304 PROF. T. a. BONNET ON THE SOUTHERN ORIGIN [Aug. I907, 



experiment, 1 where there was one obstacle, the slope of the original 

 material was between 6° and 7°, and the final slope of the mass, 

 after loading and movement, about the same. With two obstacles 2 

 (when nappes were produced), the slope of the original wedge 

 slightly exceeded 12°, and the average slope of the final mass 

 was about 6° or 7°. Contrast these conditions, especially the last, 

 with those which may have existed in the Alps. At the beginning 

 of loading, a wedge instead of an oblong : the distance to be 

 traversed 6 times the maximum thickness of the sedimentaries 

 instead of about 30 times ; and the slope of the added mass (to 

 which in the Alpine case all the movement must be due) about 

 double. And the material in that case is ordinary rock — limestone 

 and shale — instead of cobbler's wax ! 3 



Pigs. 4 & 5 (p. 300) are exaggerated in Prof. Lugeon's favour ; 

 for, if drawn to the actual scale, either the details would have been 

 much crowded vertically or the length would have been inconvenient, 

 so here the vertical scale (A'D, B'C) is double the horizontal. But 

 notwithstanding this, they will, I hope, give some idea of the differ- 

 ence between art and Nature. The one (fig. 4) is an attempt at 

 representing the condition of the Alps at the end of the first great 

 uplift (in late Oligocene and early Miocene times) when the northern 

 range was hardly developed (the central and western part of the 

 chain being more like the eastern one at the present day). 4 The 

 outlines of the crystalline core and its sedimentary covering are of 

 course, inferential. The other (fig. 5) is a very rough sketch of 

 the outlines at the present day, which, allowing for a not unim- 

 portant amount of denudation, represents them as they were left by 

 the post-Miocene uplift. As in fig. 3 the loaded part, by which the 

 movements of transference were caused, lies farther to the south- 

 east, and must have ceased to be operative as soon as the rising 

 crystalline crest of the Pennine Alps broke the continuity of the 

 overlying sedimentaries. 



* Quart. Journ. Geol. Soc. vol. lxii (1906) p. 718, fig. 1. 



2 Ibid. fig. 2. 



3 [My friend the Rev. Edwin Hill, F.G-.S., suggested to me, when we were 

 discussing this subject, that I might have put the mechanical difficulty, 

 involved in Prof. Lugeon's hypothesis, yet more strongly. A mass of sedi- 

 ment deposited at or near sea-level (for the earlier Tertiaries are mainly 

 marine in the circum- Alpine region) is supposed to have forced plastic 

 material, across the rising crystalline ridges, until it has rested some thousands 

 of feet above that level on the northern flank of the chain. But the con- 

 ditions under which motion becomes possible can be inferred from fig. 2 of 

 Prof. Sollas's first paper (Quart. Journ. Geol. Soc. vol. li, 1895, p. 363) ; while 

 the well-known experiment with a fluid at rest in a U-tube shows that its 

 surface cannot be at sea-level in one end, and very much above it in the other. 

 In Prof. Sollas's experiments, the masses of cobbler's wax were in all cases 

 wedge-shaped, and that made motion possible, but this condition did not exist 

 in the Alps. The only escape from this dilemma, so far as I can see, would be to 

 assert that the physiography of the region as a whole has been entirely altered 

 since the movements came to an end, an assertion which I think hardly 

 requires to be discussed seriously. — T. G. £., June 21st, 1907.] 



4 The reasons for this are stated in lectures given at the Eoyal Institution 

 in 1888, and published in the 'Alpine Journal' vol. xiv (1889) pp. 110-17. 



