556 DR. C. S. DT7 RICBIE PRELLER ON GLACIAL [Aug. 1 896, 



33. Glacial Deposits, Preglacial Valleys, and Interglacial 

 Lake-Formations in Subalpine Switzerland. By C. S, 

 Du Eiche Preller, M.A., Ph.D., F.G.S., F.C.S., A.M.I.C.E. r 

 M.I.E.E. (Read March 11th, 1896.) 



Contents. 



Page 



I. Introduction 556 



II. Characteristics and Origin of Cavernous Nagelfluh or 



Subalpine Deckenschotter 557 



III. Glacial Deposits 560 



IV. Preglacial Subalpine Valleys 574' 



V. The Origin of Subalpine Lake-Basins 580 



VI. Conclusion 584 



I. Introduction. 



In a paper which I had the honour of reading before the Society- 

 last session, 1 as well as in another previously published in the 

 Geological Magazine, 2 I described in detail a variety of glacial 

 deposits near the lakes of Constance, Zurich, Zug, and Thun, in- 

 eluding several of those extensive and remarkable accumulations of 

 glacio-fluviatile conglomerate which are the indirect product of 

 land-ice and, together with intervening moraine and interglacial 

 deposits, afford conclusive evidence of three separate and general 

 glaciations not only of the Alps, but more or less of the Swiss 

 lowlands. 



The numerous sections of which I exhibited diagrams, revealed 

 the apparently singular phenomenon that in the Ziirich district the 

 older glacio-fluviatile conglomerates generally crown the ridges of 

 the Subalpine Molasse hills, and the younger gravel-beds occur at 

 lower levels ; while in the Lorze valley near Zug, and in the 

 Kander valley near Thun, the position is reversed — that is, the 

 older gravels, resting in the former locality on glacial clay and 

 Molasse, and in the latter on glacial clay and Triassic beds, appear 

 at the bottom, and are, with intermediate alternations of moraine, 

 overlain by the younger series. From this marked contrast, it 

 might be inferred that at the time of the deposition of the older 

 gravels, the principal valleys in the last-named districts — that is,. 

 the basins of the Reuss and Aare, — were already eroded approxi- 

 mately to their present depth, whereas the Zurich valley was as • 

 yet non-existent or, at any rate, very imperfectly formed; so much 

 so, that if this theory be correct, the oldest gravels, now commonly 

 called Deckenschotter, must have been deposited as a sheet on a 

 Molasse plateau extending practically from the present Zurich 

 valley to that of the Rhine. 



In the paper read last session, I alluded indeed, although merely 



1 Quart. Journ. Geol. Soc. vol. li. (1895) pp. 369 et seqq. 



2 Geol. Mag. 1894, pp. 27 et seqq. 



