PITCHSTONE. 271 



In tracing the progress of common pitchstone to 

 pearlstone, an obscure spheroidal concretionary struc- 

 ture will first he found, which, by an increasing dis- 

 tinctness of the concretions, causes them to pass into 

 the latter substance. In other cases, the enamel grains 

 are the centres of a spheroidal tendency ; and such va- 

 rieties are imperfect pearlstones. In those of decided 

 character, the grains, or spherules, contain a central 

 enamel ; and these have been called pearlstone por- 

 phyry. Further, in some instances, the enamel and 

 the investing pitchstone spherule seem to have entered 

 into combination ; the result being a pearlstone of a 

 peculiar character, but in which the enamel grains 

 may still, in some part or other, be discovered. Lastly, 

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

 tals consist of successive concentric prisms : a layer of 

 pitchstone being interposed between each, so as to 

 produce a compound crystal. 



The amygdaloidal structure, so common in the trap 

 rocks, is so rare in the pitchstones, that only one 

 instance of it has yet occurred. The specimen was 

 from Baffin's Bay, containing zeolites, (mesotype or 

 nadelstein,) and resembling, in all but its base, the 

 basaltic amygdaloids. 



The decomposition of Pitchstone is an interesting 

 part of its history. Though apparently compact, it 

 contains a considerable quantity of water, and is thus 

 frequently very tender and fragile, when not exposed 

 to the air. In wet places, it is easily resolved into clay; 

 a circumstance, among many others, marking its com- 

 plete distinction from obsidian, with which it has 

 sometimes been confounded. But even when highly 

 tender, it resumes its original firmness after being 

 dried by exposure. Like some of the trap rocks, it 

 occasionally betrays, on the exposed surfaces, marks 

 of a concretionary structure which would not other- 



