Produced by WebRover, MWS, Robert Tonsing, and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.) CLIMATE AND TIME [Illustration: FRONTISPIECE N. WINTER SOLSTICE IN APHELION. N. WINTER SOLSTICE IN PERIHELION. W. & A. K. Johnston, Edinb^r. and London.] CLIMATE AND TIME _IN THEIR GEOLOGICAL RELATIONS_ A THEORY OF SECULAR CHANGES OF THE EARTHâS CLIMATE BY JAMES CROLL OF H.M. GEOLOGICAL SURVEY OF SCOTLAND LONDON DALDY, ISBISTER, & CO. 56, LUDGATE HILL 1875 LONDON: PRINTED BY VIRTUE AND CO., CITY ROAD. PREFACE. In the following pages I have endeavoured to give a full and concise statement of the facts and arguments adduced in support of the theory of Secular Changes of the Earthâs Climate. Considerable portions of the volume have already appeared in substance as separate papers in the Philosophical Magazine and other journals during the past ten or twelve years. The theory, especially in as far as it relates to the cause of the glacial epoch, appears to be gradually gaining acceptance with geologists. This, doubtless, is owing to the greatly increased and constantly increasing knowledge of the drift-phenomena, which has induced the almost general conviction that a climate such as that of the glacial epoch could only have resulted from cosmical causes. Considerable attention has been devoted to objections, and to the removal of slight misapprehensions, which have naturally arisen in regard to a subject comparatively new and, in many respects, complex, and beset with formidable difficulties. I have studiously avoided introducing anything of a hypothetical character. All the conclusions are based either on known facts or admitted physical principles. In short, the aim of the work, as will be shown in the introductory chapter, is to prove that secular changes of climate follow, as a necessary effect, from admitted physical agencies, and that these changes, in as far as the past climatic condition of the globe is concerned, fully meet the demand of the geologist. The volume, though not intended as a popular treatise, will be found, I trust, to be perfectly plain and intelligible even to readers not familiar with physical science. I avail myself of this opportunity of expressing my obligations to my colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge, jun., and also to Mr. James Paton, of the Edinburgh Museum of Science and Art, for their valuable assistance rendered while these pages were passing through the press. To the kindness of Mr. James Bennie I am indebted for the copious index at the end of the volume, as well as for many of the facts relating to the glacial deposits of the West of Scotland. JAMES CROLL. EDINBURGH, _March, 1875_. CONTENTS. CHAPTER I. INTRODUCTION. PAGE The Fundamental Problem of Geology.âGeology a Dynamical Science.âThe Nature of a Geological Principle.âTheories of Geological Climate.âGeological Climate dependent on Astronomical Causes.âAn Important Consideration overlooked.âAbstract of the Line of Argument pursued in the Volume 1 CHAPTER II. OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE GLOBE. The absolute Heating-power of Ocean-currents.âVolume of the Gulf-stream.âAbsolute Amount of Heat conveyed by it.âGreater Portion of Moisture in Inter-tropical Regions falls as Rain in those Regions.âLand along the Equator tends to lower the Temperature of the Globe.âInfluence of Gulf-stream on Climate of Europe.âTemperature of Space.âRadiation of a Particle.âProfessor Dove on Normal Temperature.âTemperature of Equator and Poles in the Absence of Ocean-currents.âTemperature of London, how much due to Ocean-currents 23 CHAPTER III. OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE GLOBE.â(_Continued._) Influence of the Gulf-stream on the Climate of the Arctic Regions.âAbsolute Amount of Heat received by the Arctic Regions from the Sun.âInfluence of Ocean-currents shown by another Method.âTemperature of a Globe all Water or all Land according to Professor J. D. Forbes.âAn important Consideration overlooked.âWithout Ocean-currents the Globe would not be habitable.âConclusions not affected by Imperfection of Data 45 CHAPTER IV. OUTLINE OE THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES OF CLIMATE. Eccentricity of the Earthâs Orbit; its Effect on Climate.âGlacial Epoch not the direct Result of an Increase of Eccentricity.âAn important Consideration overlooked.âChange of Eccentricity affects Climate only indirectly.âAgencies which are brought into Operation by an Increase of Eccentricity.âHow an Accumulation of Snow is produced.âThe Effect of Snow on the Summer Temperature.âReason of the Low Summer Temperature of Polar Regions.âDeflection of Ocean-currents the chief Cause of Secular Changes of Climate.âHow the foregoing Causes deflect Ocean-currents.âNearness of the Sun in Perigee a Cause of the Accumulation of Ice.âA remarkable Circumstance regarding the Causes which lead to Secular Changes of Climate.âThe primary Cause an Increase of Eccentricity.âMean Temperature of whole Earth should be greater in Aphelion than in Perihelion.âProfessor Tyndall on the Glacial Epoch.âA general Reduction of Temperature will not produce a Glacial Epoch.âObjection from the present Condition of the Planet Mars 54 CHAPTER V. REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE NORTHERN. AdhÃ©marâs Explanation.âAdhÃ©marâs Theory founded upon a physical Mistake in regard to Radiation.âProfessor J. D. Forbes on Underground Temperature.âGenerally accepted Explanation.âLow Temperature of Southern Hemisphere attributed to Preponderance of Sea.âHeat transferred from Southern to Northern Hemisphere by Ocean-current the true Explanation.âA large Portion of the Heat of the Gulf-stream derived from the Southern Hemisphere 81 CHAPTER VI. EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.âLIEUT. MAURYâS THEORY. Introduction.âOcean-currents, according to Maury, due to Difference of Specific Gravity.âDifference of Specific Gravity resulting from Difference of Temperature.âDifference of Specific Gravity resulting from Difference of Saltness.âMauryâs two Causes neutralize each other.âHow, according to him, Difference in Saltness acts as a Cause 95 CHAPTER VII. EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.âLIEUT. MAURYâS THEORY.â(_Continued._) Methods of determining the Question.âThe Force resulting from Difference of Specific Gravity.âSir John Herschelâs Estimate of the Force.âMaximum Density of Sea-Water.âRate of Decrease of Temperature of Ocean at Equator.âThe actual Amount of Force resulting from Difference of Specific Gravity.