Mac Culloch ou Pitchstoiie. 49 



when such joints are very near, the parts separate into irre- 

 gular spheroiddl forms. In some rare cases of the jointed 

 columnar structure, a central atom of feldspar or of enamel, is 

 found in each joint. This structure presents a considerable 

 resemblance to that of pearl-stone, here considered as a variety 

 of pitch-stone, and the specimens of this nature actually pass 

 into that substance. 



Analogous to this is a spheroidal concretionary structure 

 more or less perfect, and varying in size, but commonly minute. 

 By a transition from this, or by the greater perfection of the 

 spheroidal structure, pitch-stone becomes at length an ag- 

 gregate of irregular grains ; and it thus passes, as in the 

 former case, into pearl-stone, which, from this and the pre- 

 ceding fact, can only be considered as a variety of pitch-stone, 

 and is accordingly ranked here as a variety. The spherules 

 or grains of pearl-stone frequently also contain a central atom 

 of feldspar or enamel as already noticed, like the variety men- 

 tioned in the preceding paragraph ; a circumstance which con- 

 firms this view of its nature, and which is perhaps very inti- 

 mately connected with the peculiarity of its structure. 



The last obvious variety of structure to be noticed in pitch- 

 stone, is the porphyritic, already mentioned in pointing out its 

 analogy to the trap rocks. This varies much in the magni- 

 tude, perfection, or number of the included crystals, producing 

 corresponding differences of aspect. It was already observed, 

 that 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. 



Independently of these obvious varieties of structure, pitch- 

 stone sometimes gives indications of an internal arrangement, 

 which, as in the case of the trap rocks, is only detected on 

 wethering. Many of the varieties become of a pure white on 

 the surface after exposure to air, scaling off in successive crusts 

 that resemble white enamel, as they still retain the vitreous 

 aspect and fracture. At the same time the exposed surfaces are 

 covered with undulating lines, resembling certain varieties of 



Vol. XII. E 



