Ch. IX.] VEINS IN PRIMARY ROCKS. 169 



other ends : but this is not always the case ; and, indeed, even 

 where one part of a vein appears to be in the former pre- 

 dicament, another part is bounded by a distinct line, on each 

 side of which the substance of the vein and the rock are 

 strongly contrasted. 



So, when the rock has suffered from the decomposing 

 action of the elements, these veins often exhibit perfect walls, 

 or even an open seam or crevice, the chemical change being 

 more rapid at those points where substances of a different 

 nature come into contact: but in those veins, or in those 

 parts of veins, as in the case just mentioned, in which the 

 junction is accompanied by a perfect transition, decomposi- 

 tion does not develope this disunion of parts. 



As to size, length, and other dimensions, these veins 

 exhibit every variety within the limits of the containing block 

 or concretion : as regards their form, they are either straight 

 or tortuous, more or less uniform in breadth throughout 

 their course, or, tapering at one or both ends, they terminate 

 in one or many filaments ; and, lastly, when they meet in 

 opposite directions, some appear to traverse others, and the 

 disconnected veins either continue in the same lines on both 

 sides of the interposed veins, or in parallel lines, at some 

 distance from each other, on the opposite sides of the latter 

 veins ; in short, exhibiting on this small scale all the phe- 

 nomena which have been observed in the largest veins ; and 

 sometimes these characters are distinctly marked, even in 

 hand specimens, as in the slate of St. Agnes, and in the 

 granite of Carclaze, in both of which the minute veins are 

 metalliferous. 



Let us now advance a step farther, and we shall find that 

 when these rock concretions are not individually contem- 

 plated, but in the aggregate, as united into a layer or bed, 

 the same appearances are still exhibited : larger veins, but 

 similar in composition to those just described, traverse dif- 

 ferent concretions, not unfrequently penetrating through their 

 very substance, and even intersecting and anastomosing with 



