OCCLUSION OF IGNEOUS ROCK 415 



bic or triangular, or present six-sided outlines like cross-sections 

 of octahedra. Some are mere skeleton aggregates of loosely 

 adhering granules by imperfect development, with hollow in- 

 teriors occupied by serpentine. Even the massive grains are 

 commonly intersected by sharp clefts filled with serpentine, 

 whose parallelism indicates the plane of strain, coinciding with 

 the schist-plane of the surrounding rock. 



Pleonaste is not a common accessory, though very abundant 

 in occasional thin sections in grayish green angular grains, 

 translucent and isotropic, some of which show six-sided out- 

 lines. Many granules of chromite are included, sometimes to 

 half or more of the volume of the pleonaste, and the grains of 

 the latter may thus pass along a row into pure chromite and 

 beyond back again into spinel. 



Granules of dolomite, with a little relief, are often mixed in 

 the interstices of tremolite or gathered into nests, 0.8 to 2 mm. 

 across, colorless, grayish or reddish white, with clouded mar- 

 gins (by hydromagnesite ?). Outlines sometimes projecting, 

 with an obscure fibrous texture, Newton colors along the inter- 

 vening lines, and many included particles of magnetite. The 

 usual characteristic cleavage and high birefringence. Extinc- 

 tion symmetrical. In convergent light, the bars of an uniaxial 

 cross. 



Some clear grains with more relief were referred to calcite. 



The paragenetic relationship of the minerals offers the follow- 

 ing order : chromite and pleonaste, diallage, bronzite, actinolite 

 and tremolite, magnetite, bastite and hydrous amphibole, sec- 

 ondary tremolite, brucite and serpentine, talc and chlorite, 

 dolomite and calcite, hematite. It has been stated that the 

 mineral of higher birefringence, associated with serpentine and 

 chlorite in the veinlets, must be brucite or its fibrous form, 

 nemalite. Where uniformly diffused, its minute scales or fibers, 

 lying partly in planes normal to the visual direction and there- 

 fore isotropic, must blend indistinguishably with its serpentinous 

 matrix of feeble birefringence. This may account for the ap- 

 parently greater proportion of serpentine observed in the thin 

 sections than that indicated by the chemical analysis. But 



