376 



DR. C. S. DTJ KICHE PRELLER Otf FLCTVIO-GLACIAL [Aug. 1 89 5, 



offer no outlet to rain-water, so that the latter percolates through 

 the upper moraine and the conglomerate, and collects in copious 



springs on the Molasse, where it 

 S„ ^, has, from the same cause, also 

 formed the extensive stalactite- 

 caves of the Lorze ravine. As 

 shown in the accompanying section 

 (fig. 7), this deposit is intersected 

 in two places : (1) by the river Sihl, 

 which rises in the Schwyz Alps 

 about 12 miles beyond the Convent 

 of Einsiedeln, and now discharges 

 into the river Limmat immediately 

 below the lake and town of Zurich : 

 and (2) by the Lorze torrent, which 

 drains the mountain-lake of Aegeri 

 and discharges into the Lake of Zug 

 and, after leaving the latter, into 

 the Reuss. The imposing gravel- 

 deposit through which the Lorze 

 has thus cut its bed down to the 

 Molasse marks the ancient deflected 

 1 1 , ^"| course of the Sihl. This river was 

 1 1 £J originally the main feeder and 

 (|l q i, (I llli I pre-Glacial erosive agent of the 



S ho vo /'°t^M5i1ii nil Zurich lake-valley, into which it 



discharged practically as a waterfall 

 near the upper end of the present 

 lake. 1 The first advance of the 

 Linth glacier deflected the Sihl and 

 its glacier to the left — that is, to 

 the Reuss basin. During the second 

 glaciatiou, its new course was in its 

 turn banked up by the Reuss glacier, 

 and it again found its way into the 

 Zurich basin, though at a some- 

 what lower point than before — that 

 is, at the present island of Au 

 (11, fig. 2, p. 370), as evidenced by 

 an extensive deposit of gravel-con- 

 glomerate younger than the Pliocene 

 Nagelfluh of the Uetliberg. The 

 third advance of the Linth glacier 

 again forced the Sihl to the left ; 

 but the old outlet to the Reuss 

 basin being banked up, it was com- 



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1 The old point of outflow near Sehindellegi is clearly indicated by the topo- 

 graphical features of the locality. The altitude of the Sihl at Schindellegi is 

 about 800 metres, tbat of the lake 409 metres above sea-level ; but the hori- 

 zontal distance is only 3 kilometres; hence the fall must have been about 

 400 metres, or at the rate of 1 in 8. 



