THE PARALLEL ROADS OF GLEN ROY. 311 



the terraces, he saw that if the mouth of Glen Gluoy were stopped by 

 a barrier the waters from the surrounding mountains would be col- 

 lected in the valley until they had reached the water-shed, when any 

 further rise would be prevented by the branch-valley, which would 

 carry the additional water off to Glen Roy. As long, then, as the 

 barrier remained, there would be a lake in Glen Gluoy, at the exact 

 level of the road, which, by constant action upon the loose drift, 



Parallel Roads op Glen Roy. 



would be sufficient to produce the road. Now, if the mouth of Glen 

 Roy should also be barred at the same time by a sufficiently high bar- 

 rier, the waters would be collected behind it, the surface of the lake 

 would rise till it reached the water-shed dividing Glen Roy from Glen 

 Spey, when the superabundant water would flow into the latter val- 

 ley. In this way the highest shelf of Glen Roy would be formed. If 

 its barrier were now to be partly removed, so as to establish a connec- 

 tion between it and the upper part of Glen Spean, while the lower 

 part remained blocked up, upper Glen Spean and Glen Roy would 

 then be occupied by a continuous lake, the level of which would be 

 determined by the water-shed discovered in Glen Spean. The water 

 in Glen Roy would take a level corresponding to its new pbice of es- 

 cape, and the lowest parallel road would be formed. The conclusions 

 thus drawn would be strictly logical, if proof could be offered as to 

 the existence of the barriers. 



In Glen Spean there is a large quantity of detritus, and Sir Thomas 

 Dick-Lander supposed that this had at one time been heaped up by 

 some unknown convulsion. As he could not account for the middle 



