118 Geological Society : — 



Hence he contended that at the advent of the first glaciation 

 the Zurich Valley was already eroded, and thai, consequently, the 

 term ' Deckenschotter/ or plateau-gravel, was not strictly applicable 

 to the Pliocene glacio-fluviatile deposits of the Swiss lowlands. In his 

 view, the isolated high-level deposits were formed during the inter- 

 mittent shrinkage of the Upper Pliocene ice-sheet, while the low- 

 level deposits were formed during the subsequent recession of 

 individual glaciers left in the several valleys. 



IV. The author reconstructed the pre-Glacial floor of the Zurich 

 Valley upon the evidence of the solid rock and of the low-level 

 Pliocene nageiduh deposits, with the result that the depth of the 

 lower part of the Valley was approximately that of the present day, 

 while the floor of the upper part was at a higher level (maximum, 

 300 feet above present lake-level), and was subsequently lowered 

 by earth-movements. He further adduced evidence that the Sub- 

 alpine valleys of the Reuss, Aare, and Rhine were likewise excavated 

 before the first glaciation. By calculation, he arrived at an estimate 

 of the time required for the excavation of the Zurich Valley, and 

 contrasted the erosive energy of the river with the impotence, on 

 mechanical grounds, of a glacier 7000 times larger in volume. 



V. The author showed that the Lake of Zurich owes its origin, in 

 the first instance, to a zonal subsidence (probably between the first 

 and second glaciation) of about 1000 feet, as evidenced by the 

 reversed dip of the disturbed molasse-strata between the lakes 

 of Zurich and Zug. During the second and third Ice-periods, the 

 original Jake-basin was gradually filled with glacial and fluviatile 

 deposits at both ends, and was finally restricted to its present 

 dimensions by a post-Glacial bar deposited at its lower end by 

 a tributary river. In the author's view, the other Subalpine lakes, 

 extending from the Lake of Constance to Lac Bourget in Savoy, 

 owe their origin and present limits, in the main, to the operation of 

 similar causes. 



VI. With regard to the main question, the author averred that 

 the Lower and Middle Pliocene period was, in Switzerland, entirely 

 one of erosion and denudation on a prodigious scale. Irrespective of 

 the evidence he had adduced, he was therefore driven to the conclusion 

 that at the advent of the first Ice-period in Upper Pliocene times, 

 the principal Subalpine valleys must have been already excavated 

 approximately to their present depth, and that ever since then the 

 action of the great Alpine and Subalpine rivers has been, as it is 

 still in our own day, mainly directed to regaining the old valley- 

 floors by removing those enormous accumulations of glacial and 

 glacio-fluviatile material, which are respectively the direct and 

 indirect products of three successive and general glaciations. 



3. ' Notes concerning certain Linear Marks in a Sedimentary 

 Rock.' By Prof. J. E. Talmage, D.Sc, F.G.S. 



The marks described in the paper occur in a fine-grained argil- 



