222 FKAGMENTS OF SCIENCE. 



milder weathel set in, consumption would be in excess, 

 a lowering of the barriers and a retreat of tbe ice being 

 the consequence. But for a long time the conflict 

 between supply and consumption would continue, re- 

 tarding indefinitely the disappearance 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 Koy, which probably entered the glen 

 sufficiently far to block up Glen Glaster, would 

 gradually retreat. Glen Glaster and its col being 

 opened, the subsidence of the lake eighty feet, from 

 the level of the highest to that of the second parallel 

 road, would follow as a consequence. I think this the 

 most probable course of things, but it is also possible 

 that Glen Glaster may have been blocked by a glacier 

 from Glen Trieg. The ice dam continuing to retreat, 

 at length permitted Glen Roy to connect itself with 

 upper Glen Spean. A continuous lake then filled both 

 glens, the level of which, as already explained, was 

 determined by the col at Makul, above the head of 

 Loch Laggan. The last to yield was the portion of the 

 glacier which derived nutrition from Ben Nevis, and 

 probably also from the mountains north and south of 

 Loch Arkaig, But it at length yielded, and the waters 

 in the glens resumed the courses which they pursue 

 to-day. 



For the removal of the ice barriers no cataclysm is 

 to be invoked ; the gradual melting of the dam would 

 produce the entire series of phenomena. In sinking 

 from col to col the water would flow over a gradually 

 melting barrier, the surface of the imprisoned lake not 

 remaining sufficiently long at any particular level to 

 produce a shelf comparable to the parallel roads. By 

 temporary halts in the process of melting due to atmo- 



