DEFORMATION OF ROCKS 193 



(Fig. 6). However, I would not assert that to some extent the 

 material had not also flowed at various times before the stresses 

 exceeded the ultimate strength of the rocks and ruptures 

 occurred. Even if the apparent gentle curves of the strata can 

 be more accurately represented diagrammatically by placing end 

 to end a large number of broken lines with slight changes of 

 direction, the curves indicated by the lines would have the 

 forms of folds given under Analysis of Folds.' In the weaker 

 shaly layers between sandstones and limestones the deformation 

 is in many cases largely that of shearing, this being due to the 

 differential movement between the two bounding strong layers. 

 In the weaker layers the jointing is therefore in two diagonal 

 sets. 



In the case of the Mississippi valley it is clear that the 

 stresses producing the jointing are locally still at work. For 

 instance, at the combined rocks at Appleton,^ a recent rupture 

 occurred which was sufficient to make considerable displacements 

 in the artificial works. Other cases of a similar kind have been 

 given by Reade.3 The foregoing cases show that the apparently 

 horizontal rocks of the Palaeozoic at the present time, are locally 

 under such stress that when a slight amount of material is 

 removed, and thus the beds not held so firmly in their position, 

 the ultimate strength is exceeded and rupture occurs. Denuda- 

 tion is ever lightening the load of the strata, and from time to 

 time, as a result of this, the abrupt deformations of jointing or 

 faulting may occur. Before the time of rupture, it may be that 

 the stresses, while not sufficient to produce rupture, may still 

 surpass the elastic limit and result in slow flowage. 



C. R. Van Hise. 



' Loc. cit, (A), pp. 603-633; (B), pp. 312-353- 



= On a recent Rock Flexure, by Frank Cramer. Am. Jour. Sci., Vol. XXXIX, 

 1890, pp. 220-225. 



3 On the Cause of active compressive Stress in Rocks and recent Rock Flexures, 

 by T. Mellarde Reade. Am. Jour. Sci., Vol. XLI, 1891, pp 409-414. 