âM. Dubuatâs Experiments 115 CHAPTER VIII. EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.âDR. CARPENTERâS THEORY. Gulf-stream according to Dr. Carpenter not due to Difference of Specific Gravity.âFacts to be Explained.âThe Explanation of the Facts.âThe Explanation hypothetical.âThe Cause assigned for the hypothetical Mode of Circulation.âUnder Currents account for all the Facts better than the Gravitation Hypothesis.âKnown Condition of the Ocean inconsistent with that Hypothesis 122 CHAPTER IX. EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.âTHE MECHANICS OF DR. CARPENTERâS THEORY. Experimental Illustration of the Theory.âThe Force exerted by Gravity.âWork performed by Gravity.âCirculation not by Convection.âCirculation depends on Difference in Density of the Equatorial and Polar Columns.âAbsolute Amount of Work which can be performed by Gravity.âHow Underflow is produced.âHow Vertical Descent at the Poles and Ascent at the Equator is produced.âThe Gibraltar Current.âMistake in Mechanics concerning it.âThe Baltic Current 145 CHAPTER X. EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULATION.âDR. CARPENTERâS THEORY.âOBJECTIONS CONSIDERED. _Modus Operandi_ of the Matter.âPolar Cold considered by Dr. Carpenter the _Primum Mobile_.âSupposed Influence of Heat derived from the Earthâs Crust.âCirculation without Difference of Level.âA Confusion of Ideas in Reference to the supposed Agency of Polar Cold.âM. Dubuatâs Experiments.âA Begging of the Question at Issue.âPressure as a Cause of Circulation 172 CHAPTER XI. THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER METHOD. Quantity of Heat which can be conveyed by the General Oceanic Circulation trifling.âTendency in the Advocates of the Gravitation Theory to under-estimate the Volume of the Gulf-stream.âVolume of the Stream as determined by the _Challenger_.âImmense Volume of Warm Water discovered by Captain Nares.âCondition of North Atlantic inconsistent with the Gravitation Theory.âDr. Carpenterâs Estimate of the Thermal Work of the Gulf-stream 191 CHAPTER XII. MR. A. G. FINDLAYâS OBJECTIONS CONSIDERED. Mr. Findlayâs Estimate of the Volume of the Gulf-stream.âMean Temperature of a Cross Section less than Mean Temperature of Stream.âReason of such Diversity of Opinion regarding Ocean-currents.âMore rigid Method of Investigation necessary 203 CHAPTER XIII. THE WIND THEORY OF OCEANIC CIRCULATION. Ocean-Currents not due alone to the Trade-winds.âAn Objection by Maury.âTrade-winds do not explain the Great Antarctic Current.âOcean-currents due to the System of Winds.âThe System of Currents agrees with the System of the Winds.âChart showing the Agreement between the System of Currents and System of Winds.âCause of the Gibraltar Current.âNorth Atlantic an immense Whirlpool.âTheory of Under Currents.âDifficulty regarding Under Currents obviated.âWork performed by the Wind in impelling the Water forward.âThe _Challengerâs_ crucial Test of the Wind and Gravitation Theories.âNorth Atlantic above the Level of Equator.âThermal Condition of the Southern Ocean irreconcilable with the Gravitation Theory 210 CHAPTER XIV. THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE OF CLIMATE. Direction of Currents depends on Direction of the Winds.âCauses which affect the Direction of Currents will affect Climate.âHow Change of Eccentricity affects the Mode of Distribution of the Winds.âMutual Reaction of Cause and Effect.âDisplacement of the Great Equatorial Current.âDisplacement of the Median Line between the Trades, and its Effect on Currents.âOcean-currents in Relation to the Distribution of Plants and Animals.âAlternate Cold and Warm Periods in North and South.âMr. Darwinâs Views quoted.âHow Glaciers at the Equator may be accounted for.âMigration across the Equator 226 CHAPTER XV. WARM INTER-GLACIAL PERIODS. Alternate Cold and Warm Periods.âWarm Inter-glacial Periods a Test of Theories.âReason why their Occurrence has not been hitherto recognised.âInstances of Warm Inter-glacial Periods.âDranse, DÃ¼rnten, Hoxne, Chapelhall, Craiglockhart, Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, Sweden, Ohio, Cromer, Mundesley, &c., &c.âCave and River Deposits.âOccurrence of Arctic and Warm Animals in some Beds accounted for.âMr. Boyd Dawkinsâs Objections.âOccurrence of Southern Shells in Glacial Deposits.âEvidence of Warm Inter-glacial Periods from Mineral Borings.âStriated Pavements.âReason why Inter-glacial Land-surfaces are so rare 236 CHAPTER XVI. WARM INTER-GLACIAL PERIODS IN ARCTIC REGIONS. Cold Periods best marked in Temperate, and Warm Periods in Arctic, Regions.âState of Arctic Regions during Glacial Period.âEffects of Removal of Ice from Arctic Regions.âOcean-currents; Influence on Arctic Climate.âReason why Remains of Inter-glacial Period are rare in Arctic Regions.âRemains of Ancient Forests in Banksâs Land, Prince Patrickâs Island, &c.âOpinions of Sir R. Murchison, Captain Osborn, and Professor Haughton.âTree dug up by Sir E. Belcher in lat. 75Â° N. 258 CHAPTER XVII. FORMER GLACIAL EPOCHS.âREASON OF THE IMPERFECTION OF GEOLOGICAL RECORDS IN REFERENCE TO THEM. Two Reasons why so little is known of Glacial Epochs.âEvidence of Glaciation to be found on Land-surfaces.âWhere are all our ancient Land-surfaces?âThe stratified Rocks consist of a Series of old Sea-bottoms.âTransformation of a Land-surface into a Sea-bottom obliterates all Traces of Glaciation.âWhy so little remains of the Boulder Clays of former Glacial Epochs.âRecords of the Glacial Epoch are fast disappearing.âIcebergs do not striate the Sea-bottom.âMr. Campbellâs Observations on the Coast of Labrador.âAmount of Material transported by Icebergs much exaggerated.âMr. Packard on the Glacial Phenomena of Labrador.âBoulder Clay the Product of Land-ice.âPalÃ¦ontological Evidence.âPaucity of Life characteristic of a Glacial Period.âWarm Periods better represented by Organic Remains than cold.âWhy the Climate of the Tertiary Period was supposed to be warmer than the present.âMr. James Geikie on the Defects of PalÃ¦ontological Evidence.âConclusion 266 CHAPTER XVIII. FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF. Cambrian Conglomerate of Islay and North-west of Scotland.âIce-action in Ayrshire and Wigtownshire during Silurian Period.âSilurian Limestones in Arctic Legions.âProfessor Ramsay on Ice-action during Old Red Sandstone Period.âWarm Climate in Arctic Regions during Old Red Sandstone Period.âProfessor Geikie and Mr. James Geikie on a Glacial Conglomerate of Lower Carboniferous Age.âProfessor Haughton and Professor Dawson on Evidence of Ice-action during Coal Period.