332 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Nothing resembling leucite or suggesting it is present. This feldspar matrix 

 is crowded with microscopic crystals of augite, magnetite, and brown biotite 

 in thin tablets and long narrow crystals which often resemble hornblende. 

 In one part of the rock ilmenite accompanies magnetite. The groundmass 

 of the edge of the dike is brown glass with microlites of augite and some 

 of plagioclase. These microlites have dark-brown clouds attached to their 

 ends, or in the case of augite are colored brown. 



The augite phenocrysts are light green in thin section and are filled with 

 irregularly shaped inclusions of crystalline groundmass containing ilmenite 

 rods, which are not generally found in the groundmass of the rock outside 

 of the augites. These ilmeuite rods within the inclusions of one augite lie 

 in several orientations, apparently with rhombohedral symmetry. Although 

 they are confined to the inclusions of groundmass and do not penetrate 

 the augite substance, their shape and arrangement suggest the microscopic 

 rod-like inclusions in the diallage and hypersthene of gabbro, which may 

 possibly be ilmenite. The augites also inclose small crystals of olivine and 



magnetite. 



The olivine phenocrysts are sharply idiomorphic and of very pure sub- 

 stance, with small inclusions of magnetite and glass and occasionally bays 

 of groundmass. It is sometimes twinned in pairs of attached crystals, the 

 twinning plane being (Oil). There is a slight serpentinization along 

 the surface and cracks in some cases. The olivine crystals are much 

 smaller than those of augite, but are more numerous. The appearance of 

 this rock in thin section is shown in PI. XXXVI, fig. 1. 



It is to be observed that the glassy groundmass of the marginal surface 

 of the dike is unlike the groundmass of the main body of the dike in 

 mineral composition. Biotite is not developed, and the only feldspars are 

 the microlites of plagioclase, which may correspond to the cores in the 

 lath-shaped orthoclases of the main body of rock. As compared with the 

 leucite-bearing rock from Ishawooa Canyon (1698), it is to be noted that 

 the phenocrysts of augite and olivine in the dike rock just described are not 

 quite so numerous as in the Ishawooa rock, while there is much more 

 augite in the groundmass, besides abundant biotite. The microscopic 

 crystals of feldspar are not .so large and distinct, and nothing corre- 

 sponding to leucite can be seen. Chemically the rock is richer in potash, 

 with about the same per cent of soda. Alumina is slightly higher, and 



