Ch. VI.] PRIMARY ROCKS. 105 



are more frequently fissile. Let us then entirely overlook 

 the nature of the internal, whilst we examine the concre- 

 tionary, structure of the schistose rocks. 



In Cornwall, all the varieties of rocks belonging to the 

 schistose group are composed of rhomboidal concretions ; 

 these exhibit various modifications, and are more distinctly 

 formed in some rocks than in others, especially in those which 

 are principally composed of compact felspar. This structure 

 is not only detected by the fracture, but it is also, on a large 

 scale, developed by a partial disintegration, by which parallel 

 systems of fissures or joints are produced, as in the case of 

 granite. These are sometimes not so evident, and are com- 

 plicated by the degradation of the slate, in the direction of its 

 internal laminae. One set of these fissures may be often seen 

 traversing these rocks, in a perpendicular manner ; as shewn 

 by the smooth upright surface of some cliffs, and by the re- 

 gular parallel surfaces, when an intermediate layer has been 

 removed by the action of the elements. And, indeed, the 

 same fact may be very frequently observed in quarries. In 

 these cases, if the adjacent parallel joints be disclosed, they 

 will be found running in opposite directions, the one at a 

 greater angle than the other. This is owing to the form of 

 the layers, resulting from the aggregation of rhomboidal con- 

 cretions ; and the nature of it will be plainly illustrated by 

 so placing a rhomboid that two of its sides may be per- 

 pendicular. In this manner are arranged layers of rocks, 

 that are sometimes massive. and sometimes fissile; or which, 

 indeed, individually partake of all the forms of internal 

 structure. 



There can be little doubt but that the primary slates of 

 other countries possess the same structure ; that we have not 

 many facts in corroboration of its universal occurrence may 

 perhaps be, in some measure, attributed to these rocks not 

 being found under circumstances so favourable for observation 

 as those of Cornwall. That this structure, however, is not 

 peculiar to the latter country may be gathered from the 



