﻿Vol. 6 1.] EXPERIMENT IN MOUNTAIN-BUILDING. 347 



so-called ' writing-desk ' form of Sir Leslie Stephen, so familiar 

 to us in the Alps, and finally breaks into fragments. In some 

 experiments these were irregular, much broken up, and in that 

 case forced irregularly into the sandy layers in a manner that 

 would be very puzzling to geologists, and may throw light on 

 some difficult problems. 



In others, they broke across in two rectangular fractures, as in 

 fig. 14 (p. 354). In such a case one half (fig. 14, a, b) was forced 

 over the other, c, d. There was also an overthrust of a over b, and 

 of c over d. Fig. 15 (p. 354) gives a lateral view, from c to r/, 

 showing the overthrust of c over d. 



The result of these fractures and overthrusts is that, instead of 

 two cross-lines, there is a three-legged ridge consisting of a single 

 main elevation, with a break at, or near, the centre, from one side 

 of which a main-cross line proceeds nearly at right angles. The 

 corresponding cross-line (fig. 1, p. 348) on the other side is lower, 

 and is shifted to some distance, according to the amount of over- 

 thrust. 



The result is, that along the lines of overthrust a boring would 

 pass twice through the originally- single calcareous stratum, and 

 near the central nucleus of fracture even four times. 



This accounts for the shifting of the lower ridge in figs. 1 & 2 

 (p. 348). One is reminded of the great arch of the Alps lying 

 at right angles to the ridge of Italy, and that of the Himalayas 

 with India, the plain of Lombardy corresponding to that of Bengal. 



In other cases, the edges of the primary fractures broke off more 

 or less regularly. The detached pieces were then pushed up, assuming 

 gradually a very steep angle (fig. 16, p. 355), or even becoming 

 vertical. The main plates of plaster having now room, were able 

 to approach one another as in fig. 17 (p. 355), without any need 

 for more fractures or folding. Such a case is illustrated in the 

 accompanying diagrams (figs. 16 & 17, p. 355). 



It will be observed also that a layer of pliable material above the 

 plaster would be thrown into one or a few extensive folds ; while 

 a similar layer below is prevented from doing so, the under side 

 being comparatively flat. It accommodates itself, therefore, to the 

 circumstances by assuming a greater number of smaller folds. 



Speaking generally, it may be said, firstly, that the thicker and 

 stiffer the materials which alternated with the sand, the bolder and 

 fewer are the folds ; secondly, that the folds are more numerous 

 and more accentuated in the lower layers ; and thirdly, that the 

 folds tend to be at right angles one to the other. 



Discussion. 



The President observed that there were some who believed that 

 laboratory-experiments threw little light upon Nature's work, but 

 we could approximate more and more closely to the conditions 

 which obtained in Nature, though we could never actually imitate 



