860 J. GEIKIE ON THE GLACIAL PHENOMENA 



of the surface. The glaciated outline is superimposed on an older 

 set of features, which in the main appear to have been the result of 

 ordinary atmospheric and aqueous erosion guided by the structural 

 peculiarities of the rock. In my remarks upon the physical features 

 of the islands, frequent reference has been made to the fact that the 

 strike and dip of the gneiss have greatly influenced the conformation 

 of the ground. And this, of course, is only natural, and what we 

 usually expect to find. Long before the ice-sheet overflowed the 

 Hebrides, the surface of those islands was marked in many places by 

 escarpments and intervening hollows that traversed the land pre- 

 cisely in the line of strike. This latter being generally north-west 

 and south-east, we find that such also is the prevailing trend of the 

 features referred to. We see this notably in the district of Pairc or 

 Park, and the neighbourhood of Loch Roag in Lewis, and through- 

 out the larger portion of the whole Long Island ; and the direction 

 of most of the sounds and channels, and of the great majority of the 

 sea-lochs, is the same. All these features, there can be little doubt, 

 owe their origin to the long-continued action of ordinary subaerial 

 denudation ; even the hollows through which the sea now ebbs and 

 flows probably came into existence at a time when the whole of 

 these islands stood at a greater elevation above the sea. At present, 

 indeed, the sea, instead of eroding, is silting-up the channels between 

 the various islands. This is sufficiently proved by the presence of 

 great shoals of sand, which, in the case of the North and South 

 Fords, connect Benbecula at low water with North and South Uist. 



Severe also as the glaciation of the Hebrides has been, we cannot 

 attribute the transverse hollows exclusively to the action of the ice- 

 sheet. Indeed it is impossible not to see that the features in 

 question date their origin, as I have said, to preglacial times ; for 

 they do not always coincide in direction with the trend of the 

 glaciation. Thus to the west of Stornoway the escarpments and 

 intervening depressions run nearly at right angles to the glaciation ; 

 and the same appearance may be seen in some of the islands further 

 south, particularly in South ITist. 



The ice-sheet, therefore, only modified, and did not originate, the 

 peculiar transverse ridges and hollows that characterize so large a 

 portion of the Long Island. But the " modification " was somewhat 

 extreme. Sharp-rimmed escarpments were smoothed off, and when 

 the glaciation coincided, as it so frequently did, with the strike of 

 the gneiss, the long transverse hollows were deepened and widened. 

 That all, or nearly all, the lakes in the Long Island were likewise 

 the proximate result of glaciation. I have endeavoured to show, so 

 that some of the most characteristic scenic features of the Outer 

 Hebrides were due to the action of the ice-sheet. 



XII. SUMMAEY OE RESULTS AND CONCLUSION. 



1. Primary or General Glaciation. — The evidence now brought 

 forward demonstrates that the extent and thickness of the ice-sheet 

 that buried Western Scotland during the climax of the glacial 

 period was much greater than geologists have hitherto supposed. 



