126 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



have a narrow border of magnetite or one of small crystals of pyroxene, 

 feldspar, and magnetite. There are all gradations, from rounded horn- 

 blendes with opaque borders to small angular pieces of hornblende sur- 

 rounded by comparatively large crystals of pyroxene, feldspar, and some 

 magnetite, which form a group of interlocked crystals in the glassy ground- 

 mass. The angular outline of the hornblende and its penetration between 

 the crystals of feldspar and pyroxene would militate against the supposi- 

 tion that the hornblende is a remnant of a previous crystal that had been 

 partially resorbed in the groundmass, were it not for the occurrence in one 

 thin section of a group of different crystals with a hexagonal outline, cor- 

 responding to the cross section of the hornblende remnants contained in it 

 which are properly oriented for such a section. The greater part of the 

 group consists of feldspar and pyroxene with some magnetite. It is not to 

 be supposed that these minerals crystallized out of the melted hornblende 

 substance without interchange of material from the surrounding magma. 

 The larger groups in the same rock section exhibit no definite outward 

 form, but are bounded by the outlines of the outer crystals, so that we 

 may conclude that the process of resorption of the hornblende phenocrysts 

 was in some cases accompanied by the immediate formation of grains of 

 magnetite and the absorption of the other chemical constituents by the 

 magma, while in other cases the melted hornblende recrystallized in situ as 

 pyroxene and magnetite. But in the instances just mentioned the partial 

 resorption of the hornblende was followed by a greater tendency toward 

 crystallization in the magma immediately surrounding the melted horn- 

 blende, which led to the development of a group of all the minerals then 

 capable of forming. These minerals are the same in size and character 

 as the small crystals scattered through the glassy groundmass. 



In only two sections of the andesites examined was biotite found. It 

 was in small crystals with compound borders similar to those around horn- 

 blende. 



The remaining rock sections from this breccia represent hornblende- 

 pyroxene-andesites with varying amounts of the ferromagnesian minerals, 

 forming a series with increasing hornblende and decreasing pyroxene. In 

 these andesites the microscopical characters of the pyroxenes are the same 

 as in the rocks just described. The hornblende varies in different rocks, 



