584 dr. c. s. du riche preller on glacial [Aug. 1896, 



du Bourget by the Rhone. 1 In all these cases, then, a delta and 

 superposed detritus-cone were thrown across the lake-valley practi- 

 cally at right angles, and thus the four successive main phases in the 

 formation of Subalpine lakes of our own day may be summed up as 

 follows : — fluviatile erosion of the Preglacial valleys; subsidence of 

 the valley-floors in their upper parts by unequal earth-movements in 

 the Alps ; partial filling of the original lake-basins by glacial and 

 glacio-fluviatile deposit at both ends ; and, finally, restriction of the 

 lower ends of the lakes to their present limits by fluviatile Post- 

 glacial bars. 



VI. Conclusion". 



It will be observed that the conclusions at which I have arrived 

 in this paper differ in some respects widely from the views recently 

 enunciated by several Swiss geologists. More especially does this 

 difference relate to the question whether the principal Subalpine 

 valleys were excavated before or after the first or Upper Pliocene 

 glaciation. 



Apart from the evidence I have adduced, my conclusion that the 

 first glaciation found the principal Subalpine valleys already eroded 

 derives substantial support from the very history of the Tertiary 

 epoch. In Eocene times the Alps had already emerged from the 

 surrounding sea, though as yet only as a group of low islands. 

 The Miocene period witnessed the principal thrusting and folding 

 of the Jura and the Alps : and the products of increased denu- 

 dation — namely, sand and clay, afterwards hardened to Molasse, and 

 fluviatile detritus subsequently cemented to compact conglomerate — 

 were deposited between the two ranges in two freshwater and one 

 intermediate marine series, the latter being formed in an arm of 

 the sea which, towards the end of the Miocene period, still reached 

 from the Mayence basin to the foot of the Alps. The process of 

 folding continuing, the Subalpine Molasse and the enormous banks 

 of conglomerate, too, were raised, notably near the Alps ; the sea 

 receded ; the lakes formed in the shallow depressions of the Molasse 

 plateau dried up, and, consequent upon the increased steepness of 

 the Alpine slopes, denudation and erosion set in on a greatly 

 enhanced scale; rapids and ravines formed; the great Alpine rivers 

 effected in the Miocene Nagelfluh walls those breaches which later 

 on afforded easy and convenient passages to glaciers ; and broad 

 valleys were eroded in the soft Molasse strata of the Swiss lowlands, 

 the general direction of discharge being towards the natural col- 

 lecting-channel along the foot of the Jura, and thence to the Rhine. 

 Thus, the Lower and Middle Pliocene period in Subalpine Switzer- 

 land did not witness the deposition of any new rock-formations, but 



1 Prof. F. A. Forel {op. cit. p. 246) is disposed to regard even the alluvium 

 of the River Po as having been instrumental in forming the contour of the 

 Italian lakes at their lower ends ; but the formation of these southern lake-basins 

 is a much more complex question than that of the lakes at the northern base of 

 the Alps, and 1 have refrained from dealing with them in this paper, as they 

 are outside its immediate scope. 



