SOME EXPERIMENTS IN FOLDING 497 



should be thinned. As folds with drawn-out upper limbs are not 

 commonly found, Hobbs was led to conclude that the arcuate chains 

 have been developed as underthrusts from without. In an earlier 

 paper he attempted to show that underfolding is easier and more 

 natural than overfolding.' To quote: "The active force (thrust) 

 which produces rock folds, instead of operating from behind and 

 above the anticline, as so generally supposed, is applied below and in 

 front. Continuation of the process yields therefore not overturned 

 and overthrust, but underturned and underthrust flexures." WiUis, 

 however, contends that overthrusting and underthrusting are deter- 

 mined by local conditions, among which the relative vertical posi- 

 tions of the active pressure and the passive resistance are most 

 influential. If the line of pressure is directed above the resistance, 

 the result is overthrusting. If the pressure be directed below the 

 resistance, underthrusting results.^ 



Experimental results. — In many different experiments on folding 

 in the course of these studies, it was found that either overfolds or 

 underfolds may develop, though not with equal readiness. Appar- 

 ently some local peculiarities, if of the right sort, may determine 

 which way the fold will turn. These may be in the nature of slight 

 differences in material, unevenness in the thickness of the layers, or 

 a slight initial dip, which in the preparation of the layers is not 

 always easy to eliminate. Variable resistance from the sand on the 

 sides of the blocks of strata, or variability in the overburden, may 

 also be influencing factors. But in a large number of trials such 

 fortuitous variables should tend to eliminate themselves according 

 to the law of averages, and the relative number of overfolds and 

 underfolds should decide which is the more normal type. In these 

 tests overfolds formed in much greater numbers than underfolds. 

 Figure 4 (p. 498) shows the preference for overfolding. 



It was found that the nature of the folding can be controlled to 

 a certain extent according to the principle stated by Willis. The 

 layers to be folded were attached to the movable blocks in such a way 

 as to slope upward from the pressure block to the resistance block, 



' W. H. Hobbs, "Mechanics of the Formation of Arcuate Mountains," Jour, of 

 Geol., Vol. XXII (1914), pp. 166-81. 



^ Bailey Willis, Bull, of Geol. Soc. of Am., Vol. XXXII (1921), p. 31. 



