824 THE JOURNAL OF GEOLOGY. 



of the gabbro points to the same conclusion, viz., that the hyper- 

 sthene and olivine occupy similar places in the rock's constitu- 

 tion. Where the one is present the other is usually absent. 

 They seem to be complementary components. It is also notice- 

 able that when hypersthene is present and olivine absent the rock 

 contains more magnetite than in the case where the conditions 

 are reversed. 



In some of the beds the percentage of the iron oxide increases 

 as has already been said, until it reaches 80 per cent, or even 95 

 per cent, of the rock mass. In some of the sections made from 

 these rocks nearly the entire field of the microscope may be occu- 

 pied by a single mass of compact magnetite. On its edges this 

 mass often breaks up into small grains that are cemented together 

 by a large plate of green pyroxene. This observation is valuable 

 as showing that the beds composed almost exclusively of mag- 

 netite have the same origin as those in which this mineral con- 

 stitutes only a small part of the rock mass. The latter are cer- 

 tainly phases of the gabbro, hence the former must also be 

 phases of the same rock. Whether most of the magnetite in these 

 varieties is primary or secondary cannot be told. Much of it is 

 certainly primary. 



Granulitic Pyroxene Rock. — An interesting variety of the basic 

 rocks is No. M. 1334. It is slightly schistose, and is composed 

 almost exclusively of colorless pyroxene and green hornblende, 

 in small grains with rounded contours. Its structure is granu- 

 litic. A brief description of the rock is given in the chapter on 

 the granulitic gabbros. 



The Relation of the Basic Rocks to the Normal Gabbro. — From 

 the descriptions that have preceded, it is seen that the basic 

 rocks along the northern periphery of the gabbro are composed 

 almost exclusively of the more basic constituents of the normal 

 rock — viz., magnetite, olivine, and pyroxene. The feldspar of 

 the main mass of the gabbro is entirely lacking in them. 

 The accumulation of the basic portions of the gabbro-magma 

 on its periphery may be accounted for by its differentiation 

 during cooling. Such a differentiation of a gabbro-magma 



