244 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



oriented augite, magnetite, and biotite, which correspond to more definite 

 paramorphs after olivine in some of the other rocks. 



Similar rocks occur in dikes within .he margin of the core. They are 

 slightly coarser grained. One (1378) forms a 4-foot dike near the base of 

 the pinnacled spur on the west, and a still coarser-grained form (1418) 

 occurs in a 12-foot dike on the same spur, somewhat nearer the center of 

 the core; its grade of crystallization is about 20. The microstructure 

 becomes more pronounced as the constituent crystals are larger and more 

 distinct. The labradorite has the same kinds of inclusions, but the outlines 

 are in part serrated by the interference of adjacent grains in the groundmass. 

 The rusted paramorphs have outlines more characteristic of olivine. The 

 massive intrusive sheet on the eastern side of the core (1372) is like the last 

 in composition and microstructure, but there is a little more biotite. There 

 is some orthoclase as margins around prisms of labradorite, precisely like 

 the occurrence of orthoclase in shoshonite, but its amount is small. The 

 chemical analysis, No. II on page 261, shows a relatively high percent- 

 age of potash, and the rock is closely related to shoshonite, but is a little 

 higher in lime and magnesia and a little lower in alkalies. It is almost a 

 shoshonite-porphyry or monzonite-porphyry. The two rocks are quite fresh, 

 and exhibit no signs of crushing or other indications of alteration, the large 

 feldspars being glassy and not cracked. The microstructure of 1372 is 

 shown in PI. XXXVII, fig. 2. This rock is pyroxene-diorite-porphyry 

 approaching monzonite-porphyry. 



Another variety of massive intrusive rock is exposed at the bottom of 

 the southwest spur of the core (1377, 1379, 1383). ' It appears to be a dense 

 aphanitic form of the gabbro, and is probably an apophysis or the margin 

 of the core. It is a basalt- porphyry. The porphyrin cal crystals of augite 

 rise above the groundmass of the rock on weathered and fractured surfaces. 

 It is to be borne in mind that the chemical composition of this rock (analysis 

 4 on page 260) is the same as that of some of the basalts of the district, 

 so that the rock is a special phase of crystallization of this magma. 



In thin section the finest-grained forms have an andesitic structure in 

 the groundmass. A slightly more crystalline variety is still andesitic, but 

 consists of lath-shaped and rectangular plagioclase in a matrix of grains of 

 feldspar, augite, hypersthene, and magnetite, besides minute crystals of 

 light-brown biotite. In places the ferromagnesian minerals preponderate. 



