THE GREAT ICE AGE.*
AN endeavour is here made by Mr. James Geikie to correlate the glacial epoch with the advent of man. The pre-historic evidences -of the existence of the human race, at a time when our country was tenanted by animals now extinct, acquire greater precision and fresh interest when viewed in connection with changes of -climate. There can be no doubt that the book before us shows everywhere the marks'of accurate observation, wide research, and sound reasoning. It presents in a readable form the chief features 'of the great Ice Age, and illustrates them very amply from those great tracts of Scotland in which glaciation has left its most distinct and most enduring marks.
We shall offer in our account of Mr. Geikie's work what must prove a most inadequate outline of hisevidence and his arguments. It is, however, quite impossible to deal adequately with a complex scientific question.in the space at' our disposal. We must there- fore be satisfied with a summary of the contents of the present work, noting more especially the order in which the evidences of a glacial period are discussed, and the conclusions to which that discussion tends.
• Tile Great Ice Age. By James Oekic. Loudon: Isbister. IS74
The latest or superficial formations of the earth's surface, commonly called Post-tertiary or Quaternary, are recognised as of three periods. Of these, the oldest is known as the Till, and is considered to have been produced through the agency of a vast system of glaciers, or rather of a gigantic ice-sheet, which covered the whole of Scotland, filled up our seas, and extended even to the Outer Hebrides. The stones found in this till show marks of glaciation, and especially ice-scratches, while the subjacent rooks exhibit similar signs, often serving to indicate the direction of the flow. The precise mode in which the till originated has been often debated ; Mr. Geikie dismisses the iceberg, and other early theories in his third chapter. In the fourth, he gives Mr. Croll's theory of glacier motion ; in the fifth, he illustrates this part of the subject by a view of the glacial phenomena of Iceland ; and in the sixth and seventh chapters, he applies the lessons drawn from existing glacial actions to the origin of the Scotch till, with its accompanying rock striations and groovings.
The next four chapters of this book are devoted to the extremely important and yet most intricate subject of the causes which pro- duced those great comical changes of climate of which the till affords proof. Mr. Cron% theory, attributing these changes in- directly to an increase in the eccentricity of the earth's orbit, is adopted by our author. The earth, travelling in its elliptic orbit; receives now, whennearest to the. sun, nearly 7 per cent. more heat than when in aphelion ; but owing to the unequal pace at which our globe travels when in these two positions, each hemisphere receives at present the same tete' yearly amount of heat. But Mr. Cron concludes that, if at a period of maximum ellipticity of the orbit,
the winter of. our hemisphere should occur when the earth is in aphelion, we should then be receiving one,fifth less heat than we now do, and in the summer, on the other hand, one-fifth more. But this extra summer-heat would not (according to Mr. Cron) suffice to counteract the severity of the winter, owing partly to the foggy atmosphere which would then prevail, and partly to the fact that eighty times as much heat is required to melt ice as to warm waterone degree. We cannot here go into further details upon this point, nor upon the closely-allied question of the modifying influence upon a cosmical climate produced as above described of an.increase in the obliquity of the ecliptic. But we may sum up the main conclusions which have been reached in the words .of
Mr. Geikie
"The last great increase in the ellipticity of our earth's orbit, which began some 240,000 years ago, and terminated about 80,000 years ago, embraced a period of 160,000 years. The cold was most intense about 200,000 or 210,000 years ago, that is about 30,000 or 40,000 years after the glacial period had commenced. Now, during-the continuance of this vast age of high eccentricity, our hemisphere must have experienced several great vicissitudes of climate. Glacial periods, lasting for thousands of years, must have alternated with equally pro- longed periods of genial conditions; for the latter, no less than the for- mer, are a necessary consequence of extreme ellipticity, combined with the precession of the equinoxes."
From this quotation we learn that the glacial period 'was not con- tinuous. The great ice-sheet was broken.several times, vanishing from the low grounds and from some:Of the upland valleys, where rivers and lakes made their appearance. It was during these inter-glacial periods ormild temperature that reindeer, mammoths, oxen, and horses were able to occupy the land.. Certain deposits in Scotland and elsewhere which have been found intercalated with the till, abundantly. prove the occurrence of these milder periods, to which is lent a particular interest owing to the very great probability that-it was in one of these inter-glacial ages— possibly not the latest —that- man entered Britainrat a time when our country was joined to -the continent of Europe, across the bed
of the German Ocean.
