile (River) Deposits: Sand-reaches, Haughs, River Terraces, Alluvial Fans, Screes and Talus, Piedmont Plains, Flood Plains. Estuarine Deposits: Deltas. Lacustrine (Lake) Deposits: Lake Sediments, Disappearance of Lakes. Glacial Deposits: Unstratified (Moraines and Till), Stratified (fluvio-glacial)

CHAPTER XV.

130-147

SUBAERIAL DEPOSITS OF CHEMICAL AND ORGANIC ORIGIN.

CHEMICAL.—Calcareous: Sand-pipes, Cale-sinter, Cave Deposits, Travertine, Nodular Travertine, ‘‘ Biscuits,’’ Mound Springs. Siliceous: Siliceous Sinter, Chalcedonie Infiltrations. Saliferous: Gypsum, Salt-pans. Ferruginous: Chalybite Springs, Bog-iron. ORGANICO.—Carbonaceous: Peat, Lignite, Coal, Necessary Conditions for the Formation of a Coalfield. Caleareous: Freshwater Limestones. Siliceous: Diatomaceous Karth. Nitrogenous and Phosphatic: Nitrates, Guano, Caveearth, Phosphatic Limestones

CHAPTER XVI.

MARINE DEPOSITS.

Littoral Deposits: Rocky Beaches, Shingle Beaches, Sandy Beaches, Shore-marks. Shallow-sea Deposits: Terrigenous Sediments, Chemically-formed Marine Limestones, Concretionary Nodules of Mineral Oxides, Glauconite, Organic Deposits of the Continental Shelf. Deep-sea Deposits: Blue Mud, Caleareous Oozes.. Abyssal Deposits: Red Clay, Siliceous Oozes . eer ahr MGR areas oh %, ake ae *

148-156

156-167 xX. CONTENTS OF DIVISION I.

Part Iv.—the Origin And Classification Of Rocks

CHAPTER XVII.

CONSOLIDATION OF SEDIMENTS.

Definitions. Simple Induration of Sediments. Sub-metamorphic Alteration. Metamorphic Change. The Katamorphic Zone. The Anamorphic Zone. Contact-metamorphism... ..

CHAPTER XVIII.

SEDIMENTARY ROCKS.

Distinctive Features. Mechanically-formed Rocks: Quartzose, Argillaceous, Pyroclastic. Chemically-formed Rocks: Coneretionary Limestones, Marine Limestones of Chemical Origin, Dolomitic Limestones, Saliferous Rocks, Bedded Iron-ore Deposits, Metasomatie Deposits, Glauconitic Sands and Clays, Segregations in Sedimentary Rocks. Rocks of Organic Origin: Varieties of Coal, Organically-formed Limestones, Siliceous Rocks derived from Organic Remains

CHAPTER XIX.

METAMORPHIC ROCKS.

General Characteristics. Marbles. Quartzites. Argillites. Crystalline. Schistsn: aes ea ee ie

CHAPTER UX.X:.

IGNEOUS ROCKS.

Main Divisions. Magmas: Acid and Basie, Incipient Crystallization and Texture. Plutonic Rocks: Granite Group, Syenite Group, Diorite, Gabbro, Peridotites, Serpentine. Hypabyssal ~ kiocks: Granophyre, Porphyry, Diabase, Lamprophyre. Volcanie Rocks: Texture of Voleanic Rocks, Rhyolites, Trachytes, Andesites, Dolerites, Basalt, Tachylite, Trap, Melaphyre .

Part V.—rock Structures. Chapter XXII

Pages.

168-173

173-192

192-198

198-207

STRUCTURES PERTAINING TO SEDIMENTARY ROCKS.

Bedding. False-bedding. Inclined Strata. Contour, Dip, and Outcrop. Jointing in Sedimentary Rocks. FEissility in Sedimentary Rocks. Cataclastic or Shatter Zones. Crush Breccias. Desiccation Breccias ..

CHAPTER XXII.

STRUCTURES. PERTAINING TO IGNEOUS ROCKS.

General Characteristics —Various Forms of Igneous Intrusions: Batholiths and Stocks, Laccoliths, Bysmaliths, Phacoliths, Dykes, Sills or Sheets, Veins, Vents. Jointing in Igneous Rocks: Normal Jointing, Prismatic Jointing, Columnar Structure,.Cause of Prismatic and Columnar Structure .