âMr. W. T. Blanford on Glaciation in India during Carboniferous Period.âCarboniferous Formations of Arctic Regions.âProfessor Ramsay on Permian Glaciers.âPermian Conglomerate in Arran.âProfessor Hull on Boulder Clay of Permian Age.âPermian Boulder Clay of Natal.âOolitic Boulder Conglomerate in Sutherlandshire.â-Warm Climate in North Greenland during Oolitic Period.âMr. Godwin-Austen on Ice-action during Cretaceous Period.âGlacial Conglomerates of Eocene Age in the Alps.âM. Gastaldi on the Ice-transported Limestone Blocks of the Superga.âProfessor Heer on the Climate of North Greenland during Miocene Period 292 CHAPTER XIX. GEOLOGICAL TIME.âPROBABLE DATE OF THE GLACIAL EPOCH. Geological Time measurable from Astronomical Data.âM. Leverrierâs FormulÃ¦.âTables of Eccentricity for 3,000,000 Years in the Past and 1,000,000 Years in the Future.âHow the Tables have been computed.âWhy the Glacial Epoch is more recent than had been supposed.âFigures convey a very inadequate Conception of immense Duration.âMode of representing a Million of Years.âProbable Date of the Glacial Epoch 311 CHAPTER XX. GEOLOGICAL TIME.âMETHOD OF MEASURING THE RATE OF SUBAÃRIAL DENUDATION. Rate of SubaÃ«rial Denudation a Measure of Time.âRate determined from Sediment of the Mississippi.âAmount of Sediment carried down by the Mississippi; by the Ganges.âProfessor Geikie on Modern Denudation.âProfessor Geikie on the Amount of Sediment conveyed by European Rivers.âRate at which the Surface of the Globe is being denuded.âAlfred Tylor on the Sediment of the Mississippi.âThe Law which determines the Rate of Denudation.âThe Globe becoming less oblate.âCarrying Power of our River Systems the true Measure of Denudation.âMarine Denudation, trifling in comparison to SubaÃ«rial.âPrevious Methods of measuring Geological Time.âCircumstances which show the recent Date of the Glacial Epoch.âProfessor Ramsay on Geological Time 329 CHAPTER XXI. THE PROBABLE AGE AND ORIGIN OF THE SUN. Gravitation Theory.âAmount of Heat emitted by the Sun.âMeteoric Theory.âHelmholtzâs Condensation Theory.âConfusion of Ideas.âGravitation not the chief Source of the Sunâs Heat.âOriginal Heat.âSource of Original Heat.âOriginal Heat derived from Motion in Space.âConclusion as to Date of Glacial Epoch.âFalse Analogy.âProbable Date of Eocene and Miocene Periods 346 CHAPTER XXII. A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY ROCKS OF THE GLOBE. Prevailing Methods defective.âMaximum Thickness of British Rocks.âThree Elements in the Question.âProfessor Huxley on the Rate of Deposition.âThickness of Sedimentary Rocks enormously over-estimated.âObserved Thickness no Measure of mean Thickness.âDeposition of Sediment principally along Sea-margin.âMistaken Inference regarding the Absence of a Formation.âImmense Antiquity of existing Oceans 360 CHAPTER XXIII. THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND DURING THE GLACIAL EPOCH. Displacement of the Earthâs Centre of Gravity by Polar Ice-cap.âSimple Method of estimating Amount of Displacement.âNote by Sir W. Thomson on foregoing Method.âDifference between Continental Ice and a Glacier.âProbable Thickness of the Antarctic Ice-cap.âProbable Thickness of Greenland Ice-sheet.âThe Icebergs of the Southern Ocean.âInadequate Conceptions regarding the Magnitude of Continental Ice 368 CHAPTER XXIV. THE PHYSICAL CAUSE OF THE SUBMERGENCE AND EMERGENCE OF THE LAND DURING THE GLACIAL EPOCH.â(_Continued._) Extent of Submergence from Displacement of Earthâs Centre of Gravity.âCircumstances which show that the Glacial Submergence resulted from Displacement of the Earthâs Centre of Gravity.âAgreement between Theory and Observed Facts.âSir Charles Lyell on submerged Areas during Tertiary Period.âOscillations of Sea-Level in Relation to Distribution.âExtent of Submergence on the Hypothesis that the Earth is fluid in the Interior 387 CHAPTER XXV. THE INFLUENCE OF THE OBLIQUITY OF THE ECLIPTIC ON CLIMATE AND ON THE LEVEL OF THE SEA. The direct Effect of Change of Obliquity on Climate.âMr. Stockwell on the maximum Change of Obliquity.âHow Obliquity affects the Distribution of Heat over the Globe.âIncrease of Obliquity diminishes the Heat at the Equator and increases that at the Poles.âInfluence of Change of Obliquity on the Level of the Sea.âWhen the Obliquity was last at its superior Limit.âProbable Date of the 25-foot raised Beach.âProbable Extent of Rise of Sea-level resulting from Increase of Obliquity.âLieutenant-Colonel Draysonâs and Mr. Beltâs Theories.âSir Charles Lyell on Influence of Obliquity 398 CHAPTER XXVI. COAL AN INTER-GLACIAL FORMATION. Climate of Coal Period Inter-glacial in Character.âCoal Plants indicate an Equable, not a Tropical Climate.âConditions necessary for Preservation of Coal Plants.âOscillations of Sea-level necessarily implied.âWhy our Coal-fields contain more than One Coal-seam.âTime required to form a Bed of Coal.âWhy Coal Strata contain so little evidence of Ice-action.âLand Flat during Coal Period.âLeading Idea of the Theory.âCarboniferous Limestones 420 CHAPTER XXVII. PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS TO THE BOULDER CLAY OF CAITHNESS. Character of Caithness Boulder Clay.âTheories of the Origin of the Caithness Clay.âMr. Jamiesonâs Theory.âMr. C. W. Peachâs Theory.âThe proposed Theory.âThickness of Scottish Ice-sheet.âPentlands striated on their Summits.âScandinavian Ice-sheet.âNorth Sea filled with Land-ice.âGreat Baltic Glacier.âJutland and Denmark crossed by Ice.âSir R. Murchisonâs Observations.âOrkney, Shetland, and Faroe Islands striated across.âLoess accounted for.âProfessor Geikieâs Suggestion.âProfessor Geikie and B. N. Peachâs Observations on East Coast of Caithness.âEvidence from Chalk Flints and Oolitic Fossils in Boulder Clay 435 CHAPTER XXVIII. NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG BLOCKS. Transport of Blocks; Theories of.âEvidence of Continental Ice.âPennine Range probably striated on Summit.âGlacial Drift in Centre of England.âMr. Lacy on Drift of Cotteswold Hills.âEngland probably crossed by Land-ice.âMr. Jackâs Suggestion.âShedding of Ice North and South.âSouth of England Ice-sheet.âGlaciation of West Somerset.âWhy Ice-markings are so rare in South of England.âForm of Contortion produced by Land-ice 456 CHAPTER XXIX. EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD IN BRITAIN. Remarks on the Drift Deposits.âExamination of Drift by Borings.âBuried River Channel from Kilsyth to Grangemouth.âChannels not excavated by Sea nor by Ice.âSection of buried Channel at Grangemouth.