268 Rev. 0. Fisher — On Faulting, Jointing, and Cleavage. 



and before it was cleaved. The slate shows alternations of bedding 

 of coarse and fine sediment, and evidence of extreme commotion of 

 the sea-bottom at one interval of time. The question is, to what 

 cause may the compression be attributed which produced the faulting? 

 Possibly it was due to the contraction of two beds, above and below, 

 exceeding that of the material between them, so that the latter, 

 became compressed horizontally to the extent of 2 inches. The 

 faults pass beyond the slate, rendering it impossible to verify this, 

 explanation. But in similar cases in situ it might be tested. 



The expression (3) gives a limiting value for the ratio of P : 7F, 



for we must have ^ ^^ ^^ > — , 



or ^ >2v'-\-l-\- V\2v' + 1)--1. 



If, for instance, the co-efficient of friction has a low value, as 0"3, 



P 



which is its value for wet clay on wet clay, then — = 1"8; or the 



horizontal compressing force must be 1'8, or nearly twice, the vertical, 



to allow of reversed faulting at all. If the co-efficient of friction has 



p 

 a higher value, as 0*7, which is that for dry masonry, then ■— =2-8, 



or the horizontal force must be nearly three times the vertical to 

 allow of reversed faulting at all. 



There can be little doubt that some of the most important faults 

 are not produced by such a disposition of forces as we have con- 

 templated ; but are the result of a direct elevatory force acting upon 

 one side of the fault, or else by a failure of support upon the 

 other side. 



PAET IV. 

 On Cleavage. 



Cleavage is the property possessed by some rocks of splitting 

 readily into laminge of any desired thinness. The cleavage surfaces, 

 which are approximately planes, are, within moderate areas, parallel 

 to each other, and maintain a certain fixed direction. 



Dr. Sorby's examination of the structure of slates convinced him, 

 that the property of cleavage is due to the rearrangement of minute 

 plates of crystals, and other unequiaxed particles, on the average along 

 the planes of cleavage. The observations which he has recorded 

 ■were made on thin sections examined by transmitted light.^ If the 

 surface of an ordinary roofing slate be examined under the micro- 

 scope as an opaque object, it will be seen to be covered with minute 

 strise, that are scarcely perceivable when the light falls along them, 

 but are sufficiently apparent when it falls across them. Small frag- 

 ments of crystals, such as apparently of felspar, are arranged with 

 their longer dimensions in the direction of the striee. But there is 

 no appearance of their being distorted. In coarse slates composed 

 of volcanic agglomerate, from Borrowdale, such as is figured and 



1 Edinburgh New Phil. Joiu-n. 1853. The -writer hopes to recur to this part of 

 the subject. 



