114 



ISLAND LIFE. 



[part I. 



into new areas. But these effects must have been greatly 

 multiplied and intensified if, as there is very good reason to 

 believe, the glacial epoch itself — or at least the earlier 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-sheet of enormous 

 thickness, and when it must therefore have been, in all the parts 

 so covered, almost entirely destitute of animal and vegetable 

 life. But in several places in Scotland fine layers of sand and 

 gravel with beds of peaty matter, have been found resting on 

 " till " and again covered by " till." Sometimes these intercalated 

 beds are very thin, but in other cases they are twenty or thirty 

 feet thick, and 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 animal 

 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.^ 

 Facts of a similar nature have been observed in other parts of 

 our islands. In the east of England, Mr. Skertchly (of the 

 Geological Survey) enumerates four distinct boulder clays with 

 intervening deposits of gravels and sands.^ 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.^ Elsewhere he says : " Looking at the presence of 

 such fluviatile mollusca as Cyrena fiitmincdis and Vnio littoralis 

 and of such mammalia as the hippopotamus and other great 



1 The Great 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. 



3 On the Climate of the Post-Glacial Period." Geological Magazine, 

 1872, p. 158, 16a 



