THE BASIC MASSIVE ROCKS, ETC. 823 



The small grains included within the hypersthene are bright 

 green in all positions. Now and then slight changes in the 

 shade are noticed, indicating very weak pleochroism. The color 

 is so nearly like that of the green ray of the hypersthene, that 

 the presence of the grains in the latter mineral can be detected 

 only with the greatest difficulty, when the slide is in such a 

 position that the parallel cleavage of this mineral runs in the 

 direction of the vibration plane of the lower nicol. In most of 

 the included grains a well-marked coarse cleavage is noticed, the 

 maxium extinction against which is 41 . In addition to the 

 coarse cleavage there is often observed a second and finer series 

 of cleavage lines, whose direction is parallel to that of the 

 coarser ones. The properties of this pyroxene are those of 

 diallage. 



The magnetite in this rock is very abundant. It is in large 

 and small grains imbedded in the pyroxenes, the larger ones 

 usually in the hypersthene and the smaller ones in the diallage. 

 Many of the grains are irregular in shape, but quite a number 

 show crystal forms. There is no evidence that any of the min- 

 eral is of secondary origin. It all seems to be original. A por- 

 tion of the magnetite separated from the powdered rock by 

 digestion with hydrochloric acid was tested for titanium with a 

 negative result, although specimens from other similar rocks 

 contain this element. 



The only other constituent observed in the section was a 

 bright green hornblende, whose extinction is not known to be 

 greater than 13 . It occurs in but a few flakes with irregular and 

 indefinite outlines that fade off into the surrounding pyroxenes. 

 It is probably secondary. 



A comparison of the descriptions of the types of rocks 

 above given indicates that the hypersthene in the last described 

 type has taken the place of the olivine in the others. The green 

 diallage in both cases is the same, though in the first two instances 

 it surrounds the olivine, i. e., it is younger than this mineral, 

 while in the other instance it is surrounded by the hypersthene 

 — it is older than the latter. A study of the granulitic phases 



