OF THE MORE DISTURBED ZONES OF THE EARTH S CRUST. 



449 



Relation of Cleavage to the Mechanical Constitution of the Strata. 



There is yet another law respecting cleavage ; it is the dependence of this 

 structure upon the mechanical texture, and possibly upon the chemical compo- 

 sition, of the fissured rocks. 



Geologists have for several years recognized the fact, that in formations com- 

 posed of alternations of the coarser mechanical rocks, such as silicious grits and 

 conglomerates, with fine-grained argillaceous beds, as slates, shales, or marls, 

 the coarse beds are unaffected by cleavage, while the fine-grained ones are often 

 pervaded by it. Indeed, one may observe in a given locality almost a strict 

 proportion between the degree of intimate Assuring of the rocks by cleavage planes, 

 and the degree of comminution of their particles. 



Connected probably with this interruption in the distribution of the cleavage- 

 condition through such heterogeneous groups of strata, I have observed another 

 general fact of modification of the cleavage planes, which should not be passed 

 unnoticed here. They tend in the fine grained argillaceous beds, to curve a 

 little from the normal direction into an approach to parallelism with the surfaces 

 of bedding of the adjoining coarser mechanical deposits, presenting, in a trans- 

 verse section, a kind of gentle sigmoid or double flexure. This is well shown in 

 the cleavage-traversed rocks at the base of the anthracite coal formation of Penn- 

 sylvania, especially in the transition or passage beds which connect the Umbral 

 red shales of that region, with the base of the coal-sustaining conglomerate, 

 and also where these shales alternate with the upper coarser members of the Ves- 

 pertine sandstone. The small section here appended, showing the cleavage in 

 one of these groups of alternation of red shale and sandstone, from a railway 

 cut near Ashland, in the middle anthracite coal-field, exemplifies well the pheno- 

 menon referred to. 



Fig. a. 



Beds of Red Shale with Cleavage alternating with beds of Sandstone without Cleavage; Cleavage curving toward? 

 parallelism with the bedding at its boundaries. Section near Ashland, Pennsylvania. 



The tendency, here shown, in the cleavage planes to conform to the planes of 

 bedding, where abrupt changes of composition interrupt the continuity of the 

 fissures, is but another variety of the phenomenon already adverted to, of a de- 



