492 Prof. B. Studer on the Origin of the Swiss Lakes. 



position of a great part of the ancient gravels below Geneva is 

 explained by the deposits of the Arve, and a portion of those 

 of the Lake of Zurich by the deposits of the Sihl and the Linth. 



I do not, however, conceal from myself that this solution can- 

 not be generally admitted. It does not apply to the ancient 

 alluvia to the south of the lakes of the Italian slope of the Alps ; 

 it does not explain the lignitic formation of Uznach, the level 

 of which is considerably above that of the Linth ; nor the stra- 

 tified gravels, older than the glacial deposit, to the north of the 

 Lake of Constance. Even for the Lake of Thun the explanation 

 given does not suffice, inasmuch as we find between Uttigen and 

 Kiesen, two leagues below Thun, vast accumulations of ancient 

 gravel rising to more than 100 feet above the level of the Aar. 

 In these cases we must necessarily have recourse to depressions 

 which have taken place after the deposition of the gravel. M. 

 Guyot* prefers this solution to any other for explaining the 

 origin of the lakes of the Jura, without being driven to it by 

 deposits of ancient alluvium, which do not occur on the left bank 

 of the lakes. In the ridge of Molasse known as the Molle, 

 which rises from the bottom of the Lake of Neuchatel to within 

 30 feet of the surface of the water, as also in that which, in the 

 Lake of Bienne, unites the Jolimont to the island of St. Pierre, he 

 sees submerged ridges of the same order as those of the valley 

 and the Jolimont still occupying their original position. These 

 sublacustrine ridges, moreover, cannot well accord with the sup- 

 position that the lakes had their origin by erosion, whether by 

 means of currents of water or of glaciers, the molasse not being 

 sufficiently solid to resist this. 



If we admit the formation of the basins of our lakes by de- 

 pressions, the deposits of ancient alluvium beyond these basins 

 compel us to assume that the bottom of the great crevasses which 

 form our valleys has been entirely filled up, as is still the case 

 with that of the Arve, that of the Sarine, and others. Although 

 there can have been no deficiency of debris after one of the most 

 terrible convulsions, this filling up undoubtedly requires that an 

 exceedingly long time must have elapsed between the formation 

 of the crevasses and that of the ancient alluvium. Fortunately 

 another consideration, which has nothing hypothetical about it, 

 leads us to the same conclusion, namely, the great difference be- 

 tween the fauna and flora of the last sediments of the Molasse and 

 the first of the ancient alluvium. It requires time, and indeed a 

 long time, to allow a flora analogous to that of the Southern States 

 of North America to give place to that which now grows in our 

 country ; and w r e find the latter represented even in the diluvial 

 lignites. It is probable also that much time would be necessary 

 * Mem. de Neuchdtel, vol. iii. 



