PITCHSTONE. 537 



yellow brown, fawn colour, red brown, greenish 

 brown, olives of various hues down to blackish 

 green, dark blue, and black. The colours are 

 sometimes also intermixed, and thus pale and 

 dark green, or pale and dark grey, are interla- 

 minated, producing a striped surface. The 

 colours of pearlstone are much more limited, 

 possibly because the substance is more rare. 

 Those of the porphyritic varieties are numerous, 

 as this rock is more frequently porphyritic thau 

 simple. 



As the porphyritic varieties may possess any 

 of the modifications of the simple rock as a base, 

 many additional aspects result from this cause. 



Besides the passage into chert and cherty 

 chalcedony already noticed, pitchstone passes into 

 basalt, and, as is supposed, into opal and semi - 

 opal. In the passage to chert, it is sometimes 

 found to contain minute grains or particles of 

 chalcedony, discernible only by the lens. This 

 variety often possesses a remarkable spheroidal 

 concretionary structure already noticed in the 

 geological part of this subject. 



