AN ILLUSTRATION OF THE FLEXURE OF ROCK. 



BY GEORGE H. ASHLEY. 



Exposures of the axes of anticlines or synclines often 

 show the beds of rock bent into an arc of very short 

 radius, yet without perceptible fracturing.^ As rocks 

 are generally considered inflexible, except when softened 

 by heat, a question naturally arises concerning the con- 

 ditions under which these rocks have yielded and formed 

 such close flexures without breaking. 



The old theory is that it is due, first, to the great verti- 

 cal pressure of the superincumbent beds of rock as a 

 passive factor," and second, a greater horizontal pressure 

 as the active factor.^ Thus, it is conceived that the hori- 

 zontal pressure, from whatever source, gradually accum- 

 ulates until it is able not only to overcome the resistance 

 to bending of the layers, hundreds or thousands of feet 

 thick, but also to lift the weight of all the overlying beds. 

 Then bending will ensue. But it is assumed that though 

 the layers may be under a shearing stress far beyond 

 their strength of resistance the great vertical pressure 

 will prevent rupture by immediately forcing together 

 every incipient fracture.^ These incipient fractures, 

 however, relieve the horizontal pressure which is con- 

 verted into heat^ and this heat in turn assists the vertical 

 pressure in the mending process. Thus giving way in 



' See for example, Eeade's Origin of Mountain Kanges, pp. 177, 186, 

 189, plate 24, etc.; Geol. Surv. of Ark., Vol. Ill, 1890, pi. VIII; other 

 reports on regions of much folding. 



'^ K. Clark's Tables for Engineers, 1st ed., pp. 631 and '204. 



3 Nature, XIX, (1878), p. 103; Daubree, Geol. Experim., pp. 290 et seq. 



•* Spring in Bull. Acad. Koy. Belg., 1880, pp. 171 and 325; Spring in 

 The Engineer, Apr. 9, 1886, p. 278. 



5 Prestwick's Geology, Vol. I, p. 410; Daubree's Geol. Experim., pp. 

 448 et seq.; Mallet in Phil. Trans., 1873, p. 187. 



2d Ser. Vol. HI. April 20, 1893. 



