126 EDSON S. B AS TIN 



of cleavage and is shown under the microscope to be olivine, its 

 dark color being due to the abundance of minute magnetite inclusions 

 which it contains. Scattered dark-green or bronze-colored grains 

 in the ground-mass which show distinct cleavage faces are seen 

 under the microscope to be hornblende. Small, scattered areas, 

 gray in color with a dull luster, are plagioclase feldspar. In general 

 the rock is remarkably fresh. Serpentine and chlorite, which have 

 resulted from olivine and hornblende decomposition, are confined 

 largely to certain layers which are thread-like in form as seen in cross- 

 section on a polished surface. These areas of decomposition are 

 largest between closely grouped pyrrhotite masses, and the narrower 

 strands run from one pyrrhotite mass to another and are parallel 

 in their general trend throughout the hand specimen. 



The characters and relationships of the various original and 

 secondary minerals which compose the rock are described in detail 

 below. 



ORIGINAL MINERALS 



Olivine. — This is the most abundant original constituent of the 

 rock, and estimates on six slides show that it makes up about 60 per 

 cent, by volume of the whole rock. Since the specific gravity of the 

 rock is about 3 . 42 while the average for olivine is in the neighborhood 

 of 3.35, the percentage of olivine by weight would be only slightly 

 less than this figure. The norm shows only 30.99 per cent, by 

 weight of olivine but in addition shows 22.06 per cent, by weight of 

 hypersthene, a mineral not observed in the rock itself. A part of the 

 iron belonging to the hypersthene in the norm appears in the mode 

 in the more siliceous mineral hornblende, while the remainder appears 

 in olivine, a mineral less siliceous than hypersthene. 



The olivine occurs in rounded grains ranging from i or 2"^"^ 

 to 8"^"" in length, the majority being between 3 and 4 ^'^. Most of 

 the grains are entirely fresh except for a narrow alteration zone of 

 fibrous serpentine about their borders. Certain narrow bands 

 traversing the rock in a manner already described are character- 

 ized, however, by much more extensive alteration, and in these areas 

 the olivine grains may be partly or wholly serpentinized, the alteration 

 having, as is usual, been most extensive along the irregular cracks 



