A PYRRHOTITIC PERIDOTITE 125 



into these sediments. Gabbroid and dioritic rocks whose relations 

 show them to be differentiation products from the granitic magmas 

 are of common occurrence in this region, and the pyrrhotitic peridotite 

 under discussion probably represents an extreme phase of this dif- 

 ferentiation process, although it may on the other hand be wholly 

 unconnected in origin with the granitic intrusions. 



MEGASCOPIC APPEARANCE 



The megascopic appearance of this rock is so unusual as to attract 

 immediate attention in the field. It is medium-grained (millimeter- 



fT'^i'X'^.^.iM'r^ 



-(E 



:aj! . ^ 





Fig. I. — Tracing from polished surface of peridotite. The black areas represent 

 pyrrhotite with small associated amounts of chalcopyrite and pyrite. The white areas 

 are occupied mainly by ohvine with some hornblende and plagioclase feldspar. 

 (Natural size.) 



grained), holocrystalline, and equigranular, and is composed of a 

 yellowish-gray, metallic-looking mineral, which proves to be pyrrhotite 

 scattered in very irregular masses through a ground-mass which 

 for the most part appears structureless and is dark green to nearly 

 black in color. The form and natural size of the pyrrhotite areas 

 are shown in Fig. i, which was traced from a polished surface. 



In irregular association with the pyrrhotite are small amounts 

 of chalcopyrite distinguishable by its yellower tint. Most of the 

 nearly black matrix between the sulphide masses exhibits no trace 



