44 Mac CuUoch on Pitchstone. 



jection that this structure is parallel to the sides ; since trap 

 veins occasionally exhibit the same disposition of their columnar 

 parts. 



As there is so striking an analogy in many particulars be- 

 tween pitchstone and basalt, it is very conceivable that, if not 

 found in regular strata, it may at least exist in the form of large 

 overlying masses, like the trap rocks ; and probably some of the 

 foreign examples may be referred to this variety of position. 



If the preceding remarks are not judged sufficient ground for 

 excluding pitchstone from the regularly stratified rocks, the 

 following argument will, perhaps, be considered valid. It is 

 found in granite, in red sandstone, in the more recent sand- 

 stones, and in the latest trap rocks. It thus occupies a variety 

 of discordant positions which no rock has yet been known to do 

 except the intruding substances of the trap family. In granite, 

 or trap, it is obvious that it could not be stratified ; nor are we 

 acquainted in nature with any rock which is found in the form of 

 veins, and in that of regular and true strata also. Even where 

 it occurs apparently inter- stratified among the sandstones, it 

 must therefore be considered as a parallel vein. 



Pitchstone presents some other remarkable analogies to trap. 

 It is often porphyritic, or contains imbedded crystals, or irre- 

 gular grains of feldspar, and occasionally of quartz also. This is 

 an interesting circumstance in its history, as it is found only in 

 the unstratified rocks, (with the exception of granitic gneiss,) or 

 in those which intrude among the truly stratified substances in 

 the form of veins. The porphyritic structure is, indeed, so com- 

 mon in the pitchstones of Scotland, that the instances of it far 

 exceed those of the simple rock ; particularly in the island of 

 Arran. It is, perhaps, rare to find any mass which, in some 

 place or other, does not contain imbedded crystals of feldspar, 

 or, at least, rounded and irregular grains. 



The amygdaloidal structure, which is so common in the 

 rocks of the trap family, is, however, so rare in the pitchstones, 

 that only one instance of it has yet occurred to me in the various 

 specimens which I have examined. This specimen was from 

 Baffin's Bay, and it contained zeolites, that is, mesotype or 



