170 FRAGMENTS OF SCIENCE. 
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 darfTwouTd produce 
the entire series of phenomena. In sinking from col to 
col the water would flow over a gradually melting barrier, 
AjU- .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. Jamieson, 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 
