James Geikie — On Changes of Climate. 545 



lY. — On Changes of Climate during the Glacial Epoch. 



By James Geikie, P.E-.S.E., 

 District Surveyor of the Geological Survey of Scotland. 



First Paper. 



IF one were asked to put into a few words the general results 

 wliich have been arrived at from a study of tlie Glacial deposits, 

 he would probably say that these deposits gave evidence of a severe 

 Arctic condition of things having obtained in this country, — that the 

 gradual approach of this Arctic climate caused the disappearance 

 from our area of the fauna and flora which had previously charac- 

 terized it, — that during the continuance of the cold in Britain 

 several species of mammalia appear to have died out in the more 

 southern regions of Europe, whither they had migrated, — and that 

 it was not until after our climate had become greatly ameliorated 

 that these islands were visited by what are termed the "Post-glacial 

 mammalia," several species of which, however, had been denizens 

 of Britain and northern Europe in Pre-glacial times. In short, our 

 island, throughout the Glacial period proper, is commonly supposed 

 to have remained a barren waste of snow and ice. But the evidence 

 which has been accumulating during recent years will compel us, I 

 believe, to modify materially these general inferences. So far from 

 the Glacial epoch having been one long continuous age of ice, it 

 would appear to have been broken up by many intervening periods 

 of less Arctic, and even temperate conditions, during which the 

 snow and ice disappeared from our low grounds, and the glaciers 

 shrunk back into our mountain valleys. I speak, of course, of 

 that portion of the Glacial epoch which was antecedent to the 

 general submergence, and is represented by the Till or Boulder-clay 

 of Scotland. In this short paper I propose to give an outline of the 

 facts upon which these conclusions are based. But before doing so 

 it may be well to point out the order of succession of the Scottish 

 drift deposits, which is now no longer a matter of dispute. Begin- 

 ning with the lower beds, we have the following sequence ; — 



1. Boulder-clay or Till,^ with subjacent and intercalated beds of 



^ Under this head I include those unstratified, more or less tenacious deposits of 

 clay, which are so ahundantly charged with well-polished and striated stones. They 

 are all clearly of older date than the Kame and brick-clay series. I thought at one 

 time that the less tenacious Till-beds might possibly be of later date than the tougher 

 and harder stony clays : but subsequent investigation has shown that such is not the 

 case. The character of the Till depends in large measure upon the niiture of the 

 rocks to the demolition of which it owes its origin ; and also in some degree to the 

 pressure of the ice under which it was formed. There is a coarse moraine-like 

 accumulation of earthy clay full of rough impolished angular blocks and debris, but 

 with only a few scratched stones, which has sometimes been called Boulder- clay. 

 But this deposit is clearly posterior in date to the true Till, and belongs to the upper 

 drift series. Again, some of our later glacial brick-clays contain scattered stones, 

 many of which are striated. . These clays, if met with in the more southerly districts 

 of England, would probably be called Boulder-clays : in Scotland they are all of 

 marine origin. 



VOL. VIII.— NO. xc. 35 



