56 DESCRIPTION OF THE PRIMARY [Ch. IV. 



on the eastern side, it is only six, diminishing in its course to 

 four and even three feet, when it is again heaved, by a small, 

 quartz-vein C, towards the south, to a distance equal to its 

 width. The vein B, in contact with the western elvan, is two 

 feet wide ; in some parts purely quartzose, in others porphy- 

 ritic, and even blended with irregular portions of slate, the 

 whole mass at many points passing gradually from one rock 

 into the other It continues of the same nature and of the 

 same thickness towards the eastern portion of the course ; but, 

 before reaching it, dwindles into strings, and forms a con- 

 siderable curvature : passing the elvan a second time, it also 

 involves porphyry in its composition for a few feet, then be- 

 comes quartzose and schistose, and runs a curved course for 

 seventy feet, having a variable thickness, and sending off 

 branches, and then dies away in strings. In its southern 

 course, however, it assumes a great contortion, and at the 

 same time becomes a hard compact variety of slate, in which 

 state it disappears under the sea. 



The courses or beds of granitic rocks, in the slates, some- 

 times exceed three hundred feet in breadth, and at other 

 times they dwindle to such a small size, as to be called granite 

 veins. These courses are subject to curvatures, in common 

 with the containing slate, as has been already noticed ; and 

 they often send out branches or veins during their progress, 

 as figured by Professor Buckland in the el vans of St. Agnes.* 

 They sometimes terminate abruptly, as at Mousehole and 

 Cape Cornwall, or, as in the northern end of the elvan at 

 the latter place, gradually disappear in several small veins.t 

 Lastly, near the granite or elvan-courses, these granitic rocks 

 often occur in bunches or masses of various dimensions. 



The direction of these courses is generally parallel with the 

 strata, but the dip being very commonly towards the granite 

 at a very considerable angle, varying from 45 to 75, these 

 granitic rocks cross the laminae of the slates, as in the 

 diagrams, (fig. 3.) 



* Geol, Trans., yol. ir. f See figs. 13. 16. 



