530 PITCHSTONE. 



also contain a central atom of felspar or enamel, 

 like the variety mentioned in the preceding para- 

 graph. 



The last obvious variety of structure to be 

 noticed in pitchstone, is the porphyritic. This 

 varies much in the magnitude, perfection, or 

 number, of the included crystals ; producing cor- 

 responding differences of aspect. Frequently, 

 the larger crystals are rounded, and converted 

 into a white or grey enamel on the exterior ; while, 

 in lieu of the smaller, there are only to be found 

 spheroidal grains formed entirely of the same 

 enamel. This variety also passes into pearlstone ; 

 each grain of enamel being the centre of an 

 imperfect spheroidal structure, which, acquiring 

 greater distinctness, becomes the substance in 

 question. Thus pearlstone is shown to be 

 merely a variety of pitchstone, and the association 

 here made between the two is justified. 



In some rare instances, as in Arran, the crys- 

 tals, of glassy felspar, are formed of concentric 

 prisms separated by layers of the pitchstone, so 

 that every crystal is a compound body. 



Independently of these obvious variations of 



