350 Mr Milne on the Parallel Boads of Lochaber. 



former writers, and especially Mr Darwin, suppose. It is plain, also 

 that the barrier which kept in the waters was less rapidly worn 

 down, when they stood at shelves 2 and 3, than at either of the in- 

 termediate levels. We see that at shelves 2 and 3 the waters flowed 

 over rocky ledges ; in the one case into Spey valley, in the other case 

 by Glen Glaster. Is it not fair from this to infer, that at the inter- 

 mediate shelves, the water flowed over a blockage of such a nature 

 as was capable of being more easily worn dowji and obliterated, such 

 as detrital matter ? It is, at all events, obvious, that when the 

 water sunk 14 feet, the discharge must have ceased at the east end ; 

 and that it henceforward would go on at the west end, probably near 

 the mouth of Glen Glaster. At every other place, the rocky moun- 

 tain sides rise so high, as to preclude the possibility of overflow or 

 attrition. 



Keeping these principles in view, let us suppose that the detrital 

 matter which blocked up the lower parts of Glen Roy extended a 

 very little to the east of the mouth of Glen Glaster. How easy it 

 is to suppose that this detritus was scooped away, so as to allow of 

 the recession of the waters westward, and of their flowing round the 

 east jaw of Glen Glaster, and on towards the head of that glen, from 

 which they would descend to Glen Spean ? For this purpose, it is 

 not necessary to suppose that there was any lowering of the sup- 

 posed barrier in level, even by a single foot. All that is required is 

 the scooping or wearing away of the detritus, so as to allow of the 

 extension of the lake a little to the westward ; — a few yards would 

 be suflScient. As the discharge at this first sinking, must have been 

 at the west end, it is fair to infer that the wearing away of detritus 

 took place there ; and when once a flow of water was established 

 through detrital matter, the process of removal would go on rapidly, 

 so as to allow of repeated sinkings of the lake, till it reached the 

 water-shed at the head of Glen Glaster, the rocky nature of which 

 would for a time stop any farther sinking, and thus allow of the for- 

 mation of shelf 3. 



According to the foregoing views, we see how the waters would, by 

 successive steps, sink from shelf 2 to shelf 3, and, after entering 

 Glen Glaster, form a marking on both of its sides. We see, also, 

 that the same removal of detritus which allowed the formation of 

 shelf 3 in that glen, would allow also the extension of it on Bohun- 

 tine Hill, beyond the point where shelf 2 terminates. 



Whilst this process of attrition was going on in Glen Roy, there 

 need have been no contemporaneous change in the blockage of Glen 

 CoUarig. But there also, at some time or other, a similar scooping 

 out of detritus must have taken place, to allow of the extension of 

 shelf 3 beyond the point where shelf 2 terminates. 



Nor is it difficult to conceive how this removal of detritus was 

 effected. Thus, in Glen Collarig, there are, on both sides of the 

 glen, burns of considerable size and power (from the steepness of 



