MKDLICOTT ALPS AND HTMALATAS. 51 



main gorges of the mountains (for the prevention of which alluvial 

 deposition glaciers were admitted as secondary though almost 

 essential agencies in the hypothesis) would equally well attest the 

 action of the principal agency appealed to. Alluvial flats of this 

 nature do not exist in the Himalayas ; the great rivers are torrential 

 throughout their entire course to the plains. It is evident, however, 

 that the production of such lakes is a very non-essential result of 

 the whole process now under consideration, and contingent upon a 

 number of circumstances, in degree and in kind. All other con- 

 ditions being alike, if in one case the erosion of the valleys were 

 much more complete than in the other, the same relative movement 

 would produce lake-basins in that case, while there could be no such 

 result where the fall of the main drainage was very steep. Or, the 

 same amount of vertical movement, equally efficient for the structural 

 results required, may, from unseen influences, be very differently 

 distributed in two cases ; the central subsidence might be localized 

 at and about the centre, with little or no rise along the flanks. By 

 some such plausible modification as this, the great lake-basins of the 

 central Himalaya may be the true analogues of the fringing lakes of 

 the Alps. 



I quite admit the force of the difficulty which induced Sir Charles 

 Lyell to introduce glacial action as a subsidiary agent in the for- 

 mation of the great Alpine lakes. I, too, should have thought that 

 the accumulation of torrential debris would have kept pace with 

 the formation of the basin. If the objection is sound, it quite 

 upsets the supposition I have made regarding the age of the Alpine 

 lake-basins. It rests, however, on a purely d-priori judgment, and 

 cannot outweigh a fair accumulation of evidence on the other side. 

 If that judgment proves unfounded we shall have acquired a pro- 

 visional limit and gauge for the rapidity of the crust movement*. 



There is a well-known difficulty in Swiss geology that may, I 

 think, be reduced by the supposition I have advanced as to the 



* Some years ago Mr. H. F. Blanford applied this mode of explanation to 

 some rock-basins in the Nilghiri Hills. See Mem. Geol. Survey of India, vol. i. 

 pp. 241-243: 1859. 



Although I have attempted to substitute another explanation for that given 

 by Professor Ramsay of the formation of the great lake-basins of Switzerland, I 

 fully assent to the power of glaciers to form rock-basins under certain conditions. 

 An observation I made in Switzerland removed a mistaken a-priori opinion 

 that had until then stood in my way. The observation must be patent to many, 

 though I have not seen it described ; but as the mistaken notion to which I refer 

 seems to have still greater currency (it is the principal objection m^ged against 

 Prof. Ramsay's views in a recent presidential address to the Greological 

 Society), I may notice the observation. Supposing a rock-basin formed and 

 filled with ice, it is often doubted if there could be enough of tractive force, or 

 even of vis a tergo, to exercise a scooping-action within the basin ; it is thought 

 that the upper ice would flow on, leaving that in the basin almost undisturbed. 

 The little lake of Lungern lies at the outlet from the fine amphitheatre cut in 

 the flanks of the Brunig. The rock-barrier is so steep and narrow that it has 

 been considered worth while to make a tunnel fifty feet below the rim, for the 

 sake of the land gained at the delta by the partial drainage of the lake. The 

 precipitous upward face of the rock-barrier is thus admirably exposed ; and it 

 displays numerous deep and regular grooves, the unmistakeable marks of the 

 action in question. 



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