324 G II Smyth, Jr. — Peridotite in Central New York. 



slight variations in the relative proportions of the constituents. 

 The most abundant mineral is olivine, occurring both in 

 irregular grains, and with sharp crystal outline. The diameter 

 of individuals seldom exceeds l mm though larger masses are 

 occasionally seen. Twinning parallel to Oil is rare, having 

 been noted in only two slides.. Inclusions are not very abund- 

 ant, and show no regularity of arrangement. The usual 

 alteration to serpentine and carbonates is shown along the 

 cracks, and in some cases the original substance is entirely 

 changed, but this is not common, much of the olivine generally 

 being present. 



The large mica phenocrysts rarely show hexagonal outline. 

 The color in thin sections is dark brown, with marked pleo- 

 chroism. Cleavage plates of the mineral give, with convergent 

 polarized light, a figure which differs but slightly from that of 

 a uniaxial mineral. The mica is doubtless biotite. While some 

 individuals are quite fresh, others show considerable alteration 

 to carbonates. Occasional crystals of rhombic pyroxene are 

 present, but hardly in sufficient quantity to be considered an 

 essential constituent of the rock. 



Though a large proportion of the ground-mass is distinctly 

 crystalline, a glassy substance is present in small quantity, and 

 numerous patches of nearly isotropic material probably repre- 

 sent a glass that has been slightly altered. Of the crystalline 

 portion of the ground-mass, biotite is perhaps the most 

 abundant member. It appears in irregular lath-shape parti- 

 cles, generally quite fresh and showing the usual pleochroism. 

 The arrangement of the particles sometimes shows a decided 

 flow structure. Next in order of abundance is the iron oxide, 

 which occurs both irregular and with octahedral outline, and is 

 evidently a primary constituent of the rock. As it is strongly 

 attracted by the magnet and readily soluble in acids, it is 

 undoubtedly magnetite. Indeed, the former property is so 

 marked and the mineral so abundant, that pieces of the rock \ 

 inch in diameter may be lifted by a weak magnet. Mingled 

 with the magnetite, both in the ground-mass and as inclusions, 

 are numerous small octahedra never more than 0*12 mm in diam- 

 eter and usually less than half that size. The octahedral form 

 as a rule is distinct, but irregular grains are also present. The 

 color is deep yellow and the index of refraction high, giving 

 strong marginal reflection. With crossed nicols the crystals 

 are seen to be not perfectly isotropic. When the rock powder 

 is treated with hydrochloric acid the crystals are left in the 

 residue, but are completely dissolved by sulphuric acid.' These 

 facts show the mineral to be perofskite. The same conclusion 

 is reached upon comparing the mineral with the perofskite of 

 other American peridotites, their appearance and properties 

 being precisely the same. Some sections of the rock show 



