124 STUDIES, SCIENTIFIC AND SOCIAL chap. 



and weight of the glacier and its long duration, as indi- 

 cated by the great distance to which it extended beyond 

 the site of the lake, render the excavation by it of such a 

 basin as easy to conceive as the grinding out of a small 

 alpine tarn by ice not one-fourth as thick, and in a situa- 

 tion where the grinding material in its lower strata would 

 probably be comparatively scanty. 



We have now to consider the theory of Desor, adopted 

 by M. Favre, and set forth in the recent work of M. Falsan 

 as being " more precise and more acceptable " than that 

 of Ramsay. We are first made acquainted with a fact 

 which I have not yet alluded to, and which most writers 

 on the subject either fail to notice or attempt to explain 

 by theories, as compared with which that of Ramsay is 

 simple, probable, and easy of comprehension. This fact is, 

 that around Geneva at the outlet of the lake, as well as at 

 the outlets of the other great lakes, there is spread out an 

 old alluvium which is always found underneath the houlder- 

 clay and other glacial deposits. This alluvium is, more- 

 over, admitted to be formed in every case of materials 

 largely derived from the great Alpine range. Now here 

 is a fact which of itself amounts to a demonstration that 

 the lakes did not exist before the ice age ; because, in that 

 case all the Alpine debris would be intercepted by the 

 lake (as it is now intercepted) and the alluvium below 

 the glacial deposits would be, in the case of Geneva, that 

 formed by the wash from the adjacent slopes of the Jura ; 

 while in every case it would be local not Alpine alluvium. 



Professor James Geikie informs me that he considers 

 the so-called " old alluvium " to be probably only the 

 fluvio-glacial gravels and sands swept out from underneath 

 the advancing glacier, and therefore to be no older, 

 geologically, than the moraine matter which overlies it. 

 The Swiss geologists, however, do not appear to hold this 

 view, since they have recourse to a very remarkable 

 hypothesis in order to overcome what they evidently 

 believe to be a real difficulty in the way of the pre- 

 glacial origin of the lake. The suggested explanation is 

 as follows : At the beginning of the ice age the glacier 



