264 Professors A. JSeim and A. Penck — 



Land, Penck was able to confirm the facts which had been pub- 

 lished by A. Wettstein in his work, " Geologie von Zurich und 

 Umgebung." We are completely agreed respecting the facts observed, 

 and differences can only arise on the conclusions to be drawn from 

 the facts. The great differences in the Quaternary deposits of 

 Bavaria and of Switzerland deserve special prominence. They 

 teach that the greatest prudence is necessary, in regard to generali- 

 zations based on conclusions drawn from a limited area. 



The grounds which Penck brings forward in favour of the Glacial 

 origin of the Bavarian highland lakes are : — (1) the coincidence of 

 the position of the lakes with the glacial deposits ; (2) the 

 characters of the lake valleys are such as are produced by erosion ; 

 and (3) the age of the lakes. 



Their character as products of erosion is made clear from the 

 fact that they are valley-shaped gaps in a continuous uniform series 

 of undisturbed stratified gravels, whilst their diluvial age is proved 

 by the fact that these stratified gravels end with the deposit of 

 the diluvial Nagelfluh (I. a). On the other hand, both above and 

 below the lakes, as well as round their margins, the horizontally 

 stratified gravels (Schotter) (I. b) are shown, in which the lake-basins 

 appear to form excavations. As these gravels (I. b) are, on the one 

 hand, covered by moraines, and on the other, contain a notable 

 number of pebbles of Archaean rocks, which could only have been 

 transported by glaciers from the central region of the Alps over the 

 passes of the limestone Alps, they must therefore have been formed 

 immediately before the advance of the glacier into the lake district. 

 As. however, the lake-basins are excavated in these gravels, they are 

 necessarily more recent ; they could thus not have yet been in 

 existence at the commencement of the last glaciation of this district 

 to which the gravels correspond. On the other hand, the deposition 

 of the moraines on the slopes of the lake-basins shows that these 

 latter were formed at the time of the retreat of the glacier. Their 

 origin must therefore have taken place at the time of the glaciation 

 itself. 



Of these three grounds brought forward by Penck, the first one is 

 established. As regards the second, it might be said that the gaps 

 in the south margin of the covering of Nagelfluh might have been 

 produced in it originally ; that glacial-tongues, which covered the 

 areas of the lake-basins, produced the Nagelfluh, as a fluvio-glacial 

 deposit, whilst they protected the lake-basins against the accumula- 

 tion of gravels. Against this, is, 



(«.) The high degree of uniformity in the petrographical characters 

 and the stratigraphical deposition of the Nagelfluh (I. a). 



(/3.) The absence of any morainic material below the Nagelfluh, 

 as also the non-existence of genuine glacial materials, such as striated 

 boulders, etc., within it. 



A further possible mode of formation of the lakes might consist in 

 a relatively small change of level in the lake district, by which the 

 original slope of the Nagelfluh was diminished, whilst the valleys 

 cut in it became reversed, and converted into lake-basins. This is 



