Ch. VIII.] THE GRANITIC AND SCHISTOSE ROCKS. 14-9 



that they are of cotemporaneous formation with it, and are 

 not fragments, as has been frequently maintained.* Schistose 

 rocks also occur in a more regular and extended form in 

 granite : thus, quartz-rock traverses the granite of Glen Tilt, 

 as recorded by Macculloch, being of the same nature as that 

 among the adjacent strata; and " the granite of Clis and 

 Pouliguen, in Brittany, contains thin layers or bands of schist 

 which is very micaceous, or rather of mica in mass."t 



Of all the modes, however, in which the granitic and schist- 

 ose rocks are associated together, the most curious, and the 

 most interesting in a theoretical point of view, is that of gra- 

 nite-veins, traversing the slate at the junction of these rocks ; 

 and there is, perhaps, no part of the world, where these phe- 

 nomena are better or more numerously displayed than in the 

 western part of Cornwall. We shall, therefore, select a few 

 examples from those which have already been placed on 

 record. 



" At Forth Just, near Cape Cornwall, and below the tin- 

 mine called The Little Bounds, the number of the granite- 

 veins," says Dr. Davy, " is considerable ; they were not 

 counted, but probably exceed fifty: their size also greatly 

 varies ; the largest vein being five feet thick, and the smallest 

 not more than one tenth of an inch. They run in different 

 directions ; some about W. N. W. and E. S. E., some about 

 E. and W., and some nearly opposite, as N. and S., and con- 

 sequently they occasionally intersect each other. In one 

 instance of intersection, the disjointed parts of the vein, run- 

 ning about N. and S., were heaved about half an inch out of 

 the course of the vein. The position of most of these veins is 

 perpendicular, or nearly so: in some places they approach 

 more or less the horizontal line. In composition, they differ 

 very much : some resembling the most perfect granite ; others 

 abounding in quartz, and have very much the appearance of 



* Annals of Philosophy, vol. iv. p. 419. 

 f Annales des Mines, tome iv. p. 25. 



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