THE PARALLEL ROAES OF GLEN ROY. 261 



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 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 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 superfluous water of 

 Glen Gluoy discharging itself over the col into Glen 

 Koy. 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 



