822 THE JOURNAL OF GEOLOGY. 



between the olivine grains and the pyroxene, or in the inter- 

 stices between the plates of the latter mineral, though some 

 large grains are included within the olivine. 



In the non-olivinitic types of these basic rocks, pyroxene 

 and magnetite are the principal components. These types are 

 perhaps more common than the olivinitic types, but their variety 

 is so great that a description of them would be little else than a 

 description of individual specimens. Pyroxene forms the greater 

 part of all these rocks, but the character of the pyroxene changes 

 for each individual bed, so that it is impossible to classify their 

 material with any degree of success. It must therefore suffice 

 to study one of the most striking types, and to leave the others 

 with the statement that they are composed exclusively of some 

 form of pyroxene and of magnetite. 



The most interesting of the pyroxene-magnetite rocks is that 

 already referred to as exhibiting lustre-mottling (M. 453 H). 

 In this the pyroxene is present in two forms. It occurs as large, 

 strongly pleochroic green plates, and as small, slightly pleochroic 

 green grains, the latter included in the former. The lustre 

 mottling on the hand specimen is due to the reflection from the 

 large plates, and the fine-grained texture of the rock is the result 

 of the uniform distribution of the small grains within the larger 

 ones. The entire area of a slide is often occupied by four or five of 

 the pleochroic plates, with their inclusions. They interlock with 

 irregular sutures, and thus completely cover the thin section. 

 When examined over the lower nicol the plates are yellowish- 

 pink transverse to their prominent single cleavage and light green 

 parallel to this. The axis of least elasticity in sections showing 

 parallel cleavage lines is in the direction of this cleavage, and 

 the extinction is consequently also parallel thereto. In cross 

 sections the pyroxene cleavage is plainly apparent. Here the 

 pleochroism is in yellowish pink and light wine-yellow tints, the 

 latter in the direction of the smaller of the two lateral axes of 

 elasticity. The absorption is consequently C= green >E : = yellow- 

 ish pink^tl^wine yellow. The mineral is in all probability 

 hypersthene. 



