406 MR MILNE OX THE PARALLEL ROADS OF LOCHABER. 



So far with regard to the first depression to shelf 3, at which period I sup- 

 pose 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 

 l)elow 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 Ijlockage entirely from both Glen Roy and 

 Collarig. 



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

 operation. There is no improbability whatever in the ultimate removal by rivers 

 and burns, of a blockage of the nature supposed. There flows into Glen Roy, from 

 Bohuntine hill, and at or near the very place where the blockage must have existed, 

 the Tundrun 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, 

 tills rivulet must have had nearly equal power to wash away the more superficial 

 iilockage which existed at this place previously to that event. 



The same observations apply to the detrital matter in Glen Collarig, which 

 could easily be carried aM'ay by the numerous mountain torrents flowing into that 

 glen. 



The following is the manner in which Mr Darw in alleges that the two de- 

 pressions 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, f/ive 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 mure than 212 feet vertical must he 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 

 harrier in Glen Collarig failed, the one in Glen Roy gave way the same number of feet, 

 through some strange coincidence .■* " It is plain, from this representation, that 

 Mr Darwin had not in his view, the more simple and gradual process of removal 

 which I have ventured to suggest. It is not in the least necessary to imagine, 

 that there was any sudden sweeping away of barriers of the magnitude supposed ; 

 and which would certainly imply the existence and operation of some stupendous 

 agent ; but the effect of which would, as Mr Darwin truly says, have also probably 

 obUterated the shelves. The process which I have suggested, implies the continu- 

 ous working of ordinary and natural agents, — agents which are now seen at this 

 very place, producing results similar to those required. 



Mr Darwin says, that the barrier across the Roy must have been 800 feet 

 high. This is on the assumption, that the valley of the Roy was then of its present 

 depth and form. But is there to be no allowance made, for the removal by the 



