SOME EXPERIMENTS IN FOLDING 499 



faults. There was, however, one rather unusual case where, in the 

 early stages of the deformation, an under thrust formed on one side 

 of the model and an overthrust on the other side. After further 

 compression the underthrust was truncated by an overthrust and 

 the overthrust cut by an underthrust (see Fig. 16, p. 510). 



Hobbs has expressed the belief that overturned folds, though 

 rare, may be developed in special cases (backfolding of anticlines) 

 where the material being folded thins away from the active force.^ 

 In the experiment by Daubree which he cites, an overturned fold 

 developed in unmistakable fashion, just as in general there is little 

 difficulty in producing overfolds. Our experience has been that the 

 greater difficulty lies in producing underfolds. To put the case of 

 underfolds vs. overfolds to a more severe test by utilizing this same 

 principle, a layer was prepared to reverse these conditions, by having 

 it thin steadily toward the pressure block. This should have the 

 effect of producing weakness near the pressure block, and seemingly 

 should be most favorable for the development of an underturned 

 fold. Instead of an underfold, the resulting wrinkle was a decided 

 overfold on one side of the model, and a symmetrical upright fold 

 on the other. 



From the usual behavior in this series of experiments, it would 

 seem that in the more brittle and elastic materials, where faulting 

 occurs without much preliminary deformation, the fracture may 

 go either way without great preference for the overthrust, but in 

 more plastic, incompetent material, where portions of the mass 

 yield readily to moderate differences of stress and the phenomena are 

 those of folding preceding fracture, overfolding is far more likely 

 to occur than underfolding, and the preponderance of overthrust 

 faulting over underthrust faulting is still greater. 



VARIATIONS IN FOLDING 



With varying overburden. — To see what effect variations in the 

 amount of overburden would have on the type of folding produced, 

 a series of models was prepared of equal parts of parafiin and vase- 

 line, and of equal thickness. These were deformed at the same rate 

 of speed but with different overburden. 



^ W. H. Hobbs, "Mechanics of Formation of Arcuate Mountains," Jour, of GeoL, 

 Vol. XXII (1914), pp. 185-87. 