208-219

220-227

CONTENTS OF DIVISION I.

Part Vi.—earth Movements. Chapter XXIII

ELEVATIONS, SUBSIDENCES, AND FOLDINGS OF THE

EARTH’S CRUST.

Positive and Negative Movements. Evidences that prove Recent Elevations of the Land: Coastal Examples, Inland Features. Evidences that prove Recent Subsidences of the Land: Submerged Coastal Plains, Coral Reefs and Atolls, Warpings, Causes of Changes of Level. Curvature and Folding of Strata: Monoclinal Folds, Anticlines and Synelines, Quaquaversal and Centroclinal Folds, Pounds; Oblique, Reversed, and Recumbent Folds; Open and Closed Folds, Isoclinal Folds, Geosynclines, Anticlinoria and Synelinoria, Fan Folding, Conditions that influence Folding .. ee

CHAPTER XXIV.

FRACTURES OF THE EARTH’S CRUST.

Faulting of Strata. Normal Faults. Trough Faults. Ridge Faults. Reversed Faults. Thrust planes. Strike and Dip Faults. Fault Breccia. Slickensides. Blatter. Surface Indications of a Fault. Relative Age of Faults .. .

CHAPTER XX/Y.

Pages.

228-240

240-248.

VULCANISM—CHANGES PRODUCED BY SUBTERRANEAN CAUSES.

Increase of Temperature with Depth. Volcanic Phenomena: Explosive Volcanoes, Products of Volcanic Eruptions, Non-explosive Volcanoes, Fissure Eruptions. Plutonic or Deep-seated Vulcanism .. ee Sas

CHAPTER XXVI.

EARTHQUAKES.

Microseisms. Voleanic Harthquakes. Geotectonic Harthquakes. Definitions. Seismographs and Seismometers. Depth of

Earthquake Centrum. Harthquake Waves. Simple and Twin Earthquakes. Direction of LHarth-slip. Submarine Earthquakes. Rossi-Forel Secale of Seismic Intensity. Seismographic Records of Distant Harthquakes. The EHarth’s Interior. Locating Distant Earthquakes. Geological Effects of Earthquakes. South Australian EN as pcoesePee eal Distribution of Earthquakes . é Cae ee eer

CHAPTER XXVII.

MOUNTAINS: AND BASINS.

Definitions. Types of Mountains: Volcanic, Laccolithic, Tectonie, Arcuate, Plateau (Epeirogenic), Block Mountains, Horsts, Residual or Subsequent Mountains. Basins and Sunken Areas: Geosynelines, Ocean Deeps, Land-locked Seas, Inland Basins, Rift Valleys. Mountain Building: Cosmic Contractions, Related to Coast Lines, Thick Sedimentation, eee of Mountains iy

248-256

272-285

Xi. CONTENTS OF DIVISION I.

JHAPTER XXVIII.

CHRONOLOGICAL CLASSIFICATION OF SEDIMENTARY ROCKS. Pages.

Part Vii—organic Remains. Chapter X XIX

FOSSILIZATION.

Conditions and Various Forms of Fossilization. Pseudomorphs 289-293

CHAPTER XXX.

FOSSIL TYPES.

Invertebrates: Protozoa, Porifera, Coelenterata, Wehinodermata, Vermes, Polyzoa, Brachiopoda, Mollusca, Crustacea, Insecta. Vertebrates: Pisces, Amphibia, Reptilia, Aves, Mammalia . 253-305

CHAPTER XX XI.

THE TESTIMONY OFf FOSSILS.

The Progressive Development of Living Things. The Use of Fossils in the Correlation of Strata. Contemporaneity and Homotaxis. Changes in Geological Physical Conditions .. 305-312

CuarpreR XX XII.

EXERCISES IN DRAWING GEOLOGICAL PLANS AND SECTIONS.

Signs used on Geological saat Geological Problems for Praetical Exercises .. .. . SR ee TC Aa. 2 Pee ee 312-319 APPENDICES.

IND AAMDOS INA DoornniknvANROIN SA Go S56 oo 508 bs sca ee oe oe 320

Oe Be—isostasy S25 4%2 (See Sal So ee oe ie eee eee 321

oes C.—Theories as to the cause of Vuleanism .. .. .. 391-324

e: D-—Mountain Building) 0) 2.92 2S a. ee

INDEX PA ustralian- Localities: . “os =.) ane es eee 327-329

INDEX? Subjects, Gices ten5 wes oa a ae RS ae ee

CONTENTS OF DIVISION II.

