Ch. XV.] OF THE PRIMARY ROCKS. 353 



tions of the respective rocks, which may be either con- 

 nected therewith, or entirely insulated in the traversed mass. 



That slate-veins have not been more frequently observed, 

 may be attributed to their generally possessing the same 

 colour as the containing rock, on which account they are not 

 easily distinguished ; and also to such veins having a compact 

 structure, similar to the massive varieties of rock abounding 

 in each primary slate, and which, as has been already stated, 

 only differ from each other in structure, being species of the 

 same genus. 



The apparent movements which intersecting granite- veins 

 exhibit have been adduced as a proof of the mechanical 

 origin of these veins: we will not, however, consider this 

 subject at present, as we shall shortly discuss the nature of 

 mineral veins, in which precisely the same phenomena are 

 more clearly developed. We will, therefore, conclude this 

 topic with Dr. Macculloch's very interesting remarks on the 

 arrangement of the minerals which constitute the granite of 

 Portsoy ; he considered it necessary to apologise for entering 

 into such minutiae, but we do not hesitate to quote the passage 

 at some length, as it appears to bear with no inconsiderable 

 weight on the subject under discussion. 



" The singular disposition and mutual relations of the 

 crystallised substances which form the granite of Portsoy 

 are known to have afforded to Dr. Hutton an argument for 

 its igneous origin ; and its peculiar character (called graphic) 

 has been supposed to arise from a simultaneous, or nearly 

 simultaneous, crystallisation of the several substances con- 

 tained in it. Thfe specimens about to be enumerated are 

 such as not only throw considerable doubt on this explanation, 

 but are in fact sufficient to prove a sequence of epocha even 

 in this limited space, and to show that the compound rock in 

 question has been formed by successive operations, the 

 nature of which, however, I fear we shall not easily de- 

 termine." " In one of the specimens a detached crystal of 

 shorl, of a flattened and irregular figure, has been broken into 



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