40 Mr. J. Bryce's Notices of a late Visit 



origin to masses of ice, descending into the glens, and occu- 

 pying the very sites of our supposed lakes ? On the other 

 hand, it may be stated in favour of Mr. Thomson's views, that 

 the hypothesis of Glen Spean being " filled with an enormous 

 mass of ice" which would block up Glen Roy, is more con- 

 sistent with the geography of the district, than the supposition 

 that a glacier descended from one of the high valleys of the 

 Ben Nevis group, and forced its way into the opening of Glen 

 Roy. There is nothing in the nature of the country to deter- 

 mine a glacier to follow such a course. The form of the sur- 

 face between the Lochaber glens and the Ben Nevis group is 

 such, that if a glacier descended from any one of the five 

 great glens, whose directions are inclined to that of Glen Roy 

 at an angle of 60 or 70 degrees, and reached the open country 

 at the base of the mountains, there would be nothing to deter- 

 mine its course up Glen Roy, or indeed in any one direction 

 more than another, except the slight eastward and northward 

 slope already described. Glaciers descending from these 

 glens would thus coalesce into one huge sheet, coextensive 

 with the valley of the Spean. The hypothesis of sheets of ice 

 covering the whole surface — " des grandes nappes de glace " — 

 seems also more consistent with the absence of "perched 

 blocks " and moraines, than the idea of separate glaciers. 

 These are not seen anywhere over the surface of the open 

 tract between the mountains and the river ; and the peculiar 

 detrital covering is very like that which would be formed under 

 such advancing sheets, most of it being stratified sand and 

 small gravel, the result of wearing, or decomposition in situ. 

 Mr. Thomson's explanation of the phaenomena of Glen 

 Gluoy is very ingenious. It will be remembered that these 

 are peculiar. The shelves do not correspond with those in 

 the other glens; and while in the latter each successive shelf, 

 as we descend, extends further down the glens than those that 

 are higher, in Glen Gluoy the upper shelf extends further 

 towards the mouth of the glen than the lower; and this lower 

 shelf, unlike all the others, is not in connexion with any sum- 

 mit level. If the lake theory be true, it will follow from these 

 facts that the barrier which retained the water at the lower 

 level was further up the glen than that which retained it at 

 the higher; and that when the lower shelf was forming, the 

 overflow must have taken place at the mouth of the glen. 

 Mr. Thomson supposes " that the glacier which occasioned 

 the formation of the higher of the Glen Gluoy shelves had at 

 some former period protruded a terminal moraine as far up 

 the glen as the termination of the lower shelf; that, on the 

 final retiring of the glacier, this old moraine served as a bar- 



