352 Mr Milne on the Parallel Boadu of Lochaber, 



plain, from this representation, that Mr Darwin had not in his view 

 the more simple and gradual process of removal which I have ven- 

 tured 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 opera- 

 tion of some stupendous agent ; hut the effect of which would, as Mr 

 Darwin truly says, have also probably obliterated the shelves. The 

 process which I have suggested, implies the continuous 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 Hoy 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 river Roy of detritus from 

 the valley \ It is manifest, from many appearances along its sides, 

 that the river Roy has cut down at least 200 feet below what was 

 the original bottom (whether of lake or estuary,) formed wdien the 

 waters stood at shelf 4 ; so that the height of the supposed barrier 

 to retain the waters at shelf 2 would not exceed 600 feet above the 

 bottom of the valley, and might be much less, if the valley were 

 more filled up. Mr Darwin considers it probable (p. 53,) that the 

 buttresses existing on the sides of Glen Roy indicate, that the valley, 

 upwards from Bridge of Roy, had been filled w^ith detrital matter to 

 the very level of shelf 4 ; in which case the blockage or barrier re- 

 quisite to form a lake at the level of shelf 2, would have been only 

 about 300 feet above the bottom of the valley. My belief, however, 

 is, that the whole not only of the lower part of Glen Roy, but also 

 of the district about Unachan, High Bridge, and Fort-William, was 

 blocked up with detrital matter, which, in the course of time was 

 washed away by rivers ; and that, when the blockage of Glen Roy 

 was removed, the depressed waters standing at shelf 4 were dam- 

 med back by detrital accumulations near Unachan, so as to force a 

 a discharge by the Pass of Makkul. This 4th, or lowest shelf, 

 seems to me to stretch much farther to the north, on both sides of 

 the Spean, than former observers have noticed. On the hills flank- 

 ing the east side, this shelf can be traced to within nearly a mile of 

 Spean Bridge. On the opposite side of the valley, it can be traced 

 to within 6 or 7 miles of Fort- William. The width of the valley 

 where this shelf on both sides ceases to be visible is about 4 miles. 

 Across the mouth of this valley, a little beyond a line joining the 

 extreme visible points of shelf 4, lies the high and elongated hill of 

 Tomnempearaichin, the top of which I found, by the level, to bo 

 only 50 or 60 feet below shelf 4 ; and there is no great difficulty 

 in imagining that the whole of this district, as far as Fort- William, 

 where the enclosing hills are greatly higher, was filled by detritus. 

 There are, even now, detrital remnants of enormous size, of which 

 the well-known Hill of Tomnahurich at Inverness (about 180 feet 

 high ^nd half a mile long), and a hill to the west of it (240 feet 



