Mr Milne on the Parallel lloads of Lochaber. 351 



their channels) which 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 cliffs 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. 



So far with regard to the first depression to shelf 3, at which 

 period I suppose the Collarig blockage to be still existing (scooped 

 out a little towards the west), and the blockage in Glen Roy to have 

 been, by a similar process, removed below the mouth of Glen Glaster. 

 The next well marked shelf is No. 4, which is seen on Craig Dhu 

 and Bohuntine, and on both sides of Glen Collarig, and which infers 

 the necessity of removing the blockage entirely from both Glen Roy 

 and Collarig. 



o 



This may have been, as in the case of the previous depression, a 

 gradual operation. There is no improbability whatever in the ulti- 

 mate removal by rivers and burns, of a blockage of the nature sup- 

 posed. There flows into Glen Roy, from Bohuntine Hill, and at or 

 near the very place where the blockage must have existed, the Tun- 

 drun Burn, the sides of which shew mica-slate rocks cut through by 

 it to the depth of about 70 feet, and detrital matter above these rocks 

 cut through to the depth of 130 feet. If, since the drainage of the 

 lake, it has thus cut through and removed blockage to the depth of 

 200 feet, of which one-third is solid rock, this rivulet must have had 

 nearly equal power to wash away the more superficial blockage which 

 existed at this place previously to that event. 



The same observations apply to the detrital matter in Glen Col- 

 larig, which could easily be carried away by the numerous mountain 

 torrents flowing into that glen. 



The following is the manner in which Mr Darwin alleges that 

 the two depressions must have taken place, according to the lake 

 theory. He says, that there are two barriers, one in Glen Collarig, 

 and the other in Glen Roy : ** Let one of the two barriers, we will 

 say the smaller one in Glen Collarig, give way from the effects of an 

 earthquake, or other cause, the lake will now stand at the level of 

 the middle shelf, the barriers having given way 82 feet vertically. 

 Again let it burst, and this time rather more than 212 feet vertical 

 must be swept away. Let all this have taken place, but still a 

 barrier nearly a mile long and 800 feet in height is left standing 

 across the mouth of the Roy. Must we suppose that each time the 

 barrier in Glen Collamg failed, the one in Glen Boy gave way the 

 -same number of feety through some strange coincid()nce ?'* It is 



