PERIDOTITE DIKES NEAR ITHACA, N. Y. 271 



brown mica ranges from pale green to yellowish-brown; that of the 

 highly colored border, from green to reddish-brown; and that of the 

 clear margin, from colorless to light brown. The double refraction 

 is high, but as the mica begins to alter, the interference colors appear 

 ragged and are less intense. The extinction angles are usually small. 

 The interference figures range from distinctly uniaxial to biaxial with 

 a very small axial angle. Between the scales of altered mica there 

 are frequently lenticular masses of a carbonate. The mica often 

 alters to chlorite. This alteration was most commonly met with in 

 the porphyritic variety of the rock. In several cases slender needles 

 occur at the contact of an altered mica crystal and serpentine. These 

 needles extend radially from the mica into the serpentine, and are 

 too small to give much indication of their optical properties, except 

 the fact that they are strongly double-refracting, with nearly parallel 

 extinction. They are probably an amphibole. 



The mica contained inclusions of magnetite, perofskite, apatite, 

 and picotite, showing that these minerals belonged to an earlier stage 

 of crystallization. There were a few inclusions of serpentine, having 

 a structure which indicated that the original mineral was olivine. 

 Sections in the basal plane showed many fine, dust-like particles of 

 a brown or black color. 



There were many large crystals which showed crystal outlines 

 either wholly or in part. The contact between these and the olivine 

 indicated that both were growing at the same time. The mica some- 

 times extends into irregular embayments in the pyroxene. 



Besides the micas described above, there are irregular jagged or 

 shredlike plates of a much smaller size. These surround the altera- 

 tion products of the olivine, and they may be secondary micas. 



Pyroxene. — The pyroxene is a clear, non-pleochroic mineral, con- 

 taining inclusions of magnetite and perofskite. It is thus seen to 

 belong to a later stage of crystallization than the magnetite and 

 perofskite. 



The extinction angles range from 35 to 40 , and the axis of least 

 elasticity which lies in the acute angle fi makes an angle of 36-3 7 

 with the prismatic cleavage. In converging light the mineral is seen 

 to be biaxial, with a small axial angle. Rectangular cleavage blocks 

 show the emergence of an optic axis. This fact, taken with the rela- 