âMr. Milne Homeâs Theory.âGerman Ocean dry Land.âBuried River Channel from Kilsyth to the Clyde.âJournal of Borings.âMarine Origin of the Drift Deposits.âEvidence of Inter-glacial Periods.âOscillations of Sea-Level.âOther buried River Channels 466 CHAPTER XXX. THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.âTHEORIES OF GLACIER-MOTION. Why the Question of Glacier-motion has been found to be so difficult.âThe Regelation Theory.âIt accounts for the Continuity of a Glacier, but not for its Motion.âGravitation proved by Canon Moseley insufficient to shear the Ice of a Glacier.âMr. Matthewâs Experiment.âNo Parallel between the bending of an Ice Plank and the shearing of a Glacier.âMr. Ballâs Objection to Canon Moseleyâs Experiment.âCanon Moseleyâs Method of determining the Unit of Shear.âDefect of Method.âMotion of a Glacier in some Way dependent on Heat.âCanon Moseleyâs Theory.âObjections to his Theory.âProfessor James Thomsonâs Theory.âThis Theory fails to explain Glacier-motion.âDe Saussure and Hopkinsâs âSlidingâ Theories.âM. Charpentierâs âDilatationâ Theory.âImportant Element in the Theory 495 CHAPTER XXXI. THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.âTHE MOLECULAR THEORY. Present State of the Question.âHeat necessary to the Motion of a Glacier.âIce does not shear in the Solid State.âMotion of a Glacier _molecular_.âHow Heat is transmitted through Ice.âMomentary Loss of Shearing Force.âThe _Rationale_ of Regelation.âThe Origin of âCrevasses.ââEffects of Tension.âModification of Theory.âFluid Molecules crystallize in Interstices.âExpansive Force of crystallizing Molecules a Cause of Motion.âInternal molecular Pressure the chief Moving Power.âHow Ice can excavate a Rock Basin.âHow Ice can ascend a Slope.âHow deep River Valleys are striated across.âA remarkable Example in the Valley of the Tay.âHow Boulders can he carried from a lower to a higher Level 514 APPENDIX. I. Opinions expressed previous to 1864 regarding the Influence of the Eccentricity of the Earthâs Orbit on Climate 528 II. On the Nature of Heat-Vibrations 544 III. On the Reason why the Difference of Reading between a Thermometer exposed to direct Sunshine and One Shaded diminishes as we ascend in the Atmosphere 547 IV. Remarks on Mr. J. Y. Buchananâs Theory of the Vertical Distribution of Temperature of the Ocean 550 V. On the Cause of the Cooling Effect produced on Solids by Tension 552 VI. The Cause of Regelation 554 VII. List of Papers which have appeared in Dr. A. Petermannâs _Geographische Mittheilungen_ relating to the Gulf-stream and Thermal Condition of the Arctic Regions 556 VIII. List of Papers by the Author to which Reference is made in this Volume 560 INDEX 563 LIST OF PLATES. EARTHâS ORBIT WHEN ECCENTRICITY IS AT ITS SUPERIOR LIMIT _Frontispiece._ PLATE I. SHOWING AGREEMENT BETWEEN THE SYSTEM OF OCEAN-CURRENTS AND WINDS _To face page_ 212 II. SHOWING HOW OPPOSING CURRENTS INTERSECT EACH OTHER 219 III. SECTION OF MID-ATLANTIC 222 IV. DIAGRAM REPRESENTING THE VARIATIONS OF ECCENTRICITY OF THE EARTHâS ORBIT 313 V. SHOWING PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE 449 VI. SHOWING PATH OF ICE ACROSS CAITHNESS 453 VII. MAP OF THE MIDLAND VALLEY (SCOTLAND), SHOWING BURIED RIVER CHANNELS 471 CHAPTER I. INTRODUCTION. The Fundamental Problem of Geology.âGeology a Dynamical Science.âThe Nature of a Geological Principle.âTheories of Geological Climate.âGeological Climate dependent on Astronomical Causes.âAn Important Consideration overlooked.âAbstract of the Line of Argument pursued in the Volume. _The Fundamental Problem of Geology._âThe investigation of the successive changes and modifications which the earthâs crust has undergone during past ages is the province of geology. It will be at once admitted that an acquaintance with the agencies by means of which those successive changes and modifications were effected, is of paramount importance to the geologist. What, then, are those agencies? Although volcanic and other subterranean eruptions, earthquakes, upheavals, and subsidences of the land have taken place in all ages, yet no truth is now better established than that it is not by these convulsions and cataclysms of nature that those great changes were effected. It was rather by the ordinary agencies that we see every day at work around us, such as rain, rivers, heat and cold, frost and snow. The valleys were not produced by violent dislocations, nor the hills by sudden upheavals, but were actually carved out of the solid rock, silently and gently, by the agencies to which we have referred. âThe tools,â to quote the words of Professor Geikie, âby which this great work has been done are of the simplest and most every-day orderâthe air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the sea. These tools have been at work from the earliest times of which any geological record has been preserved. Indeed, it is out of the accumulated chips and dust which they have made, afterwards hardened into solid rock and upheaved, that the very framework of our continents has been formed.â[1] It will be observedâand this is the point requiring particular attentionâthat the agencies referred to are the ordinary meteorological or climatic agencies. In fact, it is these agencies which constitute climate. The various peculiarities or modifications of climate result from a preponderance of one or more of these agencies over the rest. When heat, for example, predominates, we have a hot or tropical climate. When cold and frost predominate, we have a rigorous or arctic climate. With moisture in excess, we have a damp and rainy climate; and so on. But this is not all. These climatic agencies are not only the factors which carved out the rocky face of the globe into hill and dale, and spread over the whole a mantle of soil; but by them are determined the character of the _flora_ and _fauna_ which exist on that soil. The flora and fauna of a district are determined mainly by the character of the climate, and not by the nature of the soil, or the conformation of the ground. It is from difference of climate that tropical life differs so much from arctic, and both these from the life of temperate regions. It is climate, and climate alone, that causes the orange and the vine to blossom, and the olive to flourish, in the south, but denies them to the north, of Europe. It is climate, and climate alone, that enables the forest tree to grow on the plain, but not on the mountain top; that causes wheat and barley to flourish on the mainland of Scotland, but not on the steppes of Siberia. Again, if we compare