Mac Culloch on Pitchstone. 63 



B. Some, or all of the grains, containing a central substance 

 of another kind ; pearl-stone porphyry. 



a. With grains of feldspar, or of feldspar and enamel, or of 

 enamel only. 



b. With quartz, or quartz and chert. 



c. With both quartz and feldspar. 



d. With a central atom of clay. 



FOURTH DIVISION. Amygdaloidal. Contaunng ' 

 imbedded nodides of another Mineral. 



A. Pitch-stone containing imbedded zeolites, 



§. As yet this variety has occurred only in Baffin's Bay. 



§. The colours of pitchstone are various. They are princi- 

 pally dull white, pale ochre, pink, pale green, greenish grey, 

 ochre yellow, ochre red, yellow brown, fawn colour, red brown, 

 greenish brown, olives of various hues down to blackish green, 

 dark blue, and black. These colours are sometimes also inter- 

 mixed ; and thus, pale and dark green, or pale and dark grey, 

 are interlaminated, producing a striped surface. The colours of 

 pearl-stone are much more limited, possibly because the sub- 

 stance is more rare. Those of the porphyritic varieties are 

 numerous, as thi^ rock is m.ore frequently porphyritic than 

 simple, 



§. As the porphyritic varieties may possess any of the modi- 

 6cations 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 sup- 

 posed, into opal and semi-opal. In the passage to chert, it is 

 sometimes found to contain minute grains or particles of chal- 

 cedony, discernible only by the lens. This variety often pos- 

 sesses a remarkable spheroidal concretionary structure, which 

 Ijvas mentioned in a preceding part of this communication. 



