60 NATURAL SCIENCE. July, 



of a still more ancient lake by the sawing through of a moraine- 

 dam by the valley-stream, can scarcely be better illustrated than in 

 the case of the Rosegg-glacier. There, as in many another Alpine 

 example, the recession of the glacier leaves no trace of excavation of the 

 valley floor, but the contrary. The Muir Glacier of Alaska is also a 

 very strong case. I gratefully avail myself of the paper on this which 

 Professor Cushing, of Cleveland, Ohio, was good enough to send me. 6 

 His concluding remarks are — " Those who hold the power of glaciers 

 to vigorously erode hard rocks under most circumstances, take (it 

 seems to me) an indefensible position. At the Muir Glacier, in just the 

 position where the greatest erosion would naturally be expected, 

 soft gravels have been undisturbed by the ice. The key setting 

 forth when glaciers will erode and when not, is certainly lacking at 

 present. It is very desirable that a prolonged and detailed study 

 of the Muir Glacier should be undertaken. It is a comparatively 

 large glacier, rapidly dying out, and presents an admirable opportunity 

 for studying the behaviour of a large glacier under such circumstances. 

 Such work could not fail to be of great value." 



On one minor point I cannot follow Professor Bonney. When 

 referring to Ramsay's classical paper, he says (p. 482) : " It was proved 

 that the rock-basins of the Alpine lakes could not have been produced 

 by any local subsidence, or by fissures in the Earth's Crust." In 

 reasons given in my paper (supra cit.) on the "Origin of Valley Lakes," 

 I have given the grounds of this dissent. I believe that, in certain 

 instances, these have severally operated as important factors, though not 

 as sole cause, and my belief of ten years ago has been confirmed by 

 the observations I made in 1883, through the courtesy of F. Ritter von 

 Hauer, of models of saliferous and gypsiferous strata, beneath upland 

 valleys in the Alpine Trias, which are to be seen in the Geologische 

 Reichsanstalt of Vienna. Mr. Jukes Browne has referred to these 

 in his "Handbook of Physical Geology " (2nd edition, p. 627). Let 

 underground erosion by the solution and removal of such beds or of 

 calcareous strata proceed through a long period of time, and another 

 great glacier filling such a valley would make, by its dead weight, 

 short work of the hollow floor. Such a crushing-in of the pie-crust is, 

 however, a very different thing from what Ruskin has graphically 

 described as a "custard scooping out its own dish." I suggested this 

 ten years ago: its rationale is self-evident, but it has I suppose 

 been ignored by subsequent writers because it was not "orthodox," 

 though it commended itself to the late Professor John Morris, when I 

 suggested it to him. But subsidences can and do occur without the 

 crushing weight of glaciers. 



The old and trite argument based on the association of lakes with 

 glaciated regions may be put another way. It may, I think, be fairly 

 asked, whether it is not the association of lake-basins in great numbers 



,; American Geologist, October, 1891. 