flat countries with mountainous, highlands with lowlands, or islands with continents, we shall find that difference of climatic conditions is the chief reason why life in the one differs so much from life in the other. And if we turn to the sea we find that organic life is there as much under the domain of climate as on the land, only the conditions are much less complex. For in the case of the sea, difference in the temperature of the water may be said to constitute almost the only difference of climatic conditions. If there is one fact more clearly brought out than another by the recent deep-sea explorations, it is this, that nothing exercises so much influence on organic life in the ocean as the temperature of the water. In fact, so much is this the case, that warm zones were found to be almost equivalent to zones of life. It was found that even the enormous pressure at the bottom of the ocean does not exercise so much influence on life as the temperature of the water. There are few, I presume, who reflect on the subject that will not readily admit that, whether as regards the great physical changes which are taking place on the surface of our globe, or as regards the growth and distribution of plant and animal life, the ordinary climatic agents are the real agents at work, and that, compared with them, all other agencies sink into insignificance. It will also be admitted that what holds true of the present holds equally true of the past. Climatic agents are not only now the most important and influential; they have been so during all past geological ages. They were so during the Cainozoic as much as during the present; and there is no reason for supposing they were otherwise during the remoter Mesozoic and PalÃ¦ozoic epochs. They have been the principal factors concerned in that long succession of events and changes which have taken place since the time of the solidification of the earthâs crust. The stratified rocks of the globe contain all the records which now remain of their action, and it is the special duty of the geologist to investigate and read those records. It will be at once admitted that in order to a proper understanding of the events embodied in these records, an acquaintance with the agencies by which they were produced is of the utmost importance. In fact, it is only by this means that we can hope to arrive at their rational explanation. A knowledge of the agents, and of the laws of their operations, is, in all the physical sciences, the means by which we arrive at a rational comprehension of the effects produced. If we have before us some complex and intricate effects which have been produced by heat, or by light, or by electricity, &c., in order to understand them we must make ourselves acquainted with the agents by which they were produced and the laws of their action. If the effects to be considered be, for example, those of heat, then we must make ourselves acquainted with this agent and its laws. If they be of electricity, then a knowledge of electricity and its laws becomes requisite. This is no mere arbitrary mode of procedure which may be adopted in one science and rejected in another. It is in reality a necessity of thought arising out of the very constitution of our intellect; for the objective law of the agent is the conception by means of which the effects are subjectively united in a rational unity. We may describe, arrange, and classify the effects as we may, but without a knowledge of the laws of the agent we can have no rational unity. We have not got the higher conception by which they can be _comprehended_. It is this relationship between the effects and the laws of the agent, a knowledge of which really constitutes a science. We might examine, arrange, and describe for a thousand years the effects produced by heat, and still we should have no science of heat unless we had a knowledge of the laws of that agent. The effects would never be seen to be necessarily connected with anything known to us; we could not connect them with any rational principle from which they could be deduced _Ã priori_. The same remarks hold, of course, equally true of all sciences, in which the things to be considered stand in the relationship of cause and effect. Geology is no exception. It is not like systematic botany, a mere science of classification. It has to explain and account for effects produced; and these effects can no more be explained without a knowledge of the laws of the agents which produced them, than can the effects of heat without a knowledge of the laws of heat. The only distinction between geology and heat, light, electricity, &c., is, that in geology the effects to be explained have almost all occurred already, whereas in these other sciences effects actually taking place have to be explained. But this distinction is of no importance to our present purpose, for effects which have already occurred can no more be explained without a knowledge of the laws of the agent which produced them than can effects which are in the act of occurring. It is, moreover, not strictly true that all the effects to be explained by the geologist are already past. It falls within the scope of his science to account for the changes which are at present taking place on the earthâs crust. No amount of description, arrangement, and classification, however perfect or accurate, of the facts which come under the eye of the geologist can ever constitute a science of geology any more than a description and classification of the effects of heat could constitute a science of heat. This will, no doubt, be admitted by every one who reflects upon the subject, and it will be maintained that geology, like every other science, must possess principles applicable to the facts. But here confusion and misconception will arise unless there be distinct and definite ideas as to what ought to constitute a geological principle. It is not every statement or rule that may apply to a great many facts, which will constitute a geological principle. A geological principle must bear the same characteristics as the principles of those sciences to which we have referred. What, then, is the nature of the principles of light, heat, electricity, &c.? The principles of heat are the laws of heat. The principles of electricity are the laws of electricity. And these laws are nothing more nor less than the ways according to which these agents produce their effects. The principles of geology are therefore the laws of geology. But the laws of geology must be simply the laws of the geological