PITCHSTONE 585 



The lamellar pitchstone sometimes contains 

 lamellae of chert, or of cherty chalcedony, into 

 which it also passes. 



SECOND DIVISION. 



Porphyritic. 



A. Pitchstone porphyry of mineralogists. 



a. With distinct crystals of glassy felspar. 

 These are sometimes so transparent as to be 



nearly invisible on a fresh fracture. 



b. With distinct crystals of common felspar. 



c. With rounded, or shapeless particles of 



the same. 



d. With imbedded spherules, consisting 



of a grain of felspar surrounded by a 

 grey enamel, or of the grey enamel 

 alone. 



This enamel is sometimes blended with the 

 surrounding rock. When the spherules are nu- 

 merous, this porphyry passes into pearlstone. 



e. With imbedded crystals or grains of 



quartz, or of quartz and felspar both. 



f. With grains of quartz surrounded by 



chert, and blending with the surround- 

 ing rock ; or with grains of chert 

 . - alone. This variety is analogous to 



d. 



