CHAPTER XV.

WARM INTER-GLACIAL PERIODS.

Alternate Cold and Warm Periods.-Warm Inter-glacial Periods a Test of Theories.-Reason why their Occurrence has not been hitherto recognised. -Instances of Warm Inter-glacial Periods.-Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart, Leith-Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs, Sweden, Ohio, Cromer, Mundesley, &c., &c.-Cave and River Deposits.-Occurrence of Arctic and Warm Animals in some Beds accounted for.-Mr. Boyd Dawkins's Objections.-Occurrence of Southern Shells in Glacial Deposits.-Evidence of Warm Inter-glacial Periods from Mineral Borings.-Striated Pavements.-Reason why Inter-glacial Land-surfaces

are so rare.

Alternate Cold and Warm Periods.-If the theory developed in the foregoing chapters in reference to the cause of secular changes of climate be correct, it follows that that long age known as the glacial epoch did not, as has hitherto been generally supposed, consist of one long unbroken period of cold and ice. Neither did it consist, as some have concluded, of two long periods of ice with an intervening mild period, but it must have consisted of a long succession of cold and warm periods; the warm periods of the one hemisphere corresponding in time with the cold periods of the other and vice versa. It follows also from theory that as the cold periods became more and more severe, the warm intervening periods would become more and more warm and equable. As the ice began to accumulate during the cold periods in subarctic and temperate regions in places where it previously did not exist, so in like manner during the corresponding warm periods it would begin to disappear in arctic regions where it had held enduring sway throughout the now closing cycle. As the cold periods in the southern hemisphere became more and more

severe, the ice would continue to advance northwards in the temperate regions; but at that very same time the intervening warm periods in the northern hemisphere would become warmer and warmer and more equable, and the ice of the arctic regions would continue to disappear farther and farther to the north, till by the time that the ice had reached a maximum during the cold antarctic periods, Greenland and the arctic regions would, during the warm intervening periods, be probably free of ice and enjoying a mild and equable climate. Or we may say that as the one hemisphere became cold the other became warm, and when the cold reached a maximum in the one hemisphere, the warmth would reach a maximum in the other. The time when the ice had reached its greatest extension on the one hemisphere would be the time when it had disappeared from the other.

Inter-glacial Periods a Test of Theories.-Here we have the grand crucial test of the truth of the foregoing theory of the cause of the glacial epoch. That the glacial epoch should have consisted of a succession of cold and warm periods is utterly inconsistent with all previous theories which have been advanced to account for it. What, then, is the

evidence of geology on this subject? If the glacial epoch can be proved from geological evidence to have consisted of such a succession of cold and warm periods, then I have little doubt but the theory will soon be generally accepted. But at the very outset an objection meets us, viz., why call an epoch, which consisted as much of warm periods as of cold, a glacial epoch, or an "Ice Age," as Mr. James Geikie tersely expresses it? Why not as well call it a warm epoch as a cold one, seeing that, according to theory, it was just as much a warm as a cold epoch? The answer to this objection will be fully discussed in the chapter on the Reason of the Imperfection of Geological Records. But in the meantime, I may remark that it will be shown that the epoch known as the glacial has been justly called the glacial epoch or "Ice Age," because the geological evidences of the cold periods remain in a remarkably

perfect state, whilst the evidences of the warm periods have to a great extent disappeared. The reason of this difference in the two cases will be discussed in the chapter to which I have referred. Besides, the condition of things during the cold periods was so extraordinary, so exceptional, so totally different from those now prevailing, that even supposing the geological records of the warm periods had been as well preserved as those of the cold, nevertheless we should have termed the epoch in question a glacial epoch. There is yet another reason, however, for our limited knowledge of warm inter-glacial periods. Till very lately, little or no attention was paid by geologists to this part of the subject in the way of keeping records of cases of inter-glacial deposits which, from time to time, have been observed. Few geologists ever dreamt of such a thing as warm periods during the age of ice, so that when intercalated beds of sand and gravel, beds of peat, roots, branches, trunks, leaves, and fruits of trees were found in the boulder clay, no physical importance was attached to them, and consequently no description or record of them ever kept. In fact, all such examples were regarded as purely accidental and exceptional, and were considered not worthy of any special attention. A case which came under my own observation will illustrate my meaning. An intelligent geologist, some years ago, read a paper before one of our local geological societies, giving an account of a fossiliferous bed of clay found intercalated between two distinct beds of till. In this intercalated bed were found rootlets and stems of trees, nuts, and other remains, showing that it had evidently been an old inter-glacial land surface. In the transactions of the society a description of the two beds of till was given, but no mention whatever was made of the intercalated bed containing the organic remains, although this was the only point of any real importance.