agents, or, in other words, the methods by which they produce their effects. Any other so-called principle can be nothing more than an empirical rule, adopted for convenience. Possessing no rationality in itself, it cannot be justly regarded as a principle. In order to rationality the principle must be either resolvable into, or logically deducible from, the laws of the agents. Unless it possess this quality we cannot give the explanation _Ã priori_. The reason of all this is perfectly obvious. The things to be explained are effects; and the relationship between cause and effect affords the subjective connection between the principle and the explanation. The explanation follows from the principle simply as the effect results from the laws of the agent or cause. _Theories of Geological Climate._âWe have already seen that the geological agents are chiefly the ordinary climatic agents. Consequently, the main principles of geology must be the laws of the climatic agents, or some logical deductions from them. It therefore follows that, in order to a purely scientific geology, the grand problem must be one of geological climate. It is through geological climate that we can hope to arrive ultimately at principles which will afford a rational explanation of the multifarious facts which have been accumulating during the past century. The facts of geology are as essential to the establishment of the principles, as the facts of heat, light, and electricity are essential to the establishment of the principles of these sciences. A theory of geological climate devised without reference to the facts would be about as worthless as a theory of heat or of electricity devised without reference to the facts of these sciences. It has all along been an admitted opinion among geologists that the climatic condition of our globe has not, during past ages, been uniformly the same as at present. For a long time it was supposed that during the Cambrian, Silurian, and other early geological periods, the climate of our globe was much hotter than now, and that ever since it has been gradually becoming cooler. And this high temperature of PalÃ¦ozoic ages was generally referred to the influence of the earthâs internal heat. It has, however, been proved by Sir William Thomson[2] that the general climate of our globe could not have been sensibly affected by internal heat at any time more than ten thousand years after the commencement of the solidification of the surface. This physicist has proved that the present influence of internal heat on the temperature amounts to about only 1/75th of a degree. Not only is the theory of internal heat now generally abandoned, but it is admitted that we have no good geological evidence that climate was much hotter during PalÃ¦ozoic ages than now; and much less, that it has been becoming _uniformly_ colder. The great discovery of the glacial epoch, and more lately that of a mild and temperate condition of climate extending during the Miocene and other periods to North Greenland, have introduced a complete revolution of ideas in reference to geological climate. Those discoveries showed that our globe has not only undergone changes of climate, but changes of the most extraordinary character. They showed that at one time not only an arctic condition of climate prevailed in our island, but that the greater part of the temperate region down to comparatively low latitudes was buried under ice, while at other periods Greenland and the Arctic regions, probably up to the North Pole, were not only free from ice, but were covered with a rich and luxuriant vegetation. To account for these extraordinary changes of climate has generally been regarded as the most difficult and perplexing problem which has fallen to the lot of the geologist. Some have attempted to explain them by assuming a displacement of the earthâs axis of rotation in consequence of the uprising of large mountain masses on some part of the earthâs surface. But it has been shown by Professor Airy,[3] Sir William Thomson,[4] and others, that the earthâs equatorial protuberance is such that no geological change on its surface could ever possibly alter the position of the axis of rotation to an extent which could at all sensibly affect climate. Others, again, have tried to explain the change of climate by supposing, with Poisson, that the earth during its past geological history may have passed through hotter and colder parts of space. This is not a very satisfactory hypothesis. There is no doubt a difference in the quantity of force in the form of heat passing through different parts of space; but space itself is not a substance which can possibly be either cold or hot. If, therefore, we were to adopt this hypothesis, we must assume that the earth during the hot periods must have been in the vicinity of some other great source of heat and light besides the sun. But the proximity of a mass of such magnitude as would be sufficient to affect to any great extent the earthâs climate would, by its gravity, seriously disarrange the mechanism of our solar system. Consequently, if our solar system had ever, during any former period of its history, really come into the vicinity of such a mass, the orbits of the planets ought at the present day to afford some evidence of it. But again, in order to account for a cold period, such as the glacial epoch, we have to assume that the earth must have come into the vicinity of a cold body.[5] But recent discoveries in regard to inter-glacial periods are wholly irreconcilable with this theory. A change in the obliquity of the ecliptic has frequently been, and still is, appealed to as an explanation of geological climate. This theory appears, however, to be beset by a twofold objection: (1), it can be shown from celestial mechanics, that the variations in the obliquity of the ecliptic must always have been so small that they could not materially affect the climatic condition of the globe; and (2), even admitting that the obliquity could change to an indefinite extent, it can be shown[6] that no increase or decrease, however great, could possibly account for either the glacial epoch or a warm temperate condition of climate in polar regions. The theory that the sun is a variable star, and that the glacial epochs of the geologists may correspond to periods of decrease in the sunâs heat, has lately been advanced. This theory is also open to two objections: (1), a general diminution of heat[7] never could produce a glacial epoch; and (2), even if it could, it would not explain inter-glacial