CHAPTER XX XITI.

INTRODUCTION. Pages.

Scope of Geology. Geological Time. British Geology. Table of eee eg ee ny a ce eta RRR he reg eta cals 327-330 CHAPTER XXXIV, FUNDAMENTAL ROCKS (PRE-CAMBRIAN).

Modes of Oceurrence, Classification.

PROTEROZOIC (ALGONKIAN) DIVISION.

Historical Notes. Extent and Distribution. Lithological Features. Economie Products. Pre-Cambrian Rocks in the other Ausraat tates< a. solu. Se ris ate reek Se Saat ane 331-337 PALAEOZOIC DIVISION.

CHAPTER XXXV.

CAMBRIAN SYSTEM. ames Uncontormity with Pre-Cambrian ....-.. ...:,; .. .. +. -- 801-339

(a) Lower Cambrian Series: Basal Beds, Phyllites, Slates and Shales; Glacial Beds (Sturtian Tillite); Limestones. Lower Cambrian of the Eastern side of Mount Lofty. Lower Cambrian of the Northern Areas. Lower Cambrian of the Flinders Ranges. Lower Cambrian of the Far North. Lower Cambrian of the Western Plateau. Igneous Rocks of the Lower Cambrian. Palaeontology of the Lower Cambrian .. .. Nee ea ete id ieee eke ei OOM oO OO

(b) Upper Cambrian Series: Plane of Division from the Lower Cambrian. Extent and Distribution, Purple Slates, Quartzites, Archaeocyathinae Limestones, Obolella and Girvanella Limestones, Red Sandstones from (?) Arid Conditions. Cambrian Overlap, Tectonic Movements. Igneous Rocks of the Upper Cambrian. Palaeontology of the Upper Cambrian. Economic Products of the Lower and Upper Cambrian. Cambrian Rocks in the other Australian States; POMPE OLLI Maina aeeMTeR UNAM Minas iy ocala Sher os von Meee to) OTIS S

Xly. CONTENTS OF DIVISION II.

CHAPTER XXXVI,

ORDOVICIAN SYSTEM.

Name. Extent and Distribution. Larapintine System. Geological Section through the Series. Palaeontology of the Larapintine System. Ordovician Rocks in the other Australian States. Post-Ordovician Conglomerate .. .. .

CHAPTER XXXVII.

SILURIAN SYSTEM.

Name. Silurian Rocks in the other Australian States ..

CHAPTER XX XVIII.

DEVONIAN (OLD RED SANDSTONE) SYSTEM.

Name. Devonian Rocks in the other Australian States ..

CHAPTER XXXIX.

CARBONIFEROUS SYSTEM.

Name. Main Divisions. Carboniferous Rocks in the other Australianwstates eo at coe ee ee et.

CHAPTER XL.

PERMIAN (PERMO-CARBONIFEROUS) SYSTEM.

Name. Main Divisions. The Inmanian Glaciation: Inman Valley, King’s Point and Rosetta Head, Southern Yorke Peninsula, Kangaroo Island, Cape Jervis, Mount Compass, Hallett’s Cove. Geological Notes on the Glacial Beds. Why no Coal in South Australia. Permo-Carboniferous Rocks in the other Australian States. Palaeontology of the Permo- Carboniferous System. Gondwanaland. Permo-Carboniferous Glacial Beds outside Australia .. wt

MESOZOIC DIVISION.

CHAPTER XLI.

TRIASSIC SYSTEM (LOWER TRIAS-JURA).

Name. Main Divisions. The Flora. The Fauna. Triassic Rocks in the other Australian States ..

CHAPTER XLII.

JURASSIC SYSTEM (UPPER TRIAS-JURA).

Name. An Age of Great Reptiles. Leigh ’s Creek Basin. Other Localities for Rocks of this Age in South Australia. Jurassic System in the other Australian States. The Jurassic Continent. Economie Products .. Asha SM aigt ihe eRe

Pages.

383-389

390-391

392-395

395-398

398-424

424-428

428-436

CONTENTS OF DIVISION II.

CuHaprer XLII.

CRETACEOUS SYSTEM.