To the Great ice Age, which we have just briefly outlined, succeeded a period of which the formation known as boulder-clay affords some relics. During its accumulation by the conjoint action of glacial rivers and of the sea, the land in Scotland would appear to have stood at a lower level relatively to the sea than it does now. The subject of the boulder-clay is briefly discussed in chapter xv., while in the five chapters which follow, the third post-tertiary deposit or upper drift is described. To this formation a greater importance attaches, owing to the considerable records which it has left, but we have no space to linger over these here. In connection with this upper drift of Scotland, we may state that Mr. Geikie describes and examines the formation of the lakes and sea-lochsof that country, and then passes on to its distinctly post-glacial and recent deposits. The next chapter (xxv.) gives a brief account of the glacial deposits of England and Wales, followed in the three succeeding chapters by similar descriptions of the chief features of the Scandinavian, the Swiss, and North-American superficial deposits. Five more chapters conclude the volume ; the subjects only of the first four of these we give ; they are, —" The Cave-Deposits and Ancient River Gravels of Eng- land "; "Neolithic and Palmolithic Deposits "; "The Climate of the Palmolithic Period " ; "The Geological Age of Palo- Ethic Deposits " ; and "The Geological Position of Neolithic, PaTmolithic, and 111ammaliferous Deposits of Foreign Countries." In the concluding chapter of the work before us, a summary of re- sults and conclusions is presented in a form so compact as scarcely to admit of further concentration. The following abstract may, however, convey some idea of the present position of the inquiry as to the nature of the ancient climate of Great Britain, and of the circumstances amidst which its first human inhabitants appeared.
An intensely severe climate prevailed in our hemisphere some 200,000 years ago. North Britain and Scandinavia were then united by a vast system of glaciers, while the ice extended down to low latitudes in England as well as on the Continent, through the intervention of the various mountain ranges. To this arctic period of sterility a more genial time succeeded ; plants, such as pine-trees, grew in the south of England, and animals, such as the woolly rhinoceros and the great bear, appeared. Gradually, how- ever, the climate grew warmer, the distinction between summer and winter became less marked, and in consequence, the northern mammalia withdrew to more arctic homes. At last a kind of per- petual summer reigned, while the fauna of the country were marked by the introduction of the hippopotamus, the elephant, the lion, the tiger, and the 113 xna.
Again a series of changes occurs, and in the reverse order to that just given, until an arctic climate has brought all life to an end. We cannot say how often such cold and warm periods were repeated, nor can we be sure in which of such warm periods the men that fashioned rude implements of stone first made their appearance. It is likely that man arrived here as early as the mammoth and the rhinoceros, and his first coming may even have preceded the glacial epoch itself. But it is certain that he entered Britain during the last inter-glacial period, when there were glaciers in our mountains and arctic mammalia in our valleys. He witnessed the northward migration of these animals and the advent of the southern mammalia. Then came a period of submergence, when the British Islands were well-nigh drowned in the sea. After that the last cold period began, and in what remained of England it is not likely that palxolithic man still lingered. But the British Islands again rose from the waves ; the treeless land was soon invaded by the reindeer, the arctic fox, and the lemming, and then neolithic man entered upon the scene. So a vast lapse of time separates the men whose implements were roughly chipped from stone, from those whose implements were laboriously fashioned and polished. Gradually the climate still further improved, plants became more numerous and luxuriant, the animals of arctic regions were replaced by the ox and the sheep, while man himself slowly progressed, until he discarded stone for bronze, and ultimately discovered the mode of working iron. Thus we reach the dawn of that human history the records of which are more varied, and at the same time more easy to decipher, than the obscure relics of the non-historic ages.
It may perhaps be permitted to us, in concluding our notice of a book which possesses so much merit as the Great ice Age, to draw the attention of its author to one or two little matters of style. Mr. Geikie continually speaks of the ice-sheet or the glacier "drawing back," "retreating," or "retiring." Of course, these words are not intended to convey any idea of a backward move- ment of ice masses, but a melting-down and cutting-off of their lower and more southern parts. . Still there will be to some minds an ambiguity in the expressions. Then, again, we find in several places the word " bulk " used as a neuter verb in a manner which one would have believed to be American, rather than Scotch. We do not think this sentence has an agreeable sound :—" Those great accumulations of sand and gravel which bulk so largely in and at the mouths of our own mountain valleys."
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