periods. The only other theory on the subject worthy of notice is that of Sir Charles Lyell. Those extraordinary changes of climate are, according to his theory, attributed to differences in the distribution of land and water. Sir Charles concludes that, were the land all collected round the poles, while the equatorial zones were occupied by the ocean, the general temperature would be lowered to an extent that would account for the glacial epoch. And, on the other hand, were the land all collected along the equator, while the polar regions were covered with sea, this would raise the temperature of the globe to an enormous extent. It will be shown in subsequent chapters that this theory does not duly take into account the prodigious influence exerted on climate by means of the heat conveyed from equatorial to temperate and polar regions by means of ocean-currents. In Chapters II. and III. I have endeavoured to prove (1), that were it not for the heat conveyed from equatorial to temperate and polar regions by this means, the thermal condition of the globe would be totally different from what it is at present; and (2), that the effect of placing all the land along the equator would be diametrically the opposite of that which Sir Charles supposes. But supposing that difference in the distribution of land and water would produce the effects attributed to it, nevertheless it would not account for those extraordinary changes of climate which have occurred during geological epochs. Take, for example, the glacial epoch. Geologists almost all agree that little or no change has taken place in the relative distribution of sea and land since that _epoch_. All our main continents and islands not only existed then as they do now, but every year is adding to the amount of evidence which goes to show that so recent, geologically considered, is the glacial epoch that the very contour of the surface was pretty much the same then as it is at the present day. But this is not all; for even should we assume (1), that a difference in the distribution of sea and land would produce the effects referred to, and (2), that we had good geological evidence to show that at a very recent period a form of distribution existed which would produce the necessary glacial conditions, still the glacial epoch would not be explained, for the phenomena of warm inter-glacial periods would completely upset the theory. _Geological Climate depending on Astronomical Causes._âFor a good many years past, an impression has been gradually gaining ground amongst geologists that the glacial epoch, as well as the extraordinary condition of climate which prevailed in arctic regions during the Miocene and other periods, must some way or other have resulted from a cosmical cause; but all seemed at a loss to conjecture what that cause could possibly be. It was apparent that the cosmical cause must be sought for in the relations of our earth to the sun; but a change in the obliquity of the ecliptic and the eccentricity of the earthâs orbit are the only changes from which any sensible effect on climate could possibly be expected to result. It was shown, however, by Laplace that the change of obliquity was confined within so narrow limits that it has scarcely ever been appealed to as a cause seriously affecting climate. The only remaining cause to which appeal could be made was the change in the eccentricity of the earthâs orbitâprecession of the equinoxes without eccentricity producing, of course, no effect whatever on climate. Upwards of forty years ago Sir John Herschel and a few other astronomers directed their attention to the consideration of this cause, but the result arrived at was adverse to the supposition that change of eccentricity could greatly affect the climate of our globe. As some misapprehension seems to prevail with reference to this, I would take the liberty of briefly adverting to the history of the matter,âreferring the reader to the Appendix for fuller details. About the beginning of the century some writers attributed the lower temperature of the southern hemisphere to the fact that the sun remains about seven days less on that hemisphere than on the northern; their view being that the southern hemisphere on this account receives seven days less heat than the northern. Sir Charles Lyell, in the first edition of his âPrinciples,â published in 1830, refers to this as a cause which might produce some slight effect on climate. Sir Charlesâs remarks seem to have directed Sir John Herschelâs attention to the subject, for in the latter part of the same year he read a paper before the Geological Society on the astronomical causes which may influence geological phenomena, in which, after pointing out the mistake into which Sir Charles had been led in concluding that the southern hemisphere receives less heat than the northern, he considers the question as to whether geological climate could be influenced by changes in the eccentricity of the earthâs orbit. He did not appear at the time to have been aware of the conclusions arrived at by Lagrange regarding the superior limit of the eccentricity of the earthâs orbit; but he came to the conclusion that possibly the climate of our globe may have been affected by variations in the eccentricity of its orbit. âAn amount of variation,â he says, âwhich we need not hesitate to admit (at least provisionally) as a possible one, may be productive of considerable diversity of climate, and may operate during great periods of time either to mitigate or to exaggerate the difference of winter and summer temperatures, so as to produce alternately in the same latitude of either hemisphere a perpetual spring, or the extreme vicissitudes of a burning summer and a rigorous winter.