Name. Cretaceous Period in Europe. Extent and Distribution of Cretaceous Rocks in Australia. Main Divisions—

(a) Lower Cretaceous: Rolling Downs ‘‘ Formation,’’ the Artesian Basin. Cretaceous beds in the other Australian States. aan oe of the Lower Cretaceous... ; Sere,

(OG!) Upper —.(or aes Cretaceous: — °* Desert Sandstone,’’ Lithological Features, RSOlOB Eee Age Secondary Silicification .. .

CAINOZOIC DIVISION.

CHAPTER XLIV.

Conditions of Deposition. Main Divisions. Geologicai Age.— Basal Beds of (?) Oligocene Age: A Freshwater Series, As developed in the Northern Areas, As Developed in the Southern Areas . Sette Mego ener i ee aad Che Ach T nen Pes tas AM hes

CHAPTER XLV.

MIOCENE SYSTEM.

Lowest Marine Series.—Extent. Lithological Features. Distribution in South Austraha: the Bunda Plateau and Great Australian Bight, Adelaide and Neighborhood, Gulf St. Vineent, Spencer Gulf, Kangaroo Island, River Murray Plains, South-Eastern Region. Palaeontology of the Beds .

CHAPTER XLVI.

PLIOCENE SYSTEM.

(a) Lower Pliocene.—Second Marine Series. Unconformity with the Miocene. Distribution in South Australia: Great Australian Bight, Adelaide, Gulf St. Vincent, River Murray Cliffs. Lithological Features. Discordance with the Miocene in the respective areas of Distribution. Palaeontology of the Beds .. .. .. 5

(b) Upper Pliocene—Third Marine Series. Deposits in the Rift- Valley. Palaeontology of the Beds ..

CHapTerR XLVI.

OLDER PLEISTOCENE.

(a) The Fluviatile Stage. A Period of Great Rivers. The Highlands of Central Australia formed the main Watershed. Dead Rivers .. .. .. Bee merece CON) 3

(b) The Plateau-forming ie Uplift and Warping. The Kosciusko Period. A New Divide established. Back-waters create great Inland Lakes .. .. .

(c) A Period of Collapse. Block-faulting. Rift Vallee Terraced platforms mark the segments of Subsidence .. ..

437-447

447-452

A

452-456

456-465

465-468

469-470

470-473.

473-474

474-478

Xv. CONTENTS OF DIVISION II.

CHapTrer XLVIII.

NEWER PLEISTOCENE AND RECENT.

Voleanie Eruptions. Newer River Systems. Oscillations in the Level of the Land. Raised Sea Beaches. Age of the Mount Lofty Ranges. A Larger South Australia .. goers « f

Cainozoic Features in the other Australian States, Terrestial Conditions. Cainozoic Flora. Oldest-known Australian Marsupial. Alternations of Level. Glaciations in New South Wales and Tasmania. Volcanic Eruptions and Lava Extrusions. The Land Fauna of the Cainozoic Periods. Geographical Distribution of the Remains of the Pleistocene Taand2kauna.=-

GAP Tere lex BIBLIOGRAPHICAL REFERENCES

APPENDICES.

APPENDIX E.—Notes on the Chronological Position of the Lower Cambrian of South Australia ..

F'.—Lower Palaeozoic (or Older) Glacial Beds ..

G.—The Origin of Unfossiliferous Limestones ..

H.—The Order of Succession and Geological Age of the Cainozoic Beds of Australa .. ..

InpEx: Australian Loealities .

Pages.

478-486

486-499

499-502

DIVISION I.—AN INTRODUCTION TO GEOLOGY FROM THE AUSTRALIAN STANDPOINT.

PART L—GENERAL ASPECTS OF THE SUBJECT.

Chapter I. Introductory

The Solar System.—The earth forms one of a number of planetary bodies of various sizes that revolve around the sun as their common centre of gravity, and collectively, are known as the solar system. This group of worlds shows a certain family likeness—

(a) The same elementary substances appear to enter into the composition of each (proved by spectroscopic examination of the sun, meteorites, ete.).

(6) All the primary planets revolve around the sun in practically the same plane, which is known as the plane of the ecliptie.

(c) All travel around the sun in the same direction.

(d) All their orbits are ellipses—only slightly different from circles.

(e) All rotate on their axes in the same direction.

