Geology of the Paleozoic area of Arkansas south of the novaculite region / by George H. Ashley.
- George Hall Ashley
- Date:
- 1897
Licence: Public Domain Mark
Credit: Geology of the Paleozoic area of Arkansas south of the novaculite region / by George H. Ashley. Source: Wellcome Collection.
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![Bearing of Certain Features of the Region upon Theories of Moun- tain 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 metaraorphism, 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 their 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 exathination 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 Etigineers,]\ms, 1892, xxi, 551-566; Thirteenth An. Rep. U. S. Geol. Surv.](https://iiif.wellcomecollection.org/image/b28136342_0085.jp2/full/800%2C/0/default.jpg)
