298 THE ICE AGE AND ITS WORK. 



tliickiies.s was necessary to cause ouwaid niotiou that thickness coald 

 not fail to be produced, since it is only by the onward motion to some 

 outlet or lowland where the ice can be melted away as fast as it is 

 renewed that indefinite enlargement of a glacier is avoided. The essen- 

 tial condition for the formation of a glacier at all is that more ice should 

 be produced antiually than is melted away. So long as the quantity 

 produceti is ou the average more than that melted, the glaciers will 

 increase ; and as the more extended surface of ice, up to a certain point, 

 by forming a refrigerator, hel])S its own extension, a very small perma- 

 nent annual surplus may lead to an enormous extension of the ice 

 Hence, it at any stage in its development the end of a glacier remains 

 stationary, either owing to some obstacle in its path or to its having 

 reached a level plain wiiere it is unable to move onward, the annual 

 surplus of ice produced 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 equilibrium 

 betAveen growth and dissolution. If, therefore, at any stage in the 

 growth of a glacier, a thickness of 0,000, 7,000, or even 8,000 feet is 

 needed to bring about this result, that thickness will inevitably be 

 produced, - - - 



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 con- 

 ceive as the grinding out of a small alpine tarn by ice not one-fourth as 

 thick, and in a sitnation where the grinding material in its lower strata 

 wonld 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 Eamsay, We are flrst made 

 accpiainted with a fact which I have not yet alluded to, and wiiich 

 most writers on the subject either fail to notice or attempt to explain 

 by theories, as compared with w^hich that of Ramsay is simple, proba- 

 ble, and easy of comi)iehension. This fact is, that around Geneva at 

 tlie 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 fonnd underneath 

 tJie bowlder clay and other {/lacial dejjosits. This alluvium is moreover 

 admitted to be formed in every case of materials largely derived from 

 the great Alpine range. Nowhere is a fact which of itself amounts to 

 a demonstration that the lakes did not e.ri.st 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 alluvinm below the glacial deposits 

 would be, in the case of Geneva, that formed by the wash from the 

 adjacent s]oi)es of the Jura, ^vhile in every case it would be local not 

 Alpine alluvium. - - - 



