12 



Prof. J. Prestwicb. On the Origin 



[May 1, 



blocks that must have occurred during the growth of the great ice- 

 sheet — the neutralisation of the ice-force in one place, and the 

 centralisation of it in others — will serve to explain much that is 

 peculiar in the distribution of this sub-glacial debris or Till, not only 

 in Lochaber, but in other parts of the country, and at all levels. 



The author then points out the many mounds and terraces in the 

 Spean Yalley formed of moraine detritus, though since levelled and 

 often masked by a covering of gravel due to subsequent water action. 

 To this cause also he attributes the large accumulation of debris at the 

 entrance to Glen Roy, between Bohuntine and Glen Glaster, where he 

 shows it to be in places 200 to 300 feet deep, and to consist of a light 

 grey argillaceous unstratified matrix with angular fragments of the local 

 rocks capped by stratified gravel. The mass rises nearly to the level 

 of the lower parallel road. Mr. Milne-Home has pointed out a similar 

 deposit at the entrance to Glen Collarig, while the large mass at the 

 entrance of Glen Spean, and now forming Unachan Hill, rising to a 

 height of 613 feet, has been often described. The importance of these 

 great deposits in the position occupied by the main ice barriers cannot 

 be overlooked, for though river action and denudation has removed 

 much of them, the portions which remain show how materially they 

 may have contributed with the ice in forming the great lake barriers. 



The next question discussed is the height of the land in relation to 

 the sea at the period of the great glaciation, as it is not possible to 

 suppose that with the great changes of level which took place sub- 

 sequently, there is now a return to the status quo ante of the earlier 

 period ; and the author sees reason to conclude that the land then 

 stood at not less than from 1,000 to 1,500 feet higher than at present, 

 so that the Irish Channel was then above the sea level, and laud 

 extended a considerable distance westward from the present coast of 

 Scotland. 



This was followed by a submergence of not less than 1,200 to 

 1,500 feet in central and northern England, Wales, and Ireland, and 

 of 600 feet in the southern part of Scotland, as proved by the occur- 

 rence of marine shells at those heights. There is no similar evidence, 

 however, to show that it was more than 300 feet in Aberdeenshire, or 

 175 feet off the north-west coast. Still, it is not to be concluded on 

 negative evidence alone of the absence of marine shells that the sea 

 reached no higher over the north of Scotland. Assuming there at all 

 events a submergence of 400 to 500 feet below the present level, this, 

 added to the previous elevation of 1,000 to 1,500 feet, would establish 

 a difference of 1,500 to 2,000 feet between the period of great glacia- 

 tion and the succeeding period of submergence. 



This difference of level would produce a twofold effect upon the 

 climate — the one resulting from altitude which would be equal to a 

 rise in the mean temperature of from 4° to 6° F., and that caused by 



