52 DESCRIPTION OF THE PRIMARY [Ch. IV. 



accords with the dip observed in the mine, and we consider 

 the northern surface underlying the slate to be the termi- 

 nation of the elvan in width, for what is only seen here in 

 part probably entirely obtains at both extremities, since they 

 are lost under the schistose rocks ; and we also think that 

 some of the irregularities in the breadth of the elvan may be 

 traced to the persistence or abrasion of the slate, more par- 

 ticularly the latter, as at A and G. The elvan is traversed 

 by lines or joints, which cross it in the direction of S. by E. 

 and N. by W. : these lines are very close together at G, and 

 there the porphyry is much stained with oxide of iron, and is 

 in a decomposing state; and the rock adjoining is similarly 

 circumstanced. Where the elvan is a well characterised por- 

 phyry, the lines are farther apart, that is, the elvan is 

 composed of larger concretions. But the most important 

 circumstance concerning these lines is, that they, in innumer- 

 able places, continue their course uninterruptedly into the 

 slate, as may be well seen even at A and F, where the elvan 

 is seventy-five and ninety feet in width. The same observ- 

 ation equally applies to the longitudinal joints, where they 

 meet with the protruding slate. Lastly, it may be noticed, 

 that at H there are several bunches and elongated portions 

 of rock perfectly resembling the slate at its junction with the 

 porphyry ; and that at fifty feet north of E, the laminae of the 

 slate are very flat and undulating, sometimes rather inclining 

 S., but oftener to the N. Beds of eurite are of frequent 

 occurrence in the Cornish rocks, which it has been proposed 

 to call proteolite; and in this case also we find a similar 

 correspondence in the nature of their respective bases, both 

 consisting of a soft and fine granular compact felspar. 



Several beds of eurite occur between Camelford and Bod- 

 min. At Tremagenna, it is a white rock, composed of com- 

 pact felspar, in which are imbedded limpid grains of quartz, 

 and scales of mica ; it is also in some parts stained with rust 

 coloured spots, produced by the decomposition of some ad- 

 ventitious mineral, probably of hornblende ; and small irre- 



