Ch. VI.] PRIMARY ROCKS. 



is commonly supposed, but that the structure and arrange- 

 ment of their parts exhibit as great marks of order and design 

 as any department of the mineral kingdom. Though the three 

 systems of cross joints often afford indications of apparent 

 complication and disorder, yet we can frequently detect a 

 more regular arrangement of layers or beds than is to be 

 found in some kinds of gneiss. We also learn, that in Ireland 

 these granitic layers are inclined at various, though elevated, 

 angles ; and the same circumstance is of very common occur- 

 rence in Cornwall, the dip varying from 45 to the perpen- 

 dicular ; and in these cases the other joints are so disposed, 

 as to divide the layers into rhomboidal concretions. And 

 since the different modifications of this form have some con- 

 nection with the mineral composition, as already stated, it 

 might be expected that the inclination of the layers would 

 correspond with the nature of their component concretions ; 

 and thus the lesser angles more generally belong to eurite 

 and felsparite ; the higher ones, to shorl rock and fine-grained 

 granite ; whilst the layers of true granite are sometimes per- 

 pendicular. 



In thus asserting that the beds of granitic rocks exhibit 

 regular bearings, both of strike and dip, it must be clearly 

 understood that these can only be occasionally detected ; and 

 are often, as in the case of the schistose rocks, complicated and 

 obscured by the curvatures of the beds. If we examine the 

 sea-shore and cliffs of the Land's End district, the parallel 

 seams of the granite will be generally found more or less 

 serpentine ; and frequently their interference with each other 

 will produce beds tapering away to a certain point, and then 

 enlarging, or even terminating in wedge-shaped masses. 

 This form of the granitic beds holds good both in perpendicu- 

 lar and horizontal sections ; and it will be soon shown that the 

 schistose rocks are subject to the same apparent irregularities. 



It is particularly worthy of remark, that the beds of granite 

 exhibited in the cliffs of the Land's End district are always 

 more or less inclined; they often indeed present perpendi- 



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