1897.] ASHLEY — GEOLOGY OF ARKAXSAS. 297 



Bearing of Certain Features of the Region upon Theories of Moun- 

 taifi Making. — Reference was made in the description of Antoine 

 mountain (p. 273) to the peculiar appearance of the layers where 

 closely bent in the axis of the anticline. They have the appear- 

 ance of having broken into blocks, and of these blocks having ad- 

 justed themselves to their neighbors. The rocks on top of Little 

 Bear mountain present much the same appearance. In neither case 

 do the rocks show noticeable metamorphism, though they are 

 slightly closer grained than most of the layers. 



Probably bearing on the same subject are the many cases noted 

 where solid layers are sharply bent into acute angles without show- 

 ing any sign of fracturing. 



It has generally been assumed where such close folding without 

 fracturing occurred that it was due to the confinement put upon it 

 by the enormous vertical pressure of superincumbent beds. It be- 

 ing thought that, though the layer were under shearing stress far 

 beyond its ultimate strength, the great vertical pressure would be 

 sufficient to close every incipient fracture. 



Some interesting experiments have been performed by Mr. Bailey 

 Willis of the United States Geological Survey^ in studying the 

 effect of a horizontal pressure on soft layers under a weight of half 

 a ton of shot. Some of these conditions were produced, and a 

 study made of tiieir action by graphical statics. Very similar re- 

 sults were obtained, but they led to certain questions presenting 

 themselves. 



If we assume such a series of layers under a gradually increasing 

 horizontal pressure, the time will come when the bending compo- 

 nent of that pressure at some point of initial dip will be sufficient to 

 overcome the various resistances and forces opposing it, and the 

 layer will begin to bend. If, when the layer has bent slightly, we 

 make another examination of the components, forces and resist- 

 ances involved, we shall find that the slight bending has given the 

 bending component of the original horizontal pressure an advan- 

 tage, and it has increased at the expense of the compressing com- 

 ponent. On the other hand the resistances and forces opposing it 

 remain about the same, hence its movement would be accelerated, 

 and soon the bending would go forward with a rush until finally 

 the horizontal pressure is transmitted across the fold instead of 



^ Bailey Willis, Transactions American Institute of Mining Efigineers,]\xnQ, 

 1892, xxi, 551-566; Thirteenth An. Rep. U. S. Geol. Sitrv. 