Since the theory that the glacial epoch resulted from a high state of eccentricity of the earth's orbit began to receive some little acceptance, geologists have paid a good deal of attention to cases of intercalated beds in the till containing organic

remains, and the result is that we have already a great body of evidence of a geological nature in favour of warm inter-glacial periods, and I have little doubt that in the course of a few years the former occurrence of warm inter-glacial periods will be universally admitted.

I shall now proceed to give a very brief outline of the evidence bearing on the subject. But the cases to which I shall have to refer are much too numerous to allow me to enter into details.

Inter-glacial Beds of Switzerland.-The first geologist, so far as I am aware, who directed attention to evidence of a break in the cold of the glacial epoch was M. Morlot. It is now twenty years ago since he announced the existence of a warm period during the glacial epoch from geological evidence connected with the glacial drift of the Alps.*

The rivers of Switzerland, he found, show on their banks three well-marked terraces of regularly stratified and wellrounded shingle, identical with the modern deposits of the rivers. They stand at 50, 100, and 150 feet above the present level of the rivers. These terraces were evidently formed by the present system of rivers when these flowed at a higher level, and extend up the Alps to a height of from 3,000 to 4,000 feet above the level of the sea. There is a terrace bordering the Rhine at Camischollas, above Disentis, 4,400 feet above the level of the sea, proving that during the period of its formation the Alps were free of ice up to the height of 4,400 feet above the sea-level. It is well known that a glacial period must have succeeded the formation of these drifts, for they are in many places covered with erratics. At Geneva, for example, an erratic drift nearly 50 feet thick is seen to rest on the drift of the middle terrace, which rises 100 feet above the level of the lake. But it is also evident that a glacial period must have preceded the formation of the drift beds, for they are found to lie in many places upon the unstratified boulder

See a paper by M. Morlot, on "The Post-Tertiary and Quaternary Formations of Switzerland." Edin. Now Phil. Journal, New Series, vol. ii., 1855.

clay or till. M. Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of drift resting upon a bed of true till 40 feet thick; the latter was composed of a compact blue clay, containing worn and scratched alpine boulders and without any trace of stratification. In the gorge of Dranse, near Thoron, M. Morlot found the whole three formations in a direct superimposed series. At the bottom was a mass of compact till or boulder clay, 12 feet thick, containing boulders of alpine limestone. Over this mass came regularly stratified beds 150 feet thick, made up of rounded pebbles in horizontal beds. Above this again lay a second formation of unstratified boulder clay, with erratic blocks and striated pebbles, which constituted the left lateral moraine of the great glacier of the Rhone, when it advanced for the second time to the Lake of Geneva. A condition of things somewhat similar was observed by M. Ischer in the neighbourhood of Berne.

These facts, M. Morlot justly considers, prove the existence of two glacial periods separated by an intermediate one, during which the ice, which had not only covered Switzerland, but the greater part of Europe, disappeared even in the principal valleys of the Alps to a height of more than 4,400 feet above the present level of the sea. This warm period, after continuing for long ages, was succeeded by a second glacial period, during which the country was again covered with ice as before. M. Morlot even suggests the possibility of these alternations of cold and warm periods depending upon a cosmical cause. "Wild as it may have appeared," he says, "when first started, the idea of general and periodical eras of refrigeration for our planet, connected perhaps with some cosmic agency, may eventually prove correct."*

Shortly afterwards, evidence of a far more remarkable character was found in the glacial drift of Switzerland, namely, the famous lignite beds of Dürnten. In the vicinity of Utznach and Dürnten, on the Lake of Zurich, and near Mörschwyl, on the Lake of Constance, there are beds of coal or *Edin. New Phil. Journ., New Series, vol. ii., p. 28.