236 Reports and Proceedings — 



Structure of Tavavanaz Diabasio Tufa." By C. S. Du Riche Preller, 

 M.A., Ph.D., F.G.S., P.C.S., A.I.M.I.E., M.I.E.E. 



(1) The main object of this paper, which is the sequel to one read 

 last session, was to solve the problem whether the Pliocene glacio- 

 fluviatile conglomerates of the Swiss lowlands were deposited on 

 a plateau or in already existing valleys. For the purpose of this 

 inquiry, the author examined last summer a large additional number 

 of glacial high- and low-level deposits throughout the Zurich Valley, 

 over an area more than 40 miles in length ; and his investigations 

 further led him to important conclusions with respect to the com- 

 bination of causes which determined the formation of the lake-basins 

 lying in the same zone at the foot of the Alps. 



(2) The author established the true characteristics of the Pliocene 

 nagelfluh as distinguished from Miocene — purely fluviatile con- 

 glomerate on the one hand, and from glacio fluviatile Pleistocene 

 gravels on the other. With respect to the origin of the Pliocene 

 conglomerate, he contended that the material composing the same 

 was not transported from a great distance, but was, in the main, 

 derived from the enormous accumulations of Miocene nagelfluh at 

 the foot of the Alps. Specimens of Miocene nagelfluh-pebbles were 

 exhibited, including the so-called " Tavayanaz Sandstone," which 

 the author, in an appendix to the paper, showed to be diabasic tufa. 



(3) The author described in detail a variety of glacial exposures, 

 and showed that Pliocene nagelfluh in situ, of which he exhibited 

 numerous specimens, occurred not only on the ridges of the hills, 

 but, at a gradually ascending level, also at and near the floor of the 

 Zurich Valley. 



Hence he contended that at the advent of the first glaciation the 

 Zurich Valley was already eroded, and that, consequently, the term 

 " Dechenschotter," 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 riiocene ice-sheet, while the low- 

 level deposits were formed during the subsequent recession of 

 individual glaciers left in the several vallej's. 



(4) The author reconstructed the pre-Glacial floor of the Zurich 

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

 Pliocene nagelfluh 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 Subalpine 

 valleys of the Eeuss, Aare, and Rhine were likewise excavated 

 betore 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. 



(5) 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 



