A PYRRHOTITIC PERIDOTITE 129 



respect to the olivine grains. Nearly all of the larger hornblende 

 crystals contain small, rounded olivine inclusions, usually fresh, 

 though sometimes serpentinized. The mineral is identified as horn- 

 blende by the rhombic cleavage exhibited in some sections, by its 

 index of refraction, which is intermediate between those of olivine 

 and serpentine, and by its double refraction, which, by comparison 

 of its interference colors with those exhibited by olivine, is found to 

 be about 0.022 to 0.024. Its extinction angles in sections in which 

 the cleavage parallel to c is well defined range up to twenty-seven 

 degrees. The pleochroism is usually weak though strong in certain 

 sections. The colors vary from pale pink to pale green and, in some 

 sections, deep brown. There is frequently more or less mottling, due 

 to slight variations in color and birefringence from point to point. 

 The contacts between hornblende and olivine and between hornblende 

 and pyrrhotite are in most cases perfectly sharp, neither mineral 

 having undergone alteration. Between hornblende and feldspar, the 

 contact though normally sharp, occasionally shows evidence of some 

 re crystallization of the hornblende next the feldspar. The abundant 

 small olivine inclusions present in the larger hornblendes seem to 

 indicate that the latter mineral followed very close upon the olivine 

 in crystallization and was probably in part contemporaneous 

 with it. The pyrrhotite and feldspar probably crystallized at 

 about the same time, and followed close upon the hornblende 

 crystallization. 



Magnetite. — Magnetite is present only in minute irregular grains 

 scattered through the olivine crystals in bands of varying width, 

 many of which represent fracture planes. The magnetite as seen in 

 the thin sections seems to form from 5 to 40 per cent, by volume of 

 the olivine grains. The average is estimated at at least 10 per cent, 

 by volume or about 17 per cent, by weight. Since the olivine was 

 estimated to constitute about 60 per cent, by weight of the whole 

 rock the magnetite included in it will form 17 per cent, of 60 per cent, 

 or about 10 per cent, by weight of the whole rock. In the analysis 

 all the iron was determined as FeO but in the calculation of the norm 

 a part was apportioned to 10 per cent, of magnetite, the remaining 

 iron forming 5.64 per cent, of FeO. Many of the magnetite bands 

 extend from one olivine grain into another which is in contact with 



