THE PAR ALL KL ROADS OF GLEN ROY. 169 



brought into play are the nutrition of the glaciers by the 

 frozen material above, and their consumption in the milder 

 air below. For a period supply exceeded consumption, and 

 the ice extended, filling Glen Speau to an ever-increasing 

 height, and abutting against the mountains to the north 

 of that glen. But why, it may be asked, should the val- 

 leys south of Glen Spean be receptacles of ice at a time 

 when those north of it were receptacles of water? The 

 answer is to be found in the position and the greater eleva- 

 tion of the mountains south of Glen Spean. They first 

 received the loads of moisture carried by the Atlantic winds, 

 and not until they had been in part dried, and also warmed 

 by the liberation of their latent heat, did these winds touch 

 the hills north of the Glen. 



An instructive observation bearing upon this point is 

 here to be noted. Had our visit been in the winter we 

 should have found all the mountains covered; had it been 

 in the summer we should have found the snow all gone. 

 But happily it was at a season when the aspect of the 

 mountains north and south of Glen Spean exhibited their 

 relative powers as snow collectors. Scanning the former 

 hills from many points of view, we were hardly able to 

 detect a fleck of snow, while heavy swaths and patches 

 loaded the latter. Were the glacial epoch to return, the 

 relation indicated by this observation would cause Glen 

 Spean to be filled with glaciers from the south, while the 

 hills and valleys on the north, visited by warmer and drier 

 winds, would remain comparatively free from ice. This 

 flow from the south would be reinforced from the west, and 

 as long as the supply was in excess of the consumption the 

 glaciers would extend, the dams which closed the glens 

 increasing in height. By and by supply and consumption 

 becoming approximately equal, the height of the glacier 

 barriers would remain constant. Then, if milder weather 

 set in, consumption would be in excess, a lowering of the 

 barriers and a retreat of the ice being the consequence. 

 But for a long time the conflict between supply and con- 

 sumption would continue, retarding indefinitely the dis- 

 appearance of the barriers, and keeping the imprisoned 

 lakes in the northern glens. But however slow its retreat, 

 the ice in the long run would be forced to yield. The dam 

 at the mouth of Glen Roy, which probably entered the 

 glen sufficiently far to block up Glen Glaster, would gradu- 



