236 FRAGMENTS OF SCIENCE 



the subsidence of the lake eighty feet, from the level of 

 the highest to that of the second parallel road, would fol- 

 low 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 Boy 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 atmospheric conditions or to the charac- 

 ter of the dam itself, or through local softness in the drift, 

 small pseudo-terraces would be formed, which, to the per- 

 plexity of some observers, are seen upon the flanks of the 

 glens to-day. 



In presence then of the fact that the barriers which 

 stopped these glens to a height, it may be, of 1,600 feet 

 above the bottom of Glen Spean, have dissolved and left 

 not a wreck behind; in presence of the fact, insisted on 

 by Professor Geikie, that barriers of detritus would un- 



