450 Dr. Boase's Inquiry into the Nature 



neous rocks, we are told that the slate in contact with the 

 granite " has been modified by a similar crystalline action in 

 passing into a solid state." Now, in the preceding pages it 

 had been laid down that the folia of schists which are parallel 

 to the strata, are not true slates, but laminae or thin beds ; 

 the difference being, that one is supposed to have been formed 

 by crystalline action, the other by deposition. The demon- 

 stration of this proposition has therefore failed in the case of 

 the Carclaze slate, and through it in all parallel cases, by 

 leading to the contradictory conclusion that lamina? are not 

 laminae^ but slates; that is, that a perfect or crystalline 

 cleavage is not always transverse to the strata, which is one of 

 the essentials in the proposed definitions of these terms. This 

 result, however, is satisfactory, for it is in accordance with 

 Nature, the inclination of the laminae of rocks having no 

 fixed relation to that of their bedding or stratification. 



For reasons similar to those already advanced concerning 

 the supposed single cleavage of slate, it may be disputed that 

 granite has only one grain or plane of lamination. For, since 

 there are no precise limits to the thickness of these lamina?, 

 (as next the slate at Carclaze, they are said to be very thin, 

 while at a distance therefrom they are more largely deve- 

 loped,) we cannot deny that the granite of other places has 

 a grain when its layers are parallel to the crystalline planes 

 of the adjacent slate. Now, in all cases these laminations or 

 layers are, by the action of the elements, divided into quadran- 

 gular masses, in consequence of the development of the joints ; 

 and they may, when the joints are not visible, be cleaved or 

 subdivided not only into thinner layers, but also across their 

 planes, in two directions, so as to produce blocks similar to 

 those formed by atmospheric agency. It must be admitted 

 that those planes which have the same direction as the strike 

 of the adjoining strata, and correspond with the laminse of the 

 slate, are more easily demonstrated ; and Mr. Enys has re- 

 corded that the workmen can cleave the granite with less 

 power on this line than on the others*. But we do not know 

 any reason why one system of planes should be referred to 

 crystalline action, and the others to mechanical violence ; the 

 notion appears only to have been adduced in support of the 

 hypothesis of one true transverse cleavage in stratified rocks, 

 against which we have been contending. 



The Professor states that the grain (that is, the cleavage in 

 one direction) has had considerable influence in modifying the 

 course of fissures subsequently produced by mechanical force, 

 and by this means he accounts for the regularity of one set of 



* [See Lond. and Edinb. Phil. Mag., vol. ii. p. 323.— Edit.] 



