406 MR MILNE ON 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 
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. 
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, 
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 Collarig, 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 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, 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 treme the 
barrier 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 
obliterated 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 
