484j Prof. B. Studer on the Origin of the Swiss Lakes. 



escaped by means of the hypothesis respecting the extension of 

 the ancient glaciers; and this hypothesis in this case can no 

 longer render us the same service. 



The difficulty may be diminished by reducing as much as 

 possible the mass of the gravels the transportation of which 

 across the lakes, before the great extension of the glaciers, 

 appears to be inevitable. Indeed we know stratified quaternary 

 gravels scarcely distinct from each other in their composition, 

 but which evidently belong to very different ages. In the envi- 

 rons of Lyon's and Yienne there are incoherent gravels, identical 

 with some old alluvium, which contain marine fossils, and which 

 M. Lory believes must be united with the Molasse. On the 

 heights of the Albis near Zurich, on other ridges of Eastern 

 Switzerland, and to the north of the Lake of Constance, there 

 are very thick stratified gravels, partly cemented by calcareous 

 tufa, as to the age of which we are in doubt, some regarding 

 them as tertiaries, others as diluvian. On the other hand, in the 

 environs of Berne and Fribourg, and as far as Lausanne, we see 

 the stratum with erratics resting immediately upon the Molasse ; 

 the gravels, equal in thickness to the boulder-formation, fill up 

 the depressions beside it, and extend in many places over the 

 boulder-clay. Here, therefore, the gravel is contemporaneous 

 with, or posterior to the glacial epoch, and appears to be the 

 product of the destruction of the erratic deposits by the action 

 of rivers. Lastly, there are large masses of gravel deposited by 

 rivers which have no lakes in their course, such e asthe Sarine, the 

 two Emmes, and the Thar. In ancient times they appear to have 

 frequently changed their course. Nevertheless, after deducting 

 all these masses, there still remains the true ancient alluvium, 

 forming the base of the erratic deposit ; and of this we must 

 endeavour to explain the transportation. This gravel is well 

 developed on the banks of the Adda and the Dora Kiparia, in 

 the environs of Geneva, at the mouth of the Kander in the Lake 

 of Thun, in the neighbourhood of Uznach, and elsewhere. The 

 horizontal strata of this ancient alluvium rest upon the bevelled 

 edges of the inclined strata of Molasse; the date of their forma- 

 tion must therefore necessarily intervene between the catastrophe 

 which elevated the tertiary beds, and the period of the great 

 extension of the glaciers. 



M. de Mortillet* assumes, like M. Desor and the great ma- 

 jority of geologists, that our Alpine lakes are orographic lakes, 

 due to ruptures of the soil which took place when the Alps 

 acquired their present form. He assumes that their basins, 

 before the glacial epoch, were filled with the pebbles and sands 

 which beyond the Alps have also formed the ancient alluvium, 

 * Atti delta Soc. Ital. vol. v. (1363), November. 



