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Show 114 ISLAND LIFE. [rAnT r. -------- into new areas. But these effects must have been greatly multiplied and intensified if, as there is very goo~ reason to believe, the glacial epoch itself-or at least th~ earher and later phases of it-consisted of two or more alternatiOns of warm and cold periods. The evidence that such was the case is very remarkable. The "till," as we have seen, could only have been formed when the country was entirely buried under a large ice-she~t of enormous thickness, and when it must therefore have been, 1n all the parts so covered, almost entirely destitute of animal and vegetable life. But in several places in Scotland fine layers of sand and <Yravel with beds of peaty matter, have been found resting on ~'till" and again covered by" till." Sometimes these intercal~tec1 . beds are very thin, but in other ca.ses they are twenty or thuty feet thick, a~d in them have been found remains of the extinct ox, the Irish elk, the horse, reindeer and mammoth. Here we have evidence of two distinct periods of intense cold, and an intervening milder period sufficiently prolonged for the country to become covered with vegetation and stocked with animR l life. In some districts borings have proved the existence of no less than four distinct formations of "till'' separated from each other by beds of sand from two to twenty feet in thickness. 1 Facts of a similar nature have been observed in other parts of our islands. In the east of England, llir. Skertchly (of the Geological Survey) enumerates four distinct boulder clays with intervening deposits of gravels and sands.2 Mr. Searles V. Wood, Jun., classes the most recent (Hessle) boulder clay as "post-glacial," but he admits an intervening warmer period, characterised by southern forms of mollusca and insects, after which glacial conditions again prevailed with northern types of mollusca.3 Elsewhere he says: "Looking at the presence of such fluviatile mollusca as Cyrena fluminalis and Unio littoralis and of such mammalia as the hippopotamus and other ¥reat 1 The G1·eat Ice Age, p. 177. 2 These are named, in descending order, Hessle Boulder Clay, Purple Boulder Clay, Chalky Boulder Clay, and Lower Boulder Clay-below which is the Norwich Crag. a " On the Climate of the Post-Glacial Period." Geological Magazine, 1872, p. 158, 160. CHAP. \'Il.) '1'1-HJ GLACIAL EPOCH. 115 ------ -------------------- pachyderms, and of such a littoral Lusitanian fauna as that of the Selsea bed where it is mi ~ed up with the remains of some of those pachyderms, as well as of some other features, it has seemed to me that the climate of the earlier part of the postglacial period in England was possibly even warmer than our present climate; and that it was succeeded by a refrigeration sufficiently severe to cause ice to form all round our coasts, and glaciers to accumulate in the valleys of the mountain districts; and that this increased severity of climate was preceded, and partially accompanied, by a limited submergence, which nowhere apparently exceeded 300 feet, and reached that amount only in the northern counties of England."1 This decided admission of an alternation of warm and cold climates since the height of the glacial epoch by so cautious a geologist as Mr. Wood is very important, as is his statement of an accompanying depression of the Jand, accompanying the increased cold, because many geologists maintain that a greater elevation of the land is the true and sufficient explanation of glacial periods. Further evidence of this alternation is found both in the Isle of Man and in Ireland, where two distinct boulder clays have been described with intervening beds of gravels and sands. Palceontological evidence of alte,rnate Cold and Warm periods.Especially suggestive of a period warmer than the present, immediately following glacial' conditions, is the occurrence of the hippopotamus in caves, brick-earths, and gravels of palreoJithic age. Entire skeletons of this animal have been founJ at Leeds in a bed of dark blue clay overlaid by gravel. Further north, at Kirkdale cave, inN. Lat. 54°.15', remains of the hippopotamus occur abundantly along with those of the ox, elephant, horse, and other quadrupeds, and with countless remains of the hyrenas which devoured them; while it has also been found in cave deposits in GJamorganshire, at Durd.ham Down, near Bristol, and in the post-Pliocene drifts of Dorsetshire. It is important to note that where it is associated with other mammals in caves-which are hya::na-dens, and not mere receptacles of water-carried remains-these always imply a mild climate, the elephant and rhinoceros found with it being species character- 1 Geulogical Magazine, 1876, p. 396. I 2 |
