Vol. 52.] DEPOSITS, ETC., IN SUBALPINE SWITZEELAND. 557 



by the way, to the occurrence of Cavernous Nagelfluh or Decken- 

 schotter * in the Ziirich valley itself, that is, at a considerably lower 

 level than the Uetliberg and allied high-level deposits ; and as the 

 question involves the solution of an important problem, I took the 

 opportunity of examining last summer a large additional number 

 of glacial deposits throughout that district. 2 The object of this 

 inquiry was twofold : — 



(1) To ascertain which, if any, of the lower-level conglomerates 

 must be regarded as the glacio-fluviatile products of the first or 

 Upper Pliocene glaciation, or whether we must simply and en bloc 

 accept the view recently enunciated by several Swiss geologists 3 

 that all the lower-level gravels are, without distinction, so-called 

 Upper and Lower Terrace-gravels — that is, the indirect products of 

 the two Pleistocene glaciations. 



(2) To determine the probable and approximate depth of the 

 Zurich valley before the deposition of the Deckenschotter — in other 

 terms, to reconstruct the Preglacial valley as it appeared at the 

 advent of the first glaciation in Upper Pliocene times. In the 

 course of this enquiry, which has a direct bearing also on the 

 general Preglacial configuration of the other principal Subalpine 

 valleys, I was, moreover, led to several important conclusions with 

 respect to the combination of causes which determined the forma- 

 tion of the lake of Zurich, and, in a wider sense, of the other lakes 

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



II. Characteristics and Origin of Cavernous Nagelfluh 

 or Subalpine Deckenschotter. 



Characteristics. — The characteristics of the Subalpine Decken- 

 schotter consist, according to my observations, chiefly in the 

 occurrence of the conglomerate in isolated or continuous cliffs of 

 peculiarly rugged appearance ; in advanced cementation, not merely 

 by siliceous material, but by calcification ; in the occurrence of 

 partially leached pebbles, and more especially of cavities left by 

 pebbles which have been either wholly dissolved or have fallen out ; 

 in the generally noticeable stratification, as well as in the occurrence 

 of underlying sand as a clear proof of the fluviatile origin of the 

 deposit ; and, lastly, in the occasional occurrence of striated pebbles 

 as further proof that, in such cases, the deposit was formed at no 

 great distance from a glacier. 



1 Although, as I shall show, the term ' Deckenschotter,' or plateau-gravel, is 

 not strictly applicable to the Subalpine Cavernous Nagelfluh deposits, both 

 terms are used throughout this paper as denoting the glacio-fluviatile conglo- 

 merate which is the indirect product of the first glaciation. The palaeonto- 

 logical evidence of the Subalpine Deckenschotter having been deposited towards 

 the end of the Pliocene period rests, as already pointed out in my paper of last 

 session, mainly on the occurrence of Mastodon arvernensis in the analogous 

 ' alluvions anciennes ' of the Ehone valley above Lyons. 



2 Prof. T. G. Bonney, in his ' Ice-Work ' (London, 1890), published since 

 this paper was written, truly observes that the Zurich district is 'classic ground 

 in the history of the work of ice.' 



3 Notably by Dr. L. Du Pasquier, Beitrage zur geol. Karte d. Schweiz, 

 vol. xxxi. (1891), and by Dr. A. Aeppli, ibid. vol. xxxiv. (1894). 



