PERIDOTITE DIKES NEAR ITHACA, N. Y. 269 



The olivine. — Not a single complete grain of olivine was seen in 

 any of the slides, and in several slides it was difficult to find more 

 than a few small fragments of the mineral imbedded in a network 

 of serpentine. The olivine was colorless and contained numerous 

 fine, dust-like inclusions of a dark color, and occasionally grains of a 

 carbonate. Usually the shape of the masses of serpentine indicated 

 that the olivine was in rounded or irregular grains; but occasional 

 sections were found which had sharp crystallographic outlines. Mica 

 scales were found within the serpentine derived from olivine, and 

 some of the grains of serpentine were surrounded by mica. The 

 contact between the two minerals was usually irregular, even when 

 the same grains showed sharp crystallographic outlines elsewhere. 

 This indicates that the two minerals were in part pyrogenic. The 

 olivine enveloped grains of magnetite, perofskite, apatite, and pico- 

 tite, indicating that these minerals belonged to an earlier stage of 

 crystallization. During the alteration of olivine to serpentine much 

 magnetite was formed. This magnetite occurs in small grains, some- 

 times so abundant as to make the serpentine appear black. Many 

 of the irregular patches originally occupied by olivine were filled, 

 either wholly or in part, by carbonate. This was doubtless partly 

 dolomite, resulting from the removal of a part of the magnesia and 

 silica, the carbonation of the remaining magnesia, and the infiltration 

 of calcium carbonate. Probably in some cases the magnesia was 

 largely removed, for when the rock was treated with weak HC1 it 

 effervesced vigorously for several minutes, indicating the presence 

 of much calcite. 



Serpentine. — Serpentine is the most abundant mineral in the rock. 

 It occurs, as pseudomorphs after olivine or pyroxene, in rounded or 

 irregular masses averaging o.o64 mm in diameter. It is frequently 

 found inclosing fragments of the unaltered olivine. Two kinds occur: 

 (1) a yellowish fibrous variety, (2) a green fibrous variety. The 

 yellow serpentine has its fibers arranged nearly parallel, and hence 

 its double refraction is considerable. The extinction is parallel. 

 In converging light a biaxial interference figure appears. The axial 

 plane lies at right angles to the fibers, and the black hyperbolas 

 barely remain in the field when a No. 9 objective is used. A faint 

 pleochroism is sometimes noticeable, ranging from green parallel to 



