THE ICE AGE AND ITS WORK. 255 



where it is unable to move onward, the annual surplus of ice pro- 

 duced will go to increase the thickness of the glacier and its 

 upper slope till motion is produced. The ice then flows onward 

 till it reaches a district warm enough to bring about an equilib- 

 rium between growth and dissolution. If, therefore, at any stage 

 in the growth of a glacier a thickness of six, seven, or even eight 

 thousand feet is needed to bring about this result, that thickness 

 will inevitably be produced. We know that the glacier of the 

 Rhone did move onward to the Jura and beyond it; that the 

 northward branch flowed on beyond Soleure till it joined the 

 glacier of the Rhine ; and that its southern branch carried Alpine 

 erratics to the country between Bourg and Lyons, two hundred 

 and fifty miles from its source. We know, too, that throughout 

 this distance it moved at the bottom as well as at the top, by the 

 rounded and polished rocks and beds of stiff bowlder clay which 

 are found in almost every part of its course. 



In view, therefore, of the admitted facts, all the objections 

 alleged by the best authorities are entirely wanting in real force 

 or validity ; while the enormous size and weight of the glacier 

 and its long duration, as indicated 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 situation 

 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 bowlder clay and other gla- 

 cial deposits. This alluvium is, moreover, 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 demon- 

 stration 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. 



Prof. James Geikie informs me that he considers the so-called 

 " old alluvium " to be probably only the fluvio-glacial gravels and 



