Vol. 63.'] ATTEIBT7TED TO THE SAVOY AND SWISS ALPS, 



303 



below Obergestelen ; also on the Nufenen Pass and the southern 

 side of the Gries Pass : this being part of a strip which may be 

 traced (by the rauchwacke) through Airolo, across the mouth of 

 the Yal Canaria, and then along the Val Piora to the Lukmanier 

 Pass and Scopi, where the Jurassic group reappears in considerable 

 force. The latter is infolded, as is well known, on both sides of 

 the massif of Mont Blanc, and on the southern one the Trias 

 also is sometimes fairly developed : this second infold crossing the 

 watershed at the Col Ferret to the valley of the Ehone a little 

 east of Martigny, along which it may be traced nearly as far as 

 the Yispthal. As these Jurassic beds are certainly marine, 1 the 

 crystalline rocks, which infold and now rise far above them, must 

 have been greatly elevated since the age of the Lias. 



But the mechanical difficulties involved in Prof. Lugeon's hypo- 

 thesis become yet more conspicuous when we illustrate it by sections 

 drawn on an approximately true scale. Fig. 3, as already stated, 

 represents the block of sedimentary rock in which the travelling 

 movement is to take place, AD marking the northern border of the 

 Prealpes, and BC the corresponding margin of the Piedmontese 

 plain, say, near Ivrea. Then AD : AB = 1 : 30, for AD represents 

 2| miles and AB 75 miles. 2 Since, so far as I know, there is no 

 evidence that the sedimentaries prior to the first Tertiary rising 

 of the Alps were thicker on the southern side, we must set them 

 in movement by supposing that the oblong extended southward 



(suppose to a point B', so that BB' = -£-) and pile on the new part 



BB' a wedge-like mass of sediment, its edge coinciding with B, and 

 its other end, say, 2| miles thick. 3 Thus the slope of its upper 

 surface is rather less than 4°, and the problem practically amounts 

 to this : to transport a mass of rock (fig. 3, p. 300) from a position near 

 x to one near y by the pressure set up by the other mass to the south 

 ofBC. 



In Prof. Sollas's experiments, the length of his trough was to 

 the height as 6 : 1. In the first experiment 4 the slope of the 

 original wedge was about 12°, and the added material, though put 

 on at different times, might, I infer, be represented by a corre- 

 sponding wedge — applied to a previous surface which had become 

 by subsidence nearly level. By the joint action of the two, 

 material is forced from the lower part of the first wedge, over an 

 obstacle placed on the floor of the trough, at a distance from the 

 loaded end rather more than a third of the whole length, and in 

 height about the same proportion of the trough's depth. In the 



1 So, of course, is the Trias in many places ; but the breccias and anhydrite 

 or gypsum, frequent in the rauchwacke, make its origin more doubtfuL 



a According to Noe's map the latter is rather more than 80 miles, but this 

 avoids overstatement. 



3 As the reader can so easily form a mental picture of this, I have thought 

 it needless to insert an awkwardly long diagram. 



4 Quart. Journ. Geol. Soc. vol. li (1895) p. 361. 



