1848.] 



SHARPE ON SLATY CLEAVAGE. 



113 



without either bedding or cleavage. Many of these metamorphic slates 

 appear to have been originally of a brecciated structure, and contain 

 a confused mixture of crystals with what appear to be fragments of 

 Fig. 1. 



Fig. 2. 



foreign origin. In slaty rocks of this description all the enclosed 

 portions which can be supposed to have had a mechanical origin are 

 flattened between the planes of cleavage, but nothing of the sort can 

 be observed of the crystals, which are scattered through the mass 

 without any reference to the cleavage-planes either in their form or 

 position. 



In good roofing-slates, which are distinguished by the fineness and 

 uniformity of their grain, it is difficult to observe the forms of the 

 parts of which they are composed : yet even in these, with the assist- 

 ance of a lens, we can see that the constituent particles are flattest 

 between the cleavage-planes, and longest along the dip of the cleavage. 

 Thus, under whatever circumstances we are able to observe the forms 

 of the component parts of slate, or of the foreign bodies contained in 

 it, (whether these are pebbles, fragments of other rocks, or organic 

 remains,) we find evidence that the mass has been compressed by a 

 force which has acted in a direction perpendicular to the planes of 

 cleavage. 



The distinctive character of slate appears to he that it is composed 

 of particles of a form more or less amygdaloidal, arranged in a similar 



