344 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



otherwise the rock appears to be holocrystalline. In one specimen (1714) 

 there is less serpentine, and considerable colorless mineral with lower 

 refraction than orthoclase, which is not isotropic, but has weak double 

 refraction. The microstrncture of this rock and the character of the large 

 feldspar phenociysts are shown in PI. XXXVII, fig. 1. 



The rocks of the other two sheets (1717 to 1719) are scoriaceous and 

 vesicular, and are microlitic with much opaque mineral. They are some- 

 what altered, and their exact mineral composition can not be made out, 

 They are related in general character to the rocks associated with them. A 

 rock of the same kind as the more crystalline forms from Two Ocean Pass 

 is found on Fox Creek (1731). Shoshonite, quite like that at Two Ocean 

 Pass, with pronounced orthoclase in the groundmass, but carrying pheno- 

 crysts of hypersthene and augite and less olivine, with large labradorites, 

 occurs west of the summit of Baldy Mountain, east of Bear Gulch, just 

 north of the Yellowstone Park. Its chemical composition is shown in 

 analysis 8. It is low in magnesia and is chemically more like the sho- 

 shonite of Two Ocean Pass than the others. There are no carbonates 

 discoverable in the thin section of the rock corresponding to the consider- 

 able percentage of carbon dioxide in the analysis. The rock is very fresh, 

 and it is possible that the material analyzed contained fragments of limestone 

 or of calcite. 



Very similar rocks occur on Lamar River 2 miles above the mouth of 

 Slough Creek (1129), and also a short distance below the mouth of Soda 

 Butte Creek (1136, 1135). They are surficial lava sheets and have pheno- 

 crysts of feldspar and very small ones of augite and olivine The 

 groundmass is nearly the same as that just described. Other rocks like 

 the last named are from the spur of Mirror Plateau south of Flint Creek 

 (1147), and from the north side of Grayling Creek west of the end of 

 Crowfoot Ridge (578), and from a point west of The Crags (579). All 

 of these were lava flows that poured out upon the surface of the earth. 



A comparison of the chemical analyses and of the rocks of this group, 

 besides making evident the relationships already noted, also shows what 

 mineralogical differences may obtain for rocks of nearly the same chemical 

 composition. Some of these differences have already been described in -the 

 case of the leucite-bearing varieties. Other differences may be mentioned. 

 The lava flow from the southeast fork of Beaverdam Creek (1647) and the 

 dike rock from the ridge northeast of Indian Peak, though nearly alike 



