270 PITCHSTONE. 



details of these and other modifications I must refer 

 to the Classification. 



Pitchstone presents many other remarkable analo- 

 gies to trap in its structure. It is often porphyritic, 

 containing imbedded crystals or irregular grains, of 

 felspar, and, occasionally, of quartz also. Tins is an 

 interesting circumstance in its history ; as it is found 

 only in the unstratified rocks, with the exception of 

 porphyritic gneiss and schist, or in those which in- 

 trude among the truly stratified substances, in the 

 form of veins. The porphyritic structure is indeed so 

 common in the pitchstones of Scotland, that the in- 

 stances of it far exceed those of the simple rock ; par- 

 ticularly in Arran. It is rare to find any mass which 

 does not in some place contain imbedded crystals, or, 

 at least, rounded and irregular grains of felspar. 



Admitting, as now demonstrated, the igneous origin 

 of trap, a remarkable circumstance occurs in some of 

 these pitchstones, which, together with its other ana- 

 logies, equally indicates the igneous origin of this rock. 

 Nothing indeed of this peculiar and demonstrative na- 

 ture has occurred, even in any of the porphyries which 

 have come under my examination. In the porphy- 

 ritic examples in question, the grains of felspar are 

 rounded, but irregular ; retaining the usual plated 

 structure in the centre, but gradually becoming con- 

 fused towards the surface, and terminating in a coat- 

 ing of white enamel, such as is produced by the fusion 

 of felspar. This felspar is, in some cases, the glassy 

 variety, in others, the common ; and the change on 

 the crystal is precisely that which may be induced by 

 the regulated action of the blowpipe, while the smaller 

 grains consist entirely of the enamel. These singular 

 varieties occur in Sky and Arran; and it is interesting 

 to remark, further, the connexion which they possess 

 with pearlstone. 



