1879.] 



of the Parallel Roads of Lochaber. 



13 



the conversion of a continental area into an archipelago, for then in 

 consequence of the submergence, the Highlands of Scotland became 

 surrounded by an open sea with only the higher peaks of England, 

 Wales, and Ireland above the waters, and the warm currents of the 

 Atlantic reached the foot of the Scottish mountains. The effects of 

 the two causes could not be less than from 12° to 15° F., which is 

 about equivalent to the difference of climate between Paris and St. 

 Petersburg. There is also to be taken into account the probable 

 increase of heat consequent on the gradual diminution of those con- 

 ditions, whatever they were, to which was owing the cold of the glacial 

 period. 



The effect of those changes was to produce a thaw which gradually 

 led to the destruction of the great ice- sheet, though subsequently other 

 changes brought for a time a return of cold sufficient to maintain local 

 glaciers in the higher mountain ranges. The conditions under which 

 the ice-sheet would be placed during this period would be similar to 

 that of glaciers where they extend below the snow line. In Europe 

 the glacier gradients are usually so steep and the ice is so fissured, that 

 even in such positions water rarely lodges on the surface ; still it does, 

 at some places give rise to small pools and streams, but in the Hima- 

 layas, where glaciers descend into wide valleys with small gradients, 

 and the summer heat is considerable, the glaciers often become 

 covered with tarns and small lakes. They have been noticed by 

 Sir J. D. Hooker and more lately by Colonel God win- Austen, who 

 describes in the middle of one of these glaciers, a series of such lakes, 

 some being 500 yards in length and 200 to 300 in breadth, and of 

 great depth. As intervening barriers give way, these lakes descend 

 in course of time from lower to lower levels, and finally escape. 



In the same way the old ice-sheet must have become covered with 

 pools and lakes, for owing to the irregular surface of the ice, and the 

 inevitable absence of all channels of drainage, the water must every- 

 where have lodged, until channels were formed, and a means of escape 

 established. The extent of these bodies of water would depend upon 

 the height and permanence of the obstructions. In the Lochaber 

 district they were, owing to the causes before named, of great size 

 and permanence, such as to form high barriers at the entrance to 

 Glen Roy, Glen Spean, and Glen Gluoy, behind which the waters 

 accumulated and rose until they found a channel of escape over the cols 

 at lower levels, when a permanent water-level would be established 

 so long as the main barriers existed. 



It is well known that the Parallel Roads are terraces composed of 

 perfectly angular fragments of the local rocks with a few rounded 

 pebbles both local and foreign to the district. The former show an 

 entire absence of any prolonged beach w T ear. The wear of the latter 

 is due to other causes. There is also an entire absence of any notch 



