28 Dr. Du Riche Preller — Three Qlaciations of Switzerland. 



which intervened a genial or interglacial period. The repeated 

 advance of Swiss glaciers was first established by Escher von der 

 Linth and Heer, more especially upon the evidence of the lignite 

 slate deposits near Durnten and Wetzikon, which fringe the edge of 

 what was at one time the Limmat and is now the Grlatt valley, to 

 the N.E. of Zurich, and were formed during the interglacial period 

 referred to. But since Prof, Penck and others have been led, on 

 the strength of fluvio-glacial evidence, to recognise in the Bavarian 

 and Austrian Alps not only two, but three, alternately recurring 

 glaciations, evidence to the same effect has, within the last two 

 years, been brought forward also in Switzerland, more especially by 

 Dr. Du Pasquier ; and as the district of Zurich, owing to the build- 

 ing operations and public works constantly in progress, abounds, 

 perhaps more than any other part of Switzerland, in quarries and 

 excavations of all kinds, I i-ecently took occasion to examine in 

 this locality various glacial deposits, which may be taken as typical 

 examples of the successive glaciations. 



Introductory. — Before dealing with those glacial deposits, it will 

 be convenient to briefly refer to the molasse and nagelfluh formation 

 which immediately preceded the glacial epochs. The Miocene molasse 

 formation, composed of mud, clay, and sand, which was derived by 

 denudation from the younger rocks of the Central Alps and hardened 

 into marl, limestone and sandstone, spread more or less over the 

 whole of the Swiss lowlands, and formed a flat freshwater basin, 

 with a sub-tropical flora and fauna and a mean temperature which, 

 according to Heer's calculation, was 18-5 degrees Centigrade (68 F.) 

 or more than double that of the present mean temperature of 

 9° Centigrade or 48° F. This molasse basin was subsequently filled 

 up and more or less completely covered by that characteristic fluvial 

 conglomerate known as nagelfluh, whose pebbles were derived 

 chiefly from the detritus of the Central Alps. The nagelfluh in 

 the Canton of Appenzell, in the north-east corner of Switzerland, 

 contains pebbles derived from rocks which do not now exist on the 

 northern slopes of the Alps, but are related to rocks such as occur 

 in Southern Tyrol at Botzen, at Lugano and elsewhere on the 

 southern slopes. This fact led Prof. Heim to argue that those 

 pebbles are derived from what are now the Southern Alps, and 

 that the crest line of the Alps was at one time much more to the 

 south than it is now. That in some parts the divide of the two 

 main watersheds may have been somewhat more to the south is not 

 improbable,^ and that the upheaval of the Alps brought about im- 

 portant deflections of river courses is certain ; but there is no reason 

 why such rocks as those of Botzen and Lugano should not have existed 

 also more to the north, whence they were washed away by the Alpine 

 rivers and their debris were deposited as pebbles of the Miocene 



1 In a recent paper " On the Engacline Lakes" I showed — and this is also Prof. 

 Bonney's and Prof. Heim's view — that the crest line of the old Inn watershed was 

 about six miles more to the south than it is now; but on Prof. Heim's theory the crest 

 line of the Julier Alps and Hinter-Ehine watershed, if placed in a line with Lugano 

 and Botzen, would have been 35 miles further south than it is now, a conjecture 

 for which there is no evidence. 



