PITCHSTONE. . 267 



no less than its place above a trap posterior to the 

 secondary strata, proves that it does not belong to this 

 series. It is unquestionably the remains of a vein 

 once pervading the trap on which it seems now to 

 stand. The degradation of that rock has left the more 

 durable pitchstone in the position it now occupies ; 

 and of this degradation, there is ample evidence. The 

 columnar structure, and its parallelism to the sides of 

 the vein, occur equally also in veins of trap. As there 

 is a striking analogy between pitchstone and basalt, it 

 is conceivable, that, although unstratified, it may at 

 least exist in the form of large overlying masses, like 

 the trap rocks ; and probably some of the foreign ex- 

 amples, supposed to be stratified, are of this nature. 



If the preceding remarks are not judged sufficient 

 to exclude pitchstone from the stratified rocks, with 

 which it was formerly classed, the following argument 

 may. It is found in granite, in red sandstone, in the 

 more recent sandstones, and in the latest trap rocks ; 

 thus occupying a variety of discordant positions 

 which no rock has yet been known to do, excepting 

 'the intruding substances of the trap family. In gra- 

 nite or trap, it is obvious that it could not be strati- 

 fied ; nor is there any rock, except Serpentine, which 

 forms veins and regular strata also. Even where it. 

 occurs apparently unstratified among the sandstones, 

 it must therefore be considered as a parallel vein. 



The entanglement of fragments of the including 

 strata in trap veins, is so common as to have fallen 

 under the observation of every geologist conversant 

 with these rocks. But if in pitchstone, it is so rare 

 that I have as yet observed only one example, it oc- 

 curs in the well-known vein of Brodick wood in Arran. 

 The including strata consist of the red sandstone; 

 and fragments of it are insulated in the middle of the 



