52 Mac Culloch on Pitchstone. 



f. Very imperfectly spheroidal, concretionary; also passes 

 into the porphyritic. 



§. Sometimes different structures are intermixed in the same 

 rock, as the columnar and prismatic with the lamellar. The 

 lamellar pitch-stone sometimes contains lamellae of chert, or of 

 cherty chalcedony, into which it also passes. 



SECOND DIVISION. Porphyritic. 



A. Pitch-stone porphyry of mineralogists. 



a. With distinct crystals of glassy feldspar. 



^. These are sometimes so transparent as to be nearly invisi- 

 ble on a fresh fracture. 



h. With distinct crystals of common feldspar. 



c. With rounded, or shapeless particles of the same. 



d. With imbedded spherules, consisting of a grain of feldspar 

 surrounded by a grey enamel, or of the grey enamel alone. 



§. This enamel is sometimes blended with the surrounding 

 rock. When the spherules are numerous, this porphyry passes 

 into pearl-stone. 



e. With imbedded crystals or grains of quartz, or of quartz 

 and feldspar both. 



f. With grains of quartz surrounded by chert, and blending 

 with the surrounding rock ; or with grains of chert alone. This 

 variety is analogous to d. 



g. Porphyritic ; but the grains, which are chiefly of quartz 

 and chert, are further condensed in lamellae, which alternate 

 with a slightly porphyritic pitchstone. 



THIRD DIVISION. Concretionary-spheroidal: 

 Pearl-stone. 



A. Consisting of simple pitch-stone ; the grains irregular, and 

 compressing each other in the manner of coccolite. 



