THE PARALLEL ROADS OF GLEN ROY. 209. 



to the level of the road, and beyond the head of this 

 lake he found, as in the other two cases, a col, or water- 

 shed, at Makul, of exactly the same level as the single 

 road in Glen Spean, which, it will be remembered, is a 

 continuation of the lowest road in Glen Roy. 



Here we have a series of facts of obvious signifi- 

 cance as regards the solution of this problem. The 

 effort of the mind to form a coherent image from such 

 facts may be compared with the effort of the eyes to 

 cause the pictures of a stereoscope to coalesce. For a 

 time we exercise a certain strain, the object remaining 

 vague and indistinct. Suddenly its various parts seem 

 to run together, the object starting forth in clear and 

 definite relief. Such, I take it, was the effect of his 

 ponderings upon the mind of Sir Thomas Dick-Lauder. 

 His solution was this: Taking all their features into 

 account, he was convinced that water only could have 

 produced the terraces. But how had the water been 

 collected? He saw clearly that, supposing the mouth 

 of Glen Gluoy to be stopped by a barrier sufficiently 

 high, if the waters from the mountains flanking the 

 glen were allowed to collect, they would form behind 

 the barrier a lake, the surface of which would gradually 

 rise until it reached the level of the col at the head of 

 the glen. The rising would then cease; the superflu- 

 ous water of Glen Gluoy discharging itself over the col 

 into Glen Roy. As long as the barrier stopping the 

 mouth of Glen Gluoy continued high enough, we 

 should have in that glen a lake at the precise level of 

 its shelf, which lake, acting upon the loose drift of the 

 flanking mountains, would form the shelf revealed by 

 observation. 



So much for Glen Gluoy. But suppose the mouth 

 of Glen Roy also stopped by a similar barrier. Behind 

 it also the water from the adjacent mountains would 



