SOME EXPERIMENTS IN FOLDING 



493 



of the frame — it would produce wrinkles with essentially the same 

 trends. The above experiment did not appear to the present writers 

 to decide the question. It seemed desirable to try experiments of 

 a somewhat different sort. 



Experimental results. — A rectangular model of much greater area 

 than is possible in the pressure box was placed in position with side 

 supports and an appropriate overburden. It was arranged so that 

 pressure could be applied over a limited portion of the free side, 

 either by a single steel jackscrew, or by two jacks operating inde- 



FiG. I. — Arcuate fold produced by jack-screw in the position shown by arrow. 

 The inner side of the arc faces the thrust. 



pendently on different portions of the free side. By applying pres- 

 sure over only a short strip of the model very good arcs were devel- 

 oped. The lines of force apparently radiate outward from the locus 

 of active thrusting. The wrinkles then form more or less normal 

 to the lines of greatest compression, developing curves whose con- 

 cave sides face the thrusting. Thus it is found that arcs can be 

 developed readily enough from compressive stress directed outward 

 from a place within the inner curve of the ensuing arc (Fig. i). 



In the formation of many mountain ranges the intensity of 

 thrusting has presumably varied from place to place, as suggested 

 by the fact that the ranges are not entirely continuous. Places of 

 greatest thrusting and greatest horizontal shifting of material would 



