302 PKOF. T. G. BONNET ON THE SOUTHERN OEIG-IN [Aug. I907, 



strata in a similar position, the floor bulges up, the roof sags down, 

 but the walls only exfoliate to a slight extent. 



Thus we find no evidence to show that the conditions of the 

 experiment were ever anticipated in this part of Europe, and must 

 fall back on lateral compression to explain the folding of the Alpine 

 chain. Experiments have already been undertaken to illustrate 

 this, among others by Favre, Cadell, and Bailey Willis. It will 

 suffice to refer to the elaborate series described by the last-named. 1 

 The materials forming the successive layers were not alike in 

 hardness and thickness, so that the resisting power of the composite 

 slab (sometimes about twelve times as long as thick) varied in 

 different parts, and thus more nearly reproduced the conditions of 

 Nature. A lateral thrust acting on one end of the slab reduced 

 the distance from the other, in some cases to about a half. Those 

 original inequalities influenced the yielding, so the results exhibited 

 considerable variation ; the usual one, however, was a succession 

 of wave-like folds, the first in order being generally rather near the 

 end at which the thrust was applied. This one, as the process was 

 continued, maintained its predominance ; sometimes forming a 

 comparatively sharp and lofty fold, into which part of the bottom 

 layer (commonly thick and comparatively soft) was squeezed. 

 Sometimes a rather important second fold was produced slightly 

 beyond the first one, and the two, as the thrust continued, became 

 combined into one of a complicated character. Overfolding, as 

 might be expected, did not occur until the slab had been con- 

 siderably shortened, and then the axis of the loop generally inclined 

 towards, not away from, the end where the thrust was applied. 

 Overfolding led to thrust-faulting, but in the not very common cases 

 where this occurred, the planes of fracture followed the same rule ; 

 and in no case did these models exhibit any close resemblance to 

 those made with cobbler's wax. 



Prof. Lugeon admits that lateral pressure 2 took some part in the 

 work of mountain-building ; for, had it not so done, the crystalline 

 floor, now crumpled up in the crests of the Pennine and Oberland 

 ranges, must also have joined in the flow, and thus could hardly 

 have attained to its present elevation. Presumably the two 

 pressures, from gravitation and from contraction, acted almost 

 simultaneously; for, prior to Oligocene times, the deposition of 

 Secondary and early Tertiary strata must have been fairly con- 

 tinuous. The sea, indeed, may have been interrupted by islands, 

 for the Trias in the central region, and again towards the south- 

 west, is either very thin or absent ; but patches of it (generally the 

 characteristic rauchwacke) occur here and there in the Pennine Alps, 

 as, for instance, near Saas Fee and in the Zermatt district (below 

 the Hochthaligrat). It crowns one of the Aiguilles Rouges, while 

 infolded masses of Jurassic strata, with or without underlying 

 rauchwacke, occur on the Furka Pass and may be followed to 



1 U.S. Geol. Surv. Thirteenth Ann. Rep. 1891-92 (1893) pt. ii, pp. 217^83. 



2 Bull. Soc. geol. France, eer. 4, vol. i (1901) pp. 819, &c. 



