130 PERIDOTITE. 



The whole is traversed by a reticulated series of fissures, which in each mineral partakes 

 of its usual mode of fracturing. 



The olivine is the predominating mineral. It forms rounded irregular grains traversed 

 by numerous fissures. Larger fissures surround the main olivine masses, these veins being 

 marked by a yellowish-brown central line of earthy ferruginous and serpentinous mate- 

 rial, on each side of which extend borders of pale-green serpentine. The borders are of 

 various widths, and usually ramify in little veinlets of serpentine through the fissures 

 intersecting the olivine individual. In places, the entire olivine is altered to serpentine. 

 The serpentine in polarized light usually shows fibrous polarization, the fibres being 

 arranged perpendicular to the sides of the fissures. The yellowish-brown eartliy mate- 

 rial that marks tlie medial line of the main veins has entirely replaced the olivine in 

 some portions of the section, giving rise to brownish patches. The serpentine is filled, 

 along various planes and especially along the central line of the veins, with innu- 

 merable minute fluid cavities, so minute tliat even magnified over nine hundred diame- 

 ters they remain as fine black globulitic specks, totally reflecting the transmitted light. 

 Occasionally one larger than the re.st shows the narrow outline of the common fuU fluid 

 cavity. 



The enstatite is in elongated crystals and irregular grains, traversed by the usual fine, 

 fibrous cleavage. The surface of the crystals is somewhat smooth and silky, and the 

 principal cleavage is broken occasionally by fractures running obliquely across the crys- 

 tals. The larger enstatites frequently show a greenish fibrous alteration extending along 

 tlie fissures and sometimes reaching the main body of the crystal. 



The diallage is, like the enstatite in most of the sections, clear and colorless. It can 

 generally be distinguished from the latter mineral by the roughness and irregularity of 

 its cleavage, owing to the acute angle at which two of the cleavages meet in most of the 

 grains. Like the smaller enstatites, the diallage is in irregular grains and masses, and both 

 occasionally contain rounded grains of olivine, and crystals and grains of picotite. In one or 

 two cases grains were observed showing the cleavage of augite. Some of the diallage plates 

 have an earthy-white or cloudy appearance, marking a certain amount of alteration. 

 Sometimes both the enstatite and diallage are traversed by serpentine veins, and the 

 smaller grains surrounded by that mineral. 



Picotite occurs in yellowish-brown octahedrons, as well as in irregular masses, opaque 

 for the most part, but translucent and of a yellowish-brown color in places. How much 

 of this might properly come under the head of chromite can not be told. In the yellow- 

 ish-brown serpentine veins are arranged grains showing the lustre of magnetite, which 

 mineral is also seen in some poitions of the before-mentioned opaque irregular masses of 

 picotite (?). 



The microscopic structure of the rock is shown in figure 1, Plate V., which indicates 

 the grayish, fissured, partly altered olivine and enstatite grains, the dark picotite grains, 

 and the brownish veins traversing the rock-mass. 



This rock was described by the collector, Mr. W. A. Goodyear, as metamorphic, but 

 ■with the stratification generally almost obliterated. Mr. Goodyear prolialjly took a some- 

 what banded arrangement of the minerals, as observed in No. 3002, ami a tendency to 

 split into platy masses, for stratification. Since both of these are common in eruptive rocks, 

 the latter showing especially on alteration and weathering, further evidence is required 

 upon the subject. Microscopically and lithologically they belong to rocks which the best 

 evidence pronounces to be eruptive. It is to be hoped that future geologists, in visiting 

 the locality, will endeavor to settle the question of the origin of these most interesting 



