1010 A TREATISE ON METAMORPHISM. 



mineral particles were fractured into several pieces, or even granulated. 

 Between the larger pieces of the feldspar minute openings formed. Many 

 minute joints arid crevices also were produced and were subsequently filled 

 by cementations. 



The marked effect which the character of the rock may have upon the 

 nature of the fractures is well illustrated in the Cripple Creek district, where, 

 according to Penrose, in the hard rocks the fissures are sharp, clean-cut 

 breaks, while in the soft rocks they are ordinarily a series of very small 

 cracks, constituting a displacement of a kind which I call a distributive 

 fault. Mines which are partly in hard and partly in soft rock illustrate this, 

 as the following extract from Penrose will show : a 



The vein on which the Buena Vista, Lee, Smuggler, and Victor mines are located 

 occupies a sharp, clean-cut fissure, partly in the massive rock and partly in the hard 

 breccia; but when it passes into the soft, tufaceous breccia on the east slope of Bull 

 Hill the fissure is represented only by faint cracks occupied by no vein of importance. 

 In this case the force which caused the fissure overcame the cohesion of the harder 

 rock sufficiently to make a clean break, but in the more plastic rock it overcame 

 cohesion only to the extent of causing a series of faint fractures without -any one 

 well-defined break. 



It follows from the above that displacement may disappear at variable 

 depths. Where there are fractures with large displacement they are likely to 

 extend to very considerable depths, and in proportion as the displacement is 

 small they are likely to disappear at less depths. Thus the depths to which 

 displacements extend depend largely upon the character of the rocks. For 

 instance, in a region in which there is a shale formation at moderate depth 

 underlying brittle rocks, strong fissures in the higher formations may disap- 

 pear as they encounter the shale, being there replaced by flexures. Where 

 formations of shale are between brittle formations fissures may cease at the 

 top of the shale and other fissures appear below it. Thus may fissures not 

 only die out below, but may disappear above, the fault along the fissures 

 being replaced by a flexure in the shale, which yields by flowage. This is 

 beautifully illustrated by the Enterprise mine, of Rico, Colo., described b} T 

 Packard and Ransome (see fig. 29, p. 1208), where faulted fissures in sand- 

 stone and limestone disappear at the place where shale is encountered, the 

 shale accommodating itself to the fractures below by monoclinal flexures. 

 (See p. 1204.) 



"Cross, Whitman, and Penrose, R. A. F., Geology and mining industries of the Cripple Creek 

 district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 144. 



