106 Mk. Bryce oh the Parallel Roads of Lochaher, 



whicb has been written upon tlie Parellel Roads. The e\'idence in support 

 of his own views has been collected with the gi-eatest sagacity, and the 

 arguments founded upon it conducted with consummate skill ; while he 

 appears to me to have completely demolished both the theory of Mr. 

 Darwin, and the glacial theory, in the form proposed by M. Arjassiz. 

 The agency assigned by Agassiz will not explain all the phenomena, and 

 is positively inconsistent with many facts. But it does not hence follow 

 that glacial action is to be rejected, as explaining the blocking up of the 

 mouths of the glens, — for it is required for this purpose alone. May not 

 a form be given to the theory which will adapt it to all the exigencies of 

 the case, and thus remove from the lake theory the one great remaining 

 objection — the origin and the disappearance of the enormous earthy 

 barriers at the mouths of the glens ? Since Agassiz wrote, the question 

 has been placed ou a very different footing. The first glacialist in 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 fiUed the still higher 

 vaUeys of the Ben Nevis group of mountains ? Professor Forbes' late 

 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 comparatively 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 no- 

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

 from the hills unites itself with that which is deposited from the atmos- 

 phere on the plains ; and the whole becomes consolidated into one con- 

 tinuous 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 sufiicient 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 tem- 

 perature 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 Hoy, would be occupied by lakes " 



The state of things here supposed is extremely critical ; not likely long 

 to maintain itself under the same geographical distribution 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 



