.580 DE. C. S. DTJ EICHE PEELLEE 02T GLACIAL [Aug. 1 896, 



schotter being embedded in the valley. 1 A further striking example is 

 that of the Rhone valley, which Prof. F. A. Forel, in his well-known 

 work on the Lake of Geneva, conclusively shows to be post- 

 Miocene, but pre-Pleistocene 2 — that is, of Pliocene and Preglacial 

 age. 



There is thus abundant evidence that all the principal river- 

 valleys of Subalpine Switzerland were, in the main, already formed 

 ■and excavated before the advent of the first glaciation. 3 



V. The Oeigtn oe Subalpine Lake-Basins. 



Prom the profile of the Preglacial valley-floor, it is evident that 

 at the advent of the first glaciation the Lake of Zurich had not yet 

 come into existence. That its formation is primarily due to a sub- 

 sidence of the old valley-floor is attested, not only by the great 

 and unknown depth at which the solid rock lies buried beneath 

 moraine and gravel near the outflow of the river Limmat from the 

 Lake of Zurich, where borings have failed to reach the Molasse even 

 at 40 metres below the river-bed, but by the gradually increasing 

 depth of the lake from Zurich upwards, the deepest point, about 

 halfway, and not far from Au, being 142 metres below lake-level, 

 although even here the bottom consists not of solid rock, but of mud 

 and other lake-deposit overlying the same. From this point the 

 lake, much filled with moraine and sand, again rapidly decreases 

 in depth as far as the old Ufenau bar, where the Molasse appears at 

 lake-level ; and immediately beyond — that is, on the other side of the 

 bridge — emerges the old moraine-wall which banked the upper or 

 shallow lake-basin, the Molasse being in this basin again covered to 

 a great depth by glacial and other deposits. As already shown, the 

 solid rock appears at the confluence of the Limmat and Aare, near 

 Turgi, at contour 332, and again near Wettingen, at contours 358 

 and 362, while the deepest point of the lake lies at contour 266, or 

 66 and 94 metres lower than the solid rock near Turgi and Wet- 

 tingen respectively. This remarkable difference of level clearly 

 points to a subsidence of the old valley-floor for a distance, measured 

 longitudinally, of nearly 40 kilometres. 



On examining the profile (p. 577), it will be observed that the 

 general crest-line of the hills flanking the Ziirich valley is marked 

 by two distinct anticlines trending obliquely across the valley and 

 parallel to the general strike of the Jura and Alps, namely, the 



1 Beitrage z. geol. Karte d. Schweiz, vol. xxxi. (1891) p. 103. In another 

 place (p. 106) Dr. Du Pasquier states that the transverse valleys (e. g. the Aare 

 Valley), too, must have been already, though imperfectly, formed even in 

 Miocene times, as is shown by the occurrence of Miocene Nagelfluh. 



2 < Le Leman,' vol. i. (1892) p. 240. 



3 I have not dealt in this paper with the Eastern Alps ; but there is no doubt 

 that the Inn Valley above and below Innsbruck is also Preglacial, the old 

 gravels being deeply embedded in it. This view is also shared by Dr. Du 

 Pasquier, op. jam cit. p. 106. 