(f) All are more or less inclined in their axes to the planes of their orbits—or, in other words, to the ecliptic.

If the axis of the earth had been perpendicular to the ecliptic, the sun would have been always vertical at the equator, and there would have been equal periods of day and night throughout the year; and there would also have been no change of season. It therefore follows that the greater the inclination which a planet’s axis of rotation bears to the plane of the ecliptic the greater the extremes in the lengths of daylight and darkness in the course of the year, and also the greater the contrasts in the seasons. The earth’s axis has an inclination of

A about 23$deg., which gives well-marked seasonal differences, but Jupiter has an inclination of only 3deg., and Saturn as much as 26 5/6deg. If the physical conditions on these planets were similar to those of the earth’s at the present time, Jupiter would have practically no change of seasons, while Saturn’s seasons would be still more strongly contrasted than those of the earth.

The question of the origin of the solar system has given rise to numerous theories. The Nebular Hypothesis, which may be taken to represent, in one form or another, the modern views on this subject, had its origin in the philosophical writings of Emmanuel Kant (Konigsberg, 1755). The theory took a more definite form as the result of the brilhant researches of Sir William Herschel (1780-1815), who discovered more than 2,500 nebulae, and was further elaborated by Laplace (Paris, 1824), whose name is now generally associated with the theory.

Briefly stated, Laplace’s nebular hypothesis assumed that the matter of which the sun and planets are composed existed originally as a diffused ineandescent gas that filled the whole of the space now occupied by the solar system, and that this gaseous vapour rotated slowly on its axis. Loss of heat—beginning at its outer boundary—would tend to the concentration of the mass, which would bring about greater density and increased speed of rotation. These effects would cause the outer zone to become detached from the inner portion, and, under the control of gravitating forces, operating from one or more nuclei, the gaseous ring thus formed would gradually take the shape of a spheroid or world. This process of condensation was repeated as successive rings became detached, until a series of worlds had become evolved from the primeval fiery vapours, while the main portion at the centre has continued in a state of incandescence to the present time and constitutes the sun. This theory of Laplace is burdened with the difficulty of explaining how so highly rarefied a gas, as is supposed to have filled the space of the solar system, could have retained its ineandescence and rotation, and it also fails to account for the retrograde movements of the satellites of Uranus and Neptune in their revolution round their primaries.

When the spectroscope was applied to the nebulae it demonstrated that these mysterious objects consisted, in part at least, of glowing gases, and appeared, thus far, to confirm the prevailing theory; but the latter has undergone important modifications at — the hands of recent observers.

Professor Norman Lockyer, as the result of a series of spectroscopic laboratory examinations of meteorites, propounded the Meteoritic Hypothesis. From the close similarity in their spectra, Lockyer concluded that all self-luminous bodies in the universe, whether comets, nebulae, meteorites, or stars, owed their origin and luminosity to swarms of meteoric bodies, which, travelling along intersecting planes, came into collision with great force, and the heat generated thereby produced a gaseous incandescence by which these celestial bodies became visible. The dark constituents of a nebula would, of course, be invisible, and give no spectra. On this hypothesis worlds are evolved by the aggregation of meteoritic material gathered from space and consolidated around certain centres of attraction.

Professors Chamberlin and Moulton, of Chicago University, are the authors of a theory known as the Planetesimal or Sprral- Nebula Hypothesis. There must be some significance in the fact that the greater number of the observed nebulae have a spiral form. The authors mentioned assume that this peculiar structure has been caused by the near approach to each other of two stars and the mutual disturbances that would result therefrom in the constitution of such bodies. In such a near approach there would follow a great dispersion of heated gaseous bodies, torn from their parent orbs, and, as the stars receded from each other, gravitational control, operating from two centres, would bring about a spiral structure in the arrangement of the mass, with denser nuclei at intervals along the arms of the spiral. It is suggested that our sun was one of such stars that came into close contact with another, and thus, in becoming the centre of a spiral nebula, supplied the first stage in the origin of the solar system. The subsequent stages were the gradual passage of the ejected matter from its original molecular and gaseous state to the solid form, followed by aggregation into small accretions, and then the condensation of the more important nuclei into planets and satellites with a solar central mass. As the nebula from which the solar system was evolved consisted of mnumerable small bodies, and all such bodies are supposed to have moved nearly independently in orbits, like minute planets, Chamberlin has designated his theory the Planetesimal Hypothesis; and by way of distinguishing it from the Laplacian ring theory, Moulton has given it the name of the Spiral Hypothesis.

