Mb. Bkyce on the Parallel Roads of Lochaher. 107 



of ice covered its surface, it is not probable that the lakes in the glens, 

 at considerably higher levels, would long remain unfrozen ; and if the 

 Ben Nevis group of mountains, whose mean height we may take at some- 

 what less than 4000 feet, not only nourished glaciers in their higher 

 recesses, but were wholly enveloped in sheets of ice, can we suppose that 

 the mountains surrounding Glen Koy and Glen Gluoy, many of which 

 attain the altitude of from 2000 to 2500 feet, would not likewise give 

 origin to masses of ice, descending into the glens, and occupying 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 

 " fiUed with an enoimous mass of ice " which would block up Glen Roy, is 

 more consistent with the geogi-aphy 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 Eoy. There is nothing 

 in the nature of the country to determine a glacier to follow such a course. 

 The form of the surface 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 moun- 

 tains, there would be nothing to determine 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 phenomena 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 summit level. If the 

 lake tlieory 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 form- 

 ing, 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 



