494 R- T. CHAMBERLIN AND F. P. SHEPARD 



present some analogies to the loci of applied thrust in the experi- 

 ments just described. In such places arcs might develop with their 

 inner curves toward the active force (Fig. 2). As a general rule, it is 

 probably also true that the effects of the thrusting have presumably- 

 been somewhat greater in the middle portion of each range and have 

 died off toward the extremities. Short mountain chains, in which 

 the horizontal movement is considerable in proportion to the length, 

 might perhaps develop curving trend lines because of this fact. 



Fig. 2. — Force applied by two Jacks (arrows) produced these arcuate folds. Their 

 concave sides face the loci of applied thrust. 



To test another possibility, a model was prepared in which a 

 semicircular rigid plaster mass was incorporated in the usual rela- 

 tively plastic mixture of paraihn and vaseline as shown in Figure 3. 

 This would represent a rounded central mass of resistant old rocks, 

 flanked by a zone of less consolidated sediments, after the analogy 

 of Asia. The resistant plaster arc was placed on the side of the 

 model farthest from the pressure which was applied uniformly by 

 means of two steel jacks. It was suspected that the curvature of the 

 resisting buffer might control, or at least greatly influence, the trend 

 of the resulting fold. The result of the experiment was that while 

 the fold manifested a slight tendency to conform to the curving out- 

 line of the resisting mass where the fold came closest to it, still the 