Perhaps the most prevalent view at the present.time, as to the origin of solar systems, is that spiral nebulae have been formed, not by the disturbing influences arising from two stars coming into close contact with one another (as is supposed in the Chamberlin-Moulton theory) but by the near approach of large streams of meteoric matter which, by their mutual attractions, are caused to revolve around a common centre of gravity, in a spiral form, and by a slow process of consolidation, at certain nuclei, pass from the meteoric to the planetary form.

None of these theories as to the origin of the solar system has received universal acceptance; they form interesting speculations, but our knowledge of the universe and its mechanism is so limited that we cannot speak with confidence as to the genesis of even the sun and planets with which we are most familiar. The remarkable discoveries in radio-activity will probably, in time, throw much light on cosmic phenomena that are now beyond our ken, and give more definite data as to the origin and duration of stars and systems.

The Moon.—-With the exception of Mercury and Venus, all the primary planets possess satellites—some have several; but the moon is the largest satellite relatively to its primary in the solar system. ‘The moon by its nearness and magnitude (1/81 that of the earth’s mass) has exerted from the beginning a very great influence over certain terrestrial phenomena, and in earlier times this influence was probably much greater than at present. The most noticeable effects of the moon’s influence are seen in the tidal wave which causes high and low water, respectively, twice in a little more than 24 hours throughout the ocean. The attraction of the moon may, at one time, have caused tidal movements within the earth itself, as well as at the surface.

Professor George H. Darwin(!) has pursued some interesting 1nvestigations as to the effects of the tides in producing a retardation of the earth’s rotation on its axis. However shght it may be, the tidal wave must cause some friction on the earth’s surface, and friction exerted on a moving body compels it to slow down. By mathematical caleulations, carried backward, Darwin has made certain deductions as to the probable history of the

(1) ‘‘The Tides and Kindred Phenomena in the Solar System,’’ London, 1898, pp. 238-260.

D mutual relationships and inter-action of forces between the earth and its satellite. It is conjectured that the moon had its origin as a fragment separated from the earth’s mass by the centrifugal force of the planet’s rotation, and, in the first instance, made its revolutions in close proximity to the earth. On the calculations made it is believed that, at that early period of the moon’s separate existence, the moon revolved around the earth in a period of between three and five of our present hours, and that the earth’s rotation on its axis was accomplished in about the same length of time as it took the moon to revolve around the earth. The friction of the tides, however, caused a retardation in the earth’s rotation, which led to a lengthening of the day and at the same time a receding of the moon from the earth, by which the lunar month was also gradually lengthened. At present the day is still very slowly lengthening, as is also the lunar month—the day much faster than the month—and the final stage will be reached when the earth’s rotation on its axis will be completed in the same time that the moon takes in performing its revolution round the earth.

The chief point of interest as bearing on geology in these speculations is the greater dynamical force which the lunar tides would exert on the earth in the earlier stages of the moon’s history. Darwin estimates that when the moon was at half its present distance from the earth the tidal friction would be 64 times as efficient as it is at present. Taking into account the special conditions, viz., the stronger pull of the moon by gravity, its more rapid revolutions around the earth, and the greatly accelerated rotation of the earth on its axis, it may be supposed that the processes of waste, tidal erosion, and sedimentation would proceed in a ratio that would be in excess of the results produced by lunar influences at the present time.

The destiny of the earth is inseparable from this group of worlds which form the solar system. Its hght, heat, and energy are received from the sun. The law of gravity, acting mutually, secures the stability of the whole. The earth, in common with every other unit of the group, is passing through evolutionary changes—it therefore has a history. The earlier chapters of that history have been largely obliterated by the fusion and recrystallization of the older rocks; the last traces of the original crust of the earth have, probably, long since disappeared; but earth fragments of a high geological antiquity have been preserved, and within the periods covered by the stratified rocks the story of the earth is full of detail and equally full of romantic interest. To decipher this object-writing of Nature is the purpose of Geology.

