'Vol. 52.] DEPOSITS, ETC., IN SUBALPINE SWITZERLAND. 583 



be removed, the configuration of the subsidence-zone between the 

 two lakes would be that of a flat dome-shaped ridge which, before 

 the subsidence, was a continuation of the original Albis range, and, 

 being then 300 metres higher, was practically at the same altitude 

 as the Molasse of the latter — that is, approximately at contour 850. 

 It is therefore on this old plateau that the Altschloss Deckenschotter, 

 now 300 metres lower, was deposited, while that of Au, assuming 

 it to extend 20 metres below present lake-level (to contour 390), was 

 deposited at an altitude 207 metres higher, that is, approximately 

 at contour 660, or on the upper slope of the old ridge. 



As is seen from the profile, the Preglacial valley-floor near 

 Altschloss was at contour 500 and, after the subsidence of 300 metres, 

 at contour 200. The lowest point of the present lake-bed, com- 

 posed of lake-deposit, is at contour 266 ; but the solid rock probably 

 lies 15 metres lower, namely at contour 251. A line drawn from 

 contour 200 through contour 251 and thence prolonged, strikes the 

 Preglacial valley-floor at contour 374 — that is, near Killwangen, 

 about 17 kilometres below the end of the present lake at Ziirich. 

 Killwangen therefore marks the probable starting-point of the 

 subsidence of the Preglacial valley-floor, and consequently the 

 probable lower end of the original lake-basin. 



The formation of the lake-basin kept pace with the gradual sub- 

 sidence of the valley-floor, and was therefore very slow ; that is, 

 the river becoming gradually more sluggish, formed pools, and, 

 finally, a continuous sheet of water with an outlet at the lower 

 end. The deposit of the second, and, again, of the third glacia- 

 tion gradually filled the lower basin, and subsequently a Postglacial 

 bar, deposited by the Sihl at its inflow into the Limmat just below 

 Zurich, defined the present limits of the lake at its lower end. 



The phenomenon of similar Postglacial bars is traceable also in 

 other Subalpine lake-formations. Thus, the Rhine which formed 

 the Lake of Constance, was probably, in the first instance, barred 

 by the Danube, and subsequently deflected by moraine-walls 

 towards Schaffhausen ; the Walen Lake, originally connected with 

 the Lake of Zurich, was barred by the Linth ; the Lake of 

 Zug by the Lorze, and probably also by the Reuss ; the Lake 

 of Lucerne by the Emme, although here the evidence is not so 

 clear ; the Lake of Brienz, originally connected with that of Thun, 

 was banked by the delta of the Liitschinen, while the Lake of 

 Thun was banked by the Kander; again, the Aare banked the 

 Lakes of Neuchatel and Bienne, and the bar deposited by the 

 Arve at its inflow into the Rhone was largely instrumental in 

 forcing the Lake of Geneva back to its present limits. Similarly, in 

 Savoy, the Lake of Annecy was barred by the Fier, and the Lac 



142 and 160 metres as the depth of the subsidence, their calculation being, 

 however, based simply upon the difference of level between the Molasse near 

 Baden and the deepest point of the lake, plus the fall of the present vallev-floor, 

 or 017 in 100. 



Q. J.G. S. No. 207. 2r 



