196 A. Sarher — Glaciated Valleys in SJcye. 



fits the above evidence, I do not see room for doubt regarding its 

 identification. 



In the future, varieties of allanite will no doubt be distinguished 

 from one another, the varieties being dependent on the proportion 

 of the rare metals, and also on the proportion of manganese present 

 in them ; but the accurate determination of them will be no easy 

 matter. The collection of material, for instance, from the Lairg rock, 

 is a work of labour; and when obtained, the microscopic fragments 

 are found under high powers to be stuffed full of endo-rainerals. 



III. — Glaciated Valleys in the Cuillins, Skyb. 



By Alfred Harker, M.A., F.G.S., of H.M. Geological Survey of Scotland. 



(Communicated by permission of the Director- General.) 



'ANY geologists have recognized that valleys, which have once 

 been occupied by large glaciers, exhibit peculiarities which can 

 only be ascribed to glacial erosion modifying the earlier water-worn 

 form of the land-surface. Indeed, anyone who allows ice-action to 

 be locally a factor of importance in erosion, must expect that the 

 resulting topographical forms will differ in some measure from 

 those proper to water-erosion. Not only does ice, in virtue of its 

 physical properties, obey different laws as regards pressure and 

 flow, but, what is of equal importance, it fills the whole valley 

 instead of following certain narrow channels. We may hope that, 

 just as Gilbert and others have worked out the principles of water- 

 erosion, so the much more difficult problem of ice-erosion may 

 some day be reduced to analysis. An attempt in this direction 

 has been made by McGee in a paper on " Glacial Cafions," ^ which, 

 however, I cite here rather for its brief summary of the observed 

 characteristics of glaciated valleys. 



The object of the present note is merely to draw attention to 

 a district in which these characteristics are exhibited in their most 

 typical form. This is due to two facts. Firstly, the mountain- 

 group of the Cuillins is wholly carved out from a single rock 

 (gabbro), so that all complication arising from the juxtaposition of 

 harder and softer materials is eliminated. Secondly, there are none 

 of the difficulties which attach to a district once overridden by 

 foreign ice. The glaciation of the Cuillins was entirely local, and, 

 although the ice passed over some of the lower watersheds, the flow 

 in the main valleys was always in the natural direction. Further, 

 the topographical forms are not only of a simple ordei-, but are also 

 very clearly exposed. In the mountain tract the drift is never thick 

 enough to obscure the shape of the ground, and the higher valleys, 

 like the mountains themselves, show nothing but naked rock. Since 

 the erosion under existing conditions is a negligeable quantity, we 

 have the forms of the valleys displayed practically as they were left 

 by the ice. 



The chief characteristics of the valleys in the Cuillins are sum- 

 marily as follows. It is important to notice that, while some of 

 them (e g. rook-basins) belong to glacial erosion only, others consist 

 1 Journal of Geology, vol. ii (1894), pp. 350-364. 