The Shape of the Barth (having regard to the flattening at the poles) is usually described as an oblate spheroid. The equatorial diameter is 26.8 miles greater than the polar diameter, the centrifugal force of rotation is therefore much greater at the equator than near the poles, to which cause the bulging at the equator is probably due. The earth is not, however, a regular spheroid, but is slightly compressed, longitudinally, as well as at the poles, so that the equatorial diameter varies at different points.

q

Fig. 1. Illustrates how the arrangement of land and water on the earth’s surface may be referred to a Tetrahedral outline. One plane, occupying a horizontal position, represents the belt of land that encircles the earth in the northern hemisphere. The meridional lines correspond to the three great land masses of South America, Africa, and Australasia, converging at the South Pole, in the Antarctic Continent. The ocean outlines are complementary to that of the land, being also roughly triangular, with their apices pointing northwards, the ocean forming the great belt of the southern hemisphere, as the land does in the northern hemisphere. (After J. W. Gregory.)

The Distribution of Land and Water over the earth’s surface presents some peculiar features. The water surface is in excess of the land surface in the proportions of 3 to 1. The land is mostly aggregated in the northern hemisphere and the water in the southern. The north pole is situated in an ocean surrounded by land; the south pole is situated on land surrounded i by an ocean. All the large land areas, viz., North and South America, Africa, India, and Australia, are roughly triangular in outline, with their apices pointing southwards; while all the great oceans are also roughly triangular with their apices pointing northwards. The northern hemisphere is encircled with a great belt of mountains, the southern hemisphere is surrounded with a great ditch or collapse of the crust occupied by the ocean.

This peculiar distribution of the land and oceans has led some writers to speak of the earth as a tetrahedron (fig. 1). The use of such a term in relation to the earth’s shape can only refer, of course, to a very superficial zone. In the shrinking of the earth’s crust there has no doubt been a distinct correlation between the areas of elevation and those of subsidence. Just as a distended bladder will collapse on letting out the air and assume an uneven surface (which, theoretically, ought to be a collapse on four sides), so the relation of continents and oceans at the present time represents just such compensations under collapse. The tetrahedral outlines of the great land masses and the great oceans, correspond to the adjustments of the earth’s surface that have taken place in these respective portions of the globe under shrinkage.

CHAPTER ILI.

MAIN PHYSICAL DIVISIONS.

The material of which the earth is composed exists under three conditions—(1) Solid, forming the land areas, or Lithosphere; (2) Liquid, forming the water areas, or Hydrosphere; (3) Gaseous, forming an aerial investment, or Atmosphere.

(a) THE LITHOSPHERE.

The lithosphere consists of continents, continental islands, and oceanic islands.

Continents are great elevated platforms with their margins below sea level. Around all the great land masses there is a fringe of shallow water, of variable width, known as the conttnental shelf, having a depth gradually increasing to about 600ft.. commonly called the hundred fathom line. Beyond this submerged shelf the sea bottom sinks rapidly, on the continental slope, to profound depths (fig. 2). The coastal plains above sea-level and the marginal shallows below sea-level represent the disputed area, within recent times, between land and sea. In the frequent oscillations of coast lines these maritime shelves or benches are sometimes above and sometimes below the water-line, so that the continental borders must be drawn at the edge where the shallow water passes rapidly into deep water.

= J rs > < eS = Sy

Continental Shelf dea Level

Fig. 2. Generalized section of the Continental Margins, Continental Shelf, and Ocean Floor.

Sometimes the sea encroaches considerably over low-lying ~ areas, forming land-locked and shallow seas, as in the seas surrounding the British Islands. These islands rest on a broad expanse of the continental shelf of western Europe. Shallow seas that transgress the land in this way are known as epicontinental seas. The continental shelf of the north-east coast of Australia supports the Great Barrier Reef, but off the Heads, at Port Jackson, it has narrowed down to 15 miles; whilst, to the northward, it unites Australia to New Guinea and the Malayan Islands, and, to the southward, it unites Tasmania to the continental mass by the shallow waters of Bass Strait.

Continental Islands are the result of coastal movements of depression causing encroachments of the sea over continental areas, and isolation of the more elevated portions of the land. Islands of this type often rest on the continental shelf and possess a geological structure similar to the adjoining mainland, as in the case of Tasmania. Some occur as isolated fragments, far removed from the mainland by the submergence of extensive land surfaces, as in the cases of New Zealand and Fiji. Continental islands often take the form of festoons, as the Japanese Islands, the Philippines, the Australasian, and the British Columbian groups.