Yol. 60.] LAKE-BASINS BETWEEN THE JURA. AND THE ALPS. 65i 



6. The Age of the principal Lake-Basins between the Jura and the 

 Alps. By C. S. Du Riche Preller, M.A., Ph.D., A.M.I.C.E., 

 M.I.E.E.* F.R.S.E., F.G.S. (Read April 29th, 1903.) 



[Abstract.] 



1. In a paper rea d before the Society in 1902/ the Author 

 showed, on the evidence of extensive high-level deposits of Decken- 

 schotter in Subalpine Prance and Switzerland, that the principal 

 Swiss lake-basins could not have existed at the time when those 

 deposits were formed, during and after the first or Pliocene glaciation 

 of the Alps. In the present paper he deals with the question 

 reserved in the preceding one, that is, to which subsequent period 

 the formation of those lake-basins should be assigned. By the 

 light of further recent investigations in the different localities, he 

 first considers the conditions of the Zurich lake-valley, where the 

 successive glacial and fiuviatile deposits are clearly defined, and 

 then applies his conclusions to the other principal lake-basins 

 lying in the same zone along the edge of the Alps. 



2. The hitherto generally-accepted view that the lake-basins are 

 pre-Glacial in the old sense, or were formed during the first inter- 

 Glacial period, rests, in the main, on two arguments : (1) that the 

 alluvia at the lower ends of the lakes are all Glacial, not only from 

 their appearance, but because the materials composing them could 

 only have been transported thence by glaciers, which either passed 

 over the lakes by bridging them, or through them by completely 

 filling them with ice ; and (2) that the zonal bending of the 

 Molasse along the edge of the Alps, to which the lake-basins owe 

 their existence, occurred before the second or maximum glaciation, 

 because on the hills flanking the Lake of Zurich the younger 

 moraine-banks are undisturbed : and, further, because at a point 

 in the Lorze ravine (near the Lake of Zug) the Deckenschotter 

 conglomerate dips reversely, that is, up the valley, while the over- 

 lying, younger, loose gravel dips in the opposite direction. 



3. The Author adduces evidence to show that the deep-level 

 gravel-beds in the Limmat Valley near and below Zurich are 

 essentially fiuviatile, composed of the characteristic Alpine material 

 of the Rhine aud Linth drainage-areas, and in all other respects 

 similar to the gravel carried by the River Sihl at the present day. 

 These gravel-beds rest upon Glacial clay of the second glaciation, 

 which fills the Molasse-bed of the valley to a great depth, and are 

 overlain by the moraine-bars and secondary products of the third 

 glaciation, the latter being overlain by, and mixed with, the post- 

 Glacial alluvia of the Sihl. 



4. He further argues that it is, on mechanical grounds, difficult 



1 Quart, Journ. Geol. Soc. vol. lviii (1902) p. 450. 

 Q.J.G. S. No. 237. f 



