250 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



diorite (1399) on the southwestern spur have the same degree of crystalli- 

 zation and the same characteristics. The microstructure is like that of the 

 coarsest variety already described. The essential feldspar is labradorite with 

 a small amount of orthoclase and little micrographic quartz. There are also 

 allotriomorphic grains of quartz, against which the orthoclase is idiomorphic. 

 There is much hypersthene, augite, biotite, and magnetite, with some pri- 

 mary green hornblende. The hypersthene in certain sections exhibits a 

 very fine microscopic lamination, produced by the parallel intergrowth of 

 monoclinic and orthorhombic pyroxene, as the optical behavior proves. 

 This lamination is found throughout the hypersthene of this gabbro, but is 

 recognizable only in certain sections and may be easily overlooked. The 

 pyroxenes have the characteristic inclusions and pinacoidal cleavage. 

 Apatite and zircon occur in comparatively large crystals. The chemical 

 composition of this rock, shown in analysis 7 on page 260, is like that of 

 1396, No. 8 of the same table. 



The lowest exposure of gabbro in the west gulch is about 1,000 feet lower 

 than the lake. It is compact (1437) and dark colored, and is like that near 

 the outlet of the lake, but is coarser grained, being higher than the highest 

 grade of crystallization found at Electric Peak. One of the coarsest-grained 

 varieties (1411) occurs near the lake (PI. XXXIII, fig. 2). It is once and 

 a half as coarse as the most crystalline diorite at Electric Peak; and on 

 the crest of the west spur, at about 10,000 feet altitude, a variety (1436) 

 is found which is twice as coarse grained as that at Electric Peak. The 

 feldspars are from 2 to 3 mm. long. The structure is like that last described, 

 but there is more feldspar, and biotite and hornblende predominate over 

 the pyroxene and magnetite. By its mineral composition as well as its 

 chemical (analysis 9 on page 261) it is shown to be a diorite fades of the 

 gabbro. In some parts of the core the dark-colored gabbro .carries small 

 light-colored masses, which are highly feldspathic. 



In this connection may be described a remarkable variety of crystalline 

 rock which does not occur within the gabbro core, but is found on the 

 northeastern spur of Hurricane Mesa, near the center toward which the 

 second group of dikes converge. It is a special phase of crystallization of 

 the magma, whose chemical composition (analysis 10 on page 261) corre- 

 sponds to certain dioritic facies of the gabbro in the core. The rock (1442) 

 is compact and fine grained, with a crystalline luster and brilliant reflections 



