250 



MR ALFRED HAKKER ON 



Fig. 10. — Perched blocks on the lower slope of Sgurr a' 

 Coir' an Lochain, towards Coruisk. 



there are some of peridotite, which, in virtue of the excessively rough surface presented 

 by that rock, have been able to take a very remarkable posture (fig. 10). 



Although the glaciers while in the mountain-glens may have exercised a con- 

 siderable erosive action upon their beds, it seems evident that the blocks of gabbro 



and other rocks which constitute most 

 of the accumulations which can be con- 

 fidently referred to the later glaciation 

 were only transported, not detached, by 

 the ice. They were broken away from 

 the parent rock on slopes overlooking 

 the glaciers by subaerial agenc}^, in which 

 frost must have been the most import- 

 ant factor. To an observer approaching 

 the Cuillins by any of the principal glens, 

 or still better by Loch Scavaig, one of the 

 first things to strike the eye is the strong 

 contrast between the smooth rounded 

 form of the lower slopes and the splint- 

 ered shapes of many parts of the higher 

 ridges. It might be hastily inferred that 

 these latter have never been submerged 

 beneath ice ; but such an explanation would soon be found to break down when applied 

 in detail, and we have already seen from other considerations that it is inadmissible. 

 There are two reasons for the higher ridges and summits not showing the effects of 

 glaciation in the same way as the corries below. Firstly the ridges, acting as ice-sheds 

 at the time of the maximum glaciation, escaped erosion owing to the lack of rock- 

 debris in the ice overlying them, which left it almost powerless ; and secondly, the same 

 ridges, exposed above the ice-surface during the later glaciation, were then subjected to 

 the splintering and shattering action of frost. 



The operation of what we may call frost-erosion was not confined merely to the 

 time when the valleys were occupied by the later glaciers. The requisite conditions, 

 viz., a sufficiency of moisture and an air-temperature fluctuating above and below the 

 freezing-point, must have existed during a part of the interval between the disappear- 

 ance of the ice-cap and the birth of the later glaciers, and certainly continued for some 

 time after these glaciers had vacated at least the upper parts of the mountain glens. 

 This is part of the evidence, already alluded to, which goes to show that, in determining 

 the glaciation of this part of the country, variations in the amount of precipitation 

 were of greater moment than variations in mean annual temperature. The proof that 

 the later glaciers withdrew from at least the upper parts of the glens while a severe 

 temperature still prevailed, is afforded by a class of accumulations not hitherto 

 mentioned, viz., the huge taluses which are so conspicuous a feature of almost all the 



