Ch. xn.] 



GLACIATION OF SCOTLAND. 



153 



Me. GeiMe has arrived at the same general conclusions as Mr. 

 Janiieson, -with respect to the principal movements of the land in 

 Scotland. The great mass of till, of which in some of the lower 

 valleys the thickness exceeds 150 feet, he attributes not to icebergs, 

 but to ice action on land, for it consists of the debris of rocks, every- 

 where found in situ in the same hydrographical basin. The absence 

 of marine shells is at once accounted for if we assume it to be of 

 glacier origin. The rarity of angular stones, those in the till being 

 usually rounded or sub-angular, and the number of fragments polished 

 and striated on one or several sides, may also be explained by sup- 

 posing the till to have been shoved along under a heavy mass of ice, 

 like that of Greenland, instead of forming parts of superficial mo- 

 raines, carried down without trituration on the top of the ice. If, in 

 accordance with the views above set forth, we admit a second glacial 

 period, when the land was reelevated after the great submergence, the 

 action of ice at this later date may well be supposed to have obliter- 

 ated almost all signs of the sojourn of the sea upon the land in the 

 highest regions, where the cold was most intense ; but in the lower 

 country, some patches of marine strata with arctic shells might more 

 easily escape destruction. 



The greatest height to which marine shells have yet been traced in 

 Scotch drift is only 524 feet above the level of the sea, at which ele- 

 vation they have been observed at Airdrie, fourteen miles southeast 

 of Glasgow. At that spot they were found imbedded in stratified 

 clays with till above and below them. There appears no doubt that 

 the overlying deposit was true glacial till, as some boulders of granite 

 were observed in it, which must have come from distances of sixty 

 miles at the least.* 



Fig. 129. 

 Astarte borealis. 



Fig. 130. 

 Leda dblonga. 



Kg. 131. Fig. 132. Fig. 133. Fig. 134. 



Saxicava rugosa. Pecten island icus. Katica clausa. Trophon clathratwm.. 



Northern shells common in the drift of the Clyde, in Scotland. 



The shells here figured are only a few out of a large assemblage of 

 living species, which, taken as a whole, bear testimony to conditions 

 far more arctic than those now prevailing in the Scottish seas. But 



* Smith, of Jordan Hill, Geol. Quart. Journ., vol. vi. p. 387. 1850. 



