Mac Culloch on Pitchstone. 45 



naJelstein ; resembling, in every respect but its base, the well- 

 known basaltic amygdaloids. 



The entanglement of fragments of the including strata in 

 trap veins, has always been esteemed a proof of the intrusion of 

 these veins among the stratified rocks, and as further indicating 

 a degree of violence, simultaneous with or preceding the intru- 

 sion of the vein. This very remarkable circumstance is suffi- 

 ciently common in trap veins to have fallen under the observa- 

 tion of every geologist conversant in districts where these rocks 

 abound ; but in pitchstone veins it is so rare, that as yet I 

 have only observed one example of it. This is in the well- 

 known vein at the end of Brodick Wood in Arran. The includ- 

 ing strata consist of the red sandstone, and the fragments of the 

 same sandstone are insulated in the middle of the vein. If this 

 occurrence is as yet solitary, it must be recollected that it does 

 not happen, perhaps, in one trap vein of a thousand ; so that it 

 may be merely a question of chances, depending on the much 

 greater rarity of veins of pitchstone. The fact, even if solitary, 

 is valuable, as establishing, in another important particular, 

 the resemblance between pitchstone and the trap rocks. 



Admitting, or presuming, with most modern geologists, the 

 igneous origin of the trap rocks, there is found in some pitch- 

 stones a remarkable circumstance, which, together with the 

 analogies already pointed out to these, seem strongly to indicate 

 an igneous origin for them also. Nothing indeed of this peculiar 

 nature has occurred in any of the porphyritic traps that have 

 come under my examination. In those pitch-stones which are 

 of a porphyritic character, the crystals, or rather grains of feld- 

 spar, are rounded but irregular. When broken, it will be found 

 that the feldspar retains its usual plated structure in the centre of 

 the grain, but that it gradually becomes confused, or loses 

 its character ; while the outer surface is a coating of white 

 enamel such as is produced by the fusion of feldspar. This 

 feldspar is in some cases the glassy variety, in others the com- 

 mon ; and, the change on the crystal is precisely that which 

 may be induced by the regulated action of the blow-pipe. 

 The smaller crystals, it is also worthy of notice, are entirely 



