PERIDOTITE INTEUSIVES. 251 



Olivine. — This is present most commonly in round anhedra, and is usually 

 almost entirely altered to yellowish-green serpentine fibers. When the olivine 

 is completely altered, the mesh structure affords a ready means of recogniz- 

 ing the original mineral, especially when taken in conjunction with grains 

 of a rich-green isotropic mineral, and also octahedra and grains brown in 

 color, ^H'obably spinels, which are found included in the pseudomorphs. 



Hornblende. — The most of tlic hombleude in the peridotites is a brown 

 variety showing strong pleochroism. a is light cream-yellow, c is yellowish 

 brown, and J) is reddish brown; I)>>C>-n. Patton ^ has already called 

 attention to the pleochroism of the hornblende, which "is exceptional, 

 inasmuch as the brownest color is that of rays of light vibrating parallel to 

 the orthodiagonal axis." The brown hornblende is accompanied by a very 

 small quantity of green hornblende. Moreover, the brown hornblende 

 grades over into a light green, tlie two being in perfect crystalline conti- 

 nuity. In rare cases this brown hornblende is also intergrown with a light- 

 green pyroxene in such way as to give a mottled polarization effect. The 

 pinacoidal cleavage of the hornblende continues through the j)yi'Oxene, and 

 the extinction angle of the hornblende against this cleavage is 19 degrees, 

 while that of the pyroxene runs up to 34 degrees. The hornblende includes 

 numerous anhedra of pyroxene, somewhat fewer grains of olivine, and, less 

 commonly, ragged pieces of biotite, giving it a poikilitic character. It is 

 also very full of opaque metallic or brownish translucent plates of ilmenite. 

 In some individuals minute clear microlites, similar to those described 

 above in the hornblende of certain gabbros, are noticed in small quantity. 

 These are irregularly distributed in the hornblende, giving the crystals a 

 patchy appearance. In res^ject to its color and these inclusions, this horn- 

 blende, as noted by Patton, bears a rather striking resemblance to hyper- 

 sthene on superficial examination.^ Iron ore in large masses is fairly 

 frequent as an inclusion, and it is very commonly noticeable that where 

 such inclusions occur the zone of hornblende immediately surrounding 

 them is free from the platy inclusions mentioned above. Such a clear zone 

 is also observed at times surrounding the inclusions of biotite and olivine, 

 but never in case of pyroxene. Where these clear zones surround the 



' Microscopic .study of some Michigan roclis, by H. V. Patton; Rept. State Board of Geol. Surv. 

 for 1891-92, 1893, p. 186. 

 - Loc. cit., p. 186. 



