1 70 VRA OMENTS OF SCI KNCE. 



ally 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 the 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 character 

 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 1,500 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- 

 doubtedly have been able to maintain themselves had they 

 ever been there; in presence of the fact that great glaciers 

 once most certainly filled these valleys that the whole 

 region, as proved by Mr. Jamiesou, is filled with the traces 

 of their action; the theory which ascribes the parallel 

 roads to lakes dammed by barriers of ice has, in my 

 opinion, a degree of probability on its side which amounts 

 to a practical demonstration of its truth. 



Into the details of the terrace formation I do not enter. 

 Mr. Darwin and Mr. Jamieson on the one side, and Sir 

 John Lubbock on the other, deal with true causes The 



