248 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



connection with the cracks in the crystals, and are undoubtedly primary. 

 The biotite frequently surrounds magnetite and is attached to it and the 

 pyroxene. In one of the rocks there is a moderate amount of compact 

 green hornblende, which forms borders and intergrowths with pyroxene in 

 exactly the same manner as in the diorite of Electric Peak. In one of these 

 forms of the rock one augite has, besides the rod-like inclusions character- 

 istic of this occurrence, others which are immediately connected with distinct 

 cracks and are undoubtedly secondary. This was the only instance of the 

 kind noticed in these remarkably fresh rocks. 



The next two modifications of the rock (1392, 1393) are still coarser 

 grained, but have the same microstructure. The outline of the porphyritical 

 feldspars is lost in an irregular interlocking of adjacent crystals. The 

 poikilitic orthoclase is more distinct. In the next two phases of the rock 

 (1394, 1395) the grain is still coarser, resembling that of a fine-grained 

 granite. They have a saccharoidal texture in hand specimens, which is 

 characteristic of the greater number of all of the rocks of the core. When 

 broken they appear loosely coherent, the crystals parting along faces and 

 cleavage planes rather than in smooth planes across the rock. The micro- 

 structure of these forms of the rock resembles that of the preceding, without 

 the porphyritic development. The feldspathic minerals are in excess of the 

 ferromagnesian. The largest-sized individuals of feldspar are the poikilitic 

 orthoclases, but the lath-shaped plagioclase is far more abundant. Some 

 individuals of orthoclase have a marked micrographic and plumose arrange- 

 ment of quartz inclusions, associated with which are long hair-like needles, 

 which in places pass into lines of black dots. They also contain some 

 microcrystals of mica. The ferromagnesian minerals are partly idiomorphic 

 and consist of augite, hypersthene, green hornblende, and biotite in about 

 equal proportions. It is to be remarked that in these gabbros the color 

 of the hornblende is generally stronger and purer green than in the 

 diorite of Electric Peak. The crystals of apatite and zircon are noticeably 

 larger in these modifications of the rock than in the finer-grained ones. In 

 specimen 1395 biotite occurs in large interrupted patches. This is charac- 

 teristic of the coarsest-grained member of the series (1396), which is a gray 

 granular rock that is exposed in large rounded masses, weathering into sand 

 and resembling a crumbling granite superficially. In thin section it exhibits 

 the same microstructure as the previous variety, but is still coarser, being 



