ARGILLACEOUS SCHIST. 353 



the gravelly, and the conglomerate kinds, are 

 each characterized by textures which these terms 

 will sufficiently explain. A parallel disposition 

 of the mica, is sometimes, in the micaceous va- 

 rieties, a common cause of fissility, where no 

 such tendency is perceptible in the base. In 

 many of these rocks, it is apparent that the parts 

 are united by something more than a mechanical 

 adhesion. This seems to be the case in many of 

 the compact kinds, or the hone slates, and occa- 

 sionally, in some of those which contain abund- 

 ant grains of quartz ; but, in the greater number, 

 the indurated clay forms the cement which unites 

 all the fragments, whether great or small, into 

 a solid mass. 



The transitions by which the argillaceous 

 schists pass into some of the preceding rocks, 

 have already been mentioned ; and it will here- 

 after be seen, that, under certain circumstances, 

 they also pass into siliceous schist, and into sand- 

 stone. Respecting fracture, it is almost unneces- 

 sary to say, that it varies according to the direc- 

 tion in which it is made, and according to the 

 nature of the particular variety ; but these and 



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