272 GEORGE C MAT SON 



tion of the axis of least elasticity to the prismatic cleavage, indicates 

 that the mineral is the variety of pyroxene called diopside. A trace 

 of a pinacoidal parting shows on sections normal to the prismatic 

 cleavage. The alteration gives rise to a carbonate which often sur- 

 rounds the crystals and extends along the cleavage cracks. Some of 

 the grains of serpentine with fibers at right angles to each other are 

 doubtless the result of alteration of pyroxene. 



Magnetite and ^'/memVe.— Magnetite is scattered throughout the 

 rock, sometimes in the form of minute grains in the serpentine, and 

 at other times in octahedrons or irregular grains which are as much 

 as o.o3 mm in diameter. Cross-sections are quadratic, trigonal, or 

 irregular. The grains in the serpentine are usually very small and 

 result from the alteration of olivine, or, less commonly, pyroxene. 

 As alteration progresses, these fine grains are removed by solution. 

 In the alteration of some of the irregular grains and crystals a white, 

 semi-translucent substance, leucoxene, is produced, indicating the 

 presence of titanium in the form of titaniferous magnetite or ilmenite. 

 Almost all the original crystals appeared to be titaniferous; the only 

 pure magnetite being that resulting from the alteration of olivine and 

 other ferro-magnesian minerals. 



Apatite. — Apatite occurs in irregular grains or in slender prisms, 

 which sometimes show a basal parting. It occasionally occurs as 

 inclusions in the olivine or biotite; at other times the grains or 

 crystals are scattered about between the crystals of mica. The 

 apatite is a clear, colorless, non-pleochroic mineral, with a high index 

 of refraction and a low double refraction. Basal sections show a 

 uniaxial interference figure. No alteration takes place, but the sur- 

 face of the crystals is often pitted. 



Perofskite and picotite. — The perofskite occurs in yellowish-brown 

 crystals and grains. The cross-sections are varied, being diamond- 

 shaped, trigonal, hexagonal, or irregular. Many of the irregularities 

 are due to aggregations of crystals. Some skeleton crystals and some 

 incipient forms of growth occur. The index of refraction is high, 

 and the sections are usually entirely isotropic between crossed nicols. 

 The sections are often traversed by irregular cracks, sometimes 

 showing slight alteration. In one case a grain of perofskite which 

 had been included in apatite was partly altered to a carbonate (cal- 



