to the Parallel Roads of Lochaber. 39 



Europe, Prof. J. D. Forbes, has given it as his decided opinion 

 that glaciers formerly existed on the Cuchullin hills in Skye 

 (Ed. N. Phil. Journ., vol. xl. p. 79). Why, then, may not 

 masses of ice have filled the still higher valleys of the Ben 

 Nevis group of mountains? Professor Forbes's Jate discoveries 

 in Switzerland respecting the viscidity of glacier ice, and the 

 nature of glacier motion, appear to have suggested to Mr. 

 James Thomson the highly ingenious modification of the 

 glacial theory lately proposed by him (Ed. N. Phil. Journ., 

 vol. xlv, p. 49). The gist of this theory is contained in the 

 following passage: — 



" In Switzerland the mean temperature of the compara- 

 tively low and flat land is so much above the freezing-point, 

 that the ice no sooner descends from the mountains than it 

 melts away; and it is thus usually prevented from spreading 

 to any considerable extent over the plains. In the Antarctic 

 continent, on the contrary, the mean temperature is nowhere 

 so high as the freezing-point. The ice. therefore, which de- 

 scends from the hills unites itself with that which is deposited 

 from the atmosphere on the plains; and the whole becomes 

 consolidated into one continuous mass, of immense depth, 

 which glides gradually onwards towards the ocean .... Now 

 a climate somewhere intermediate between these extremes 

 appears to be that which would be requisite to form the shelves 

 in the glens of Lochaber. The climate of Switzerland would 

 be too warm to admit of a sufficient horizontal extension of 

 the glaciers; that of the Antarctic continent too cold to allow 

 the lakes to remain unfrozen. If the climate of Scotland were 

 again to become such that the mean temperature of Glen 

 Spean would be not much above the freezing-point, there 

 seems to be every reason to believe that that glen would again 

 be nearly filled with an enormous mass of ice; while its upper 

 parts, and also Glen Roy, would be occupied by lakes . . . ." 



The state of things here supposed is extremely critical ; not 

 likely long to maintain itself under the same geographical dis- 

 tribution of the surface as now prevails, and liable to be 

 changed by many slight causes. If the mean temperature of 

 Glen Spean was little above freezing, and wide fields 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 somewhat less than 4000 feet, not only nou- 

 rished glaciers in their higher recesses, but were wholly em - 

 loped in sheets of ice, can we suppose that the mountains sur- 

 rounding Glen Roy and Glen Gluoy, many of which attain 

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



