144 ON THE CHARACTERS AND DISPOSITION 



vacuity. The first of these cases is most common 

 with granite, the latter with trap ; although, even in 

 this last rock, veins are sometimes found intruding 

 into the schistose structure, or parallel to the stratifi- 

 cation of the conterminous strata. In both cases, an 

 incorrect or prejudiced view of these appearances has 

 given rise to the notion of trap and granite being 

 stratified ; with what ti'th will easily be judged, 

 when the following obvious marks by which these 

 are to be distinguished from real strata, are described. 



Where the courses of granite veins are parallel to 

 the laminae of the including rock, their true nature is 

 easily distinguished ; as they rarely persist, even for 

 a few inches, without some deviation or ramification. 

 In the case of trap, however, as they are often of 

 considerable extent, it requires more care to trace 

 them to their irregularities or terminations. But they 

 present other indications of their real nature, by occa- 

 sionally sending out branches, or by the effects which 

 they have produced on the adjoining strata. The 

 nature of the rock indeed, as proved from other 

 appearances, ought to be sufficient to satisfy the 

 spectator that they are veins, and not beds : but as it is 

 probable that real strata of trap do actually occur, I 

 must reserve the minute parts of this discussion to 

 the history of the trap rocks. 



Respecting the relation in point of time between 

 these rock veins and mineral, or metallic veins, it is 

 not probable that any general rules can be laid down. 

 In Cornwall, granite veins, as well as veins of por- 

 phyry traverse the mineral veins ; but as mineral 

 veins are also found in granite, even in that district, 

 it is certain that they are of a time intermediate be- 

 tween two formations of that rock. That ordinary 



