P1TCHSTONE. 



529 



in size, are either straight or curved. In some 

 cases, they are as thin as paper ; in others, they 

 are thick, and further, separable by joints at right 

 angles to the planes of the lamellae. Thus they 

 become divisible so as sometimes to present a 

 minute prismatic disposition. 



Occasionally, these prisms present also some 

 curved surfaces together with the straight ; so as 

 to form columns on a very small scale, which are, 

 further, jointed, in some very rare instances, by 

 surfaces alternately concave and convex ; and, 

 when such joints are very near, the parts separate 

 into irregular spheroidal forms. In some rare 

 cases of the jointed columnar structure, a central 

 atom of felspar, or of enamel, is found in each 

 joint. 



Analogous to this, is a spheroidal concre- 

 tionary structure, more or less perfect, and vary- 

 ing in size, but commonly, minute. By a tran- 

 sition from this, it becomes an aggregate of 

 irregular grains, and passes into pearlstone, which 

 can only be considered as a variety of pitchstone, 

 and is accordingly ranked here as a variety. 

 The spherules, or grains of pearlstone, frequently 



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