THE PAKALLEL KOADS OF GLEN ROY. 209 



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

 lake ne 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 significance 

 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-Lander. 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 Grlen 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 superfluous water of 

 ^rlen 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 moun- 

 tains, 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 

 collect. The surface of the lake thus formed would 



