Ch. XV.] OF THE PRIMARY ROCKS. 365 



How are doubtful veins affected by true veins ? Granite- 

 veins are sometimes traversed by contemporaneous veins, and 

 also by true veins, if it be admitted that the large quartz-vein, 

 intersecting both granite and slate, at Mousehole, is a true 

 vein, and, if so, a parallel vein of the same nature is crossed 

 by this granite vein at the distance of a few feet. Elvan- 

 courses are commonly traversed by true veins : sometimes the 

 latter are included in the former; but, in one instance, as at 

 Polgooth, the elvan continues uninterruptedly through several 

 metalliferous veins. The general opinion is that the elvans 

 are veins of fissure, formed by igneous injections : Mr. Carne 

 thinks that circumstances are in favour of their being true 

 veins, and the miner sometimes considers them to be lodes; 

 the facts, however, already brought forward, render this very 

 doubtful, since they both traverse and are intersected by con- 

 temporaneous as well as by true veins. 



We now proceed to show that the relative ages of the sub- 

 divisions of true veins are of the same uncertain character as 

 those of the doubtful order. 



In the first place, it may be observed, that the application 

 of the rule of intersection has led to the formation of several 

 classes which seem to be very natural arrangements, inasmuch 

 as they differ from each other either in composition, or in their 

 bearings, or in both respects. Thus, the two first classes are 

 tin lodes ; the third, fourth, and sixth, copper lodes ; the fifth 

 class, cross courses or veins, not varying more than 30 from 

 the north ; the seventh, cross jlulcam, the same as the fifth, 

 only entirely composed of clay ; the eighth and last, the slides, 

 of the same nature as the seventh, as regards composition 

 and bearing. 



Now the tin lodes are divided into two classes, because some 

 are traversed and heaved by others ; and they have been 

 termed the oldest and the more recent tin lodes : the former are 

 said to dip generally towards the north, the latter for the 

 most part towards the south. But there is no general rule ; 

 indeed such veins, underlying to the same point of the com- 



