402 PROF. J. PEESTWICH 0~N THE GLACIAL PERIOD, WITH 



of species now living in Switzerland the temperature was that of 

 Switzerland at the present day. Pinus sylvestris, Abies excelsa, 

 the Yew, the Birch, and the Oak flourish equally in Sweden and 

 far north in Siberia. On the other hand, there is one species of 

 Pinus (P. montana) which is spread over the mountain country up 

 to heights of 7000 feet, and is rare in the low lands ; while one 

 of the mosses is closely allied to a species now growing on the 

 hills of Lapland. The few species of Mammalia have a distinctly 

 northern facies. Eleplias primic/enius, E. antiquus, Ursus spelceus, 

 as also Cervus elaplius and Bos primigenius are commonly asso- 

 ciated with the lleindeer, Musk-Ox, and other Arctic animals of the 

 cold Postglacial times. Further, Loth the trees and animals are 

 those of our i( Forest-bed," the last land-survival before the setting 

 in of the extreme Glacial cold. 



Is the return, therefore, of the retreating glacier, supposing the 

 boulder-gravel above the lignites of Diirnten to be due to direct ice- 

 action, to be ascribed to anything more than a comparatively slight 

 temporary change of climate, like those that now for a succession 

 of seasons cause, from time to time, a temporary advance of the 

 glaciers, only more marked ? "We must allow, of course, for greater 

 differences and longer intervals of time than now obtain. 



Such minor vicissitudes of climate are more compatible with 

 changes in the physiography of Europe than with the cosmical causes 

 to which the Glacial epoch, as a whole, was, there is little doubt, due. 

 Ivor is it difficult to find such a cause in the extensive changes in the 

 distribution of land and water which took place in Britain and Nor- 

 thern Europe after the first great land-glaciation and the formation 

 of* the Lower Boulder-clay. The submergence of Ireland, Wales, 

 Scotland, and England (in part), and of a large area in Russia and 

 North German } T , extending to Holland, was sufficient, with the 

 influence of currents from the south (for in the shells of the Middle 

 Boulder-series there is a large percentage of southern forms and an 

 absence of extreme Arctic forms), to effect a considerable amelioration 

 of the climate, such as would lead to the temporaiy return of the 

 old Preglacial, but still northern, fauna and flora. 



With the rise of the temporarily submerged lands the climate again 

 changed, and brought the Alpine glaciers back over part of their old 

 ground, overwhelming in their course the forest-growth which had 

 sprung up in the meantime. But the beds of stratified sand, gravel, 

 and boulders overlying the lignite are more likely to have been the 

 result of glacial torrents than of the direct superposition of the ice, 

 which may have again approached, but is not proved to have covered, 

 the spot. 



For the formation of this interglacial bed a period of 6000 years 

 has been claimed ; but the claim rests on doubtful data. The lignite 

 is from 5 to 10, and rarely 12 feet thick. In the estimate the maxi- 

 mum thickness of 12 feet is taken, and it is assumed that to form this 

 12 feet of lignite it would have required 60 feet of peaty matter, or 

 that it took 5 feet of peat to form 1 foot of lignite, and that 100 years 

 would be needed for the growth of each foot of peat : thus a total of 



