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 . 1 
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 . 2 Mr. Searles Y. 
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 Jluminalis and Unio 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, 160. 
