MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 405 



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 mountain sides rise so high, as to pre- 

 clude the possibility of overflow or attrition. 



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

 Avhich 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 ne- 

 cessary to suppose, that there was any lowering of the supposed 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 sufficient. As the discharge at this first sinking, must have been 

 at the west end, it is fair to infer that the weai'ing 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 

 formation of shelf 3. 



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

 cessive steps, sink from shelf 2 to shelf 3, and, after entering Glen ( ilaster, 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 exten- 

 sion of it on Bohantine Hill, beyond the point where shelf 2 terminates. 



Whilst this process of attrition was going on in Glen Eoy, there need have 

 been no contemporaneous change in the blockage of Glen Collarig. 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 termi- 

 nates. 



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 their channels) Avhich flowed into the lake. 

 There are three of them, which now descend in that part of the glen marked by 

 shelves 2 and 3. If the detritus which formed the blockage in the lower part of 

 the valley consisted of the same loose sand and gravel which now abounds there, 

 forming cliff's from 70 to 80 feet high, nothing is more easy or natural than the 

 scooping of it out, by such means. 



The same observations apply to the blockage in Glen Roy, which, to prevent 

 the waters when at shelf 2 flowing into Glen Glaster, must have been near the 

 mouth of Glen Collarig, called Gap in the maps, out of which, from the number of 

 streams in it, a considerable current had flowed. 



