Dr. CO. S. Du R. Preller—Lakes Zurich and Wallen. 223 
1. The drainage area of the lake of Zurich comprises about 420 
square miles, the principal tributaries being the rivers Seetz and 
Linth. Both these rivers rise in the Glarner Alps and discharge 
into the Wallen lake which, in its turn, drains into the Lake of 
Zurich by means of the famous Hscher-Linth canal. In addition 
to the discharge of the Wallen lake, which is essentially Alpine in 
character, the Lake of Zurich receives the waters of the sub-Alpine 
hills; and the lower part of the whole drainage area is composed of 
Oligocene molasse and Miocene nagelfluh, the upper part of the series 
_ characteristic of the Glarner Alps, viz. Hocene “flysch” (sandstone, 
limestone and marl), Cretaceous ‘‘ seewen ” limestone, Jurassic (malm 
and dogger) limestone, and Triassic Buntsandstein, the building up 
of the successive Jurassic and Cretaceous series being especially con- 
spicuous in the Glarnisch, the central member of this group of the 
Alps. There can be no doubt that, at one time, the upper (Wallen) 
and lower (Zurich) lakes formed one lake, the subsequent division 
being due to the material brought down from the Alps and deposited 
by the river Linth, thus forming a post-Tertiary intermediate tract 
of marshy and now reclaimed land, just as the detritus carried 
down from the Iungfrau group by the River Lititschinen gradually 
separated the lakes of Brienz and Thun, and formed the intermediate 
tract of land from which Interlaken derives its name. The difference 
of level between the Wallen-lake and that of Zurich is 46 feet, the 
level of the former having been gradually raised by the action of 
the Linth as already explained, until that river, instead of emptying 
itself into, and more or less submerging, the tract between the two 
lakes, was canalized in its lower section and made to discharge into 
the Wallen-lake. A further characteristic point in relation to the 
Wallen-lake is the evident connection which at one time existed 
between it and the Rhine valley, until a low saddle of less than one 
mile in length and only 60 feet in depth was thrown across the defile 
between the Glarner and the Churfirsten Alps near Mels and Sargans 
by the detritus carried down by the river Seetz, thus separating the 
watersheds of the Rhine and Linth or Limmat systems. 
The notion which is still accepted by many, that the principal 
Alpine Lakes owe their origin to glacial erosion, is completely 
falsified by the two lakes in question. The greatest known depth 
of the Wallen-lake is 560 feet, that of the Lake of Zurich 460 feet ; 
but at that depth the bottom only consists of soft mud, and the true 
or solid bottom is at an unknown, probably much greater, depth. 
This circumstance must lead to the conclusion that the two lakes 
owe their origin, not to glacial action, but to a deep rent formed by 
the shrinkage of the earth’s crust and the consequent thrusting and 
subsidences during the great Alpine movements of Tertiary age. 
The evidence of this cleft or rent is particularly striking in the 
Wallen-lake. The axis of the fissure, running in the longitudinal 
‘direction of the lakes, is parallel to that of the (Jurassic) Churfirsten 
Alps flanking the Wallen-lake, and of the (molasse) range of hills 
flanking the Lake of Zurich, the tributary valleys of the drainage 
areas being more or less at right angles to it. It was this fissure 
