298 ASHLEY — GEOLOGY OF ARKANSAS. [May 13, 



along it. If the force not used up by the first fold were great 

 enough other similar folds might be made. 



Figs. ^^ to 35 outline the action, only the main or combined 

 forces or resistances being given in the figures. The conditions 

 assumed are similar to those of Mr. Willis' (L) model (not given 

 in the paper referred to). In this case the forces are only con- 

 sidered in reference to the main heavy layer, the others being as- 

 sumed to be soft shales with little power of transmitting pressure. 

 The question then arises, How can bending take place unless the 

 bending component be as great as assumed at the moment when 

 bending begins ? A study of the rocks of Antoine mountain and 

 elsewhere has led to this suggestion. May it not be accomplished 

 by a much smaller force by introducing the factors of great length 

 of time in connection with viscosity of solids? 



It is now generally held that mountain making is a process of 

 thousands of years, the movement being, as a rule, so slow as to be 

 imperceptible, and yet we still cling to the idea that the forces and 

 resistances involved are such as would be involved in rapid 

 laboratory experiments. Thus a bar of stone is placed in the ma- 

 chine for testing the bending strength, and the result obtained is 

 used in the study of mountain making. 



Experiments have shown that solids under pressure below their 

 ultimate strength but above tljeir elastic limit will flow in the same 

 way as will a plastic substance like putty. They have also shown 

 that the elastic limit becomes lower when more time is allowed 

 the pressure in which to act ; and examples have been found 

 of marble slabs, which have lain many years, bending^ under their 

 own weight without fractures, though when first placed in position 

 they would not have bent under several times that weight, and 

 would probably have broken before bending perceptibly. In Mr. 

 Willis' experiments it may be that the plasticity of the materials 

 made up for the shortness of time in which movement took place. 



We are led to the conclusion that the folding may be due in part 

 to flow of the cold solid rock under a comparatively small pressure 

 continued over a great length of time. 



^Winslow, Am. Jour, of Sci., iii, Vol. xliii, p. 133. Ashley, Proc. Cal. 

 Acad, of Set., 1893, p. 319. See also Gibbs, Am. iyat.,\o\. iv, Jan., 1871, No. 

 II, Salem, Mass., p. 695. 