â This opinion, however, was unfortunately to a great extent nullified by the statement which shortly afterwards appeared in his âTreatise on Astronomy,â and also in the âOutlines of Astronomy,â to the effect that the elliptic form of the earthâs orbit has but a very trifling influence in producing variation of temperature corresponding to the sunâs distance; the reason being that whatever may be the ellipticity of the orbit, it follows that equal amounts of heat are received from the sun in passing over equal angles round it, in whatever part of the ellipse those angles may be situated. Those angles will of course be described in unequal times, but the greater proximity of the sun exactly compensates for the more rapid description, and thus an equilibrium of heat is maintained. The sun, for example, is much nearer the earth when he is over the southern hemisphere than he is when over the northern; but the southern hemisphere does not on this account receive more heat than the northern; for, owing to the greater velocity of the earth when nearest the sun, the sun does not remain so long on the southern hemisphere as he does on the northern. These two effects so exactly counterbalance each other that, whatever be the extent of the eccentricity, the total amount of heat reaching both hemispheres is the same. And he considered that this beautiful compensating principle would protect the climate of our globe from being seriously affected by an increase in the eccentricity of its orbit, unless the extent of that increase was very great. âWere it not,â he says, âfor this, the eccentricity of the orbit would materially influence the transition of seasons. The fluctuation of distance amounts to nearly 1/30th of its mean quantity, and consequently the fluctuation in the sunâs direct heating power to double this, or 1/15th of the whole. Now the perihelion of the orbit is situated nearly at the place of the northern winter solstice; so that, were it not for the compensation we have just described, the effect would be to exaggerate the difference of summer and winter in the southern hemisphere, and to moderate it in the northern; thus producing a more violent alternation of climate in the one hemisphere, and an approach to perpetual spring in the other. As it is, however, no such inequality subsists, but an equal and impartial distribution of heat and light is accorded to both.â[8] Herschelâs opinion was shortly afterwards adopted and advocated by Arago[9] and by Humboldt.[10] Arago, for example, states that so little is the climate of our globe affected by the eccentricity of its orbit, that even were the orbit to become as eccentric as that of the planet Pallas (that is, as great as 0Â·24), âstill this would not alter in any appreciable manner the mean thermometrical state of the globe.â This idea, supported by these great authorities, got possession of the public mind; and ever since it has been almost universally regarded as settled that the great changes of climate indicated by geological phenomena could not have resulted from any change in the relation of the earth to the sun. There is, however, one effect that was not regarded as compensated. The total amount of heat received by the earth is inversely proportional to the minor axis of its orbit; and it follows, therefore, that the greater the eccentricity, the greater is the total amount of heat received by the earth. On this account it was concluded that an increase of eccentricity would tend to a certain extent to produce a warmer climate. All those conclusions to which I refer, arrived at by astronomers, are perfectly legitimate so far as the direct effects of eccentricity are concerned; and it was quite natural, and, in fact, proper to conclude that there was nothing in the mere increase of eccentricity that could produce a glacial epoch. How unnatural would it have been to have concluded that an increase in the quantity of heat received from the sun should lower the temperature, and cover the country with snow and ice! Neither would excessively cold winters, followed by excessively hot summers, produce a glacial epoch. To assert, therefore, that the purely astronomical causes could produce such an effect would be simply absurd. _Important Consideration overlooked._âThe important fact, however, was overlooked that, although the glacial epoch could not result _directly_ from an increase of eccentricity, it might nevertheless do so _indirectly_. Although an increase of eccentricity could have no direct tendency to lower the temperature and cover our country with ice, yet it might bring into operation physical agents which would produce this effect. If, instead of endeavouring to trace a direct connection between a high condition of eccentricity and a glacial condition of climate, we turn our attention to the consideration of what are the physical effects which result from an increase of eccentricity, we shall find that a host of physical agencies are brought into operation, the combined effect of which is to lower to a very great extent the temperature of the hemisphere whose winters occur in aphelion, and to raise to nearly as great an extent the temperature of the opposite hemisphere, whose winters of course occur in perihelion. Until attention was directed to those physical circumstances to which I refer, it was impossible that the true cause of the glacial epoch could have been discovered; and, moreover, many of the indirect and physical effects, which in reality were those that brought about the glacial epoch, could not, in the nature of things, have been known previously to recent discoveries in the science of heat. The consideration and discussion of those various physical agencies are the chief aim of the following pages. _Abstract of the Line of Argument pursued in this Volume._âI shall now proceed to give a brief abstract of the line of argument pursued in this volume. But as a considerable portion of it is devoted to the consideration of objections and difficulties bearing either directly or indirectly on the theory, it will be necessary to point out what those difficulties are, how they arose, and the methods which have been adopted to overcome them. Chapter IV. contains an outline of the physical agencies affecting climate which are brought into operation by an increase of eccentricity. By far the most important of all those agencies, and the one which mainly brought about the glacial epoch, is the _Deflection_ of Ocean-Currents. The consideration of the indirect physical connection between a high state of eccentricity and the deflection of ocean-currents, and also the enormous influence on climate which results from this deflection constitute not only the most important part of the subject, but the one beset with the greatest amount of difficulties. The difficulties besetting this part of the theory arise mainly from the imperfect state of our knowledge, (1st) with reference to the absolute amount of heat transferred from equatorial to temperate and polar regions by means of ocean-currents and the influence which the heat thus transferred has on the distribution of temperature on the earthâs surface; and (2nd) in connection with the physical cause of ocean circulation. In Chapters II. and
{'title': 'Climate and Time in Their Geological Relations by James Croll', 'source': 'Project Gutenberg', 'lang': 'en'}