348 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The rocks of analyses 1 and 2 form dikes, the second one occurring 

 on the south face of Hoodoo Mountain and the first one on the divide south, 

 the two dikes trending in the same direction, a little east of south. The 

 rock is the same in both (1296, 1309). It is light gray and aphanitic, with 

 a glistening luster. There are prominent phenocrysts of black augite and 

 rusted spots of serpentinized olivine, but no porphyritical feldspar. In the 

 rock from Hoodoo Mountain (1296) there are numerous amygxlules of white 

 stellate zeolite. 



In thin section the rocks are holocrystalline, with more feldspar than 

 ferromagnesian minerals. The feldspar is lath-shaped and tabular, with 

 irregular outlines. The crystals are simple twins with low extinction 

 angles, and are orthoclase with kernels of plagioclase, which is mostly 

 altered, the centers of the crystals being decomposed in many cases. 

 There is considerable serpentine scattered through the rock. The 

 ferromagnesian minerals of the groundmass have the same characters as in 

 the allied rocks; they are augite and biotite, with magnetite, and some 

 ilmenite in roddike shapes; apatite occurs in needles. The rocks contain 

 much analcite, forming clusters of crystals, which are partly cloudy but 

 mostly transparent. They frequently fill cavities, and sometimes occupy 

 spaces whose outlines appear to have belonged primarily to isometric 

 minerals, while in places the irregular outline and inclosure of other minerals 

 make it appear to be a part of the original groundmass of the rock. It is to 

 be remarked in this connection that basalts with abundant crystals of analcite 

 which appear to be primary minerals have been found and described by 

 Lindgren 1 in the Highwood Mountains, about 200 miles north of this district. 

 The dike on the divide south of Hoodoo Mountain is parallel to that of the 

 absarokite (1306) whose chemical composition is shown in analysis 2 of the 

 table on p. 329. 



A rock from the ridge west of Rocky Creek (1 624) is almost identical 

 with that just described. The groundmass is nearly panidiomorphic. The 

 orthoclase is pronounced, and there -is much biotite and augite, besides 

 grains of an isotropic mineral, probably analcite. Its microstructure is 

 shown in PL XXXVIII, fig. 1. 



Others of the same kind are from Hoodoo Basin (1302), from the south 



1 Tenth Census, Vol. XV, 1886, p. 727 ; also Proc. California Acad. Sci., 2d ser., Vol. Ill, p. 51. 



