276 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of Slough Creek. A remnant caps the breccia on the east side of Yellow- 

 stone Canyon, opposite the hot springs above Baronett Bridge (1126). It 

 is dark, dense, and crystalline, with a slightly resinous luster. It has large 

 tabular phenocrysts of plagioclase, some 8 or 10 mm. long, but none of augite 

 or olivine. Another sheet of basalt at the north base of Specimen Ridge 

 is dark and dense, with many medium-sized phenocrysts of tabular plagio- 

 clase, and fewer of augite (1127). A somewhat similar basalt overlies the 

 acid breccia just east of the mouth of Crystal Creek. It is dense and full 

 of medium and large phenocrysts of feldspar and augite. It is partly 

 amygdaloidal with agate and quartz (1128). Over it is another sheet of 

 dense basalt, dark and crystalline, with riiany large phenocrysts cf rec- 

 tangular and tabular feldspars, and fewer and less noticeable augites (1130). 



A similar basaltic rock occurs farther east, at the south base of Bison 

 Peak. It is dense, has a slightly resinous luster, and is filled with large 

 brilliant feldspars and numerous smaller augites, and contains some amyg- 

 dules of zeolite (1131). Similar basalt occurs at the southwestern base of 

 Druid Peak (1132). It is underlain by a massive sheet, which is dense and 

 crystalline, with abundant large phenocrysts of tabular plagioclase 10 mm. 

 long, and fewer and smaller augites (1133). Farther east, at the first gulch 

 below the mouth of Soda Butte Creek, basalt similar to the last is exposed 

 200 feet above the river (1135). Three miles up Soda Butte Valley a basalt 

 cliff exposes a sheet of dense black rock, with abundant small phenocrysts 

 of feldspar and augite (1137). 



These rocks have a peculiar mineral composition that distinguishes 

 them from ordinary basalts. They are like some of the basaltic lavas 

 occurring in the upper parts of this breccia, or overlying it, which are char- 

 acterized by a varying content of orthoclase, and since they have never 

 been described in detail their specific characters will be given in Chapter 

 IX, which is devoted to a general description of all similar rocks occurring 

 within the Yellowstone Park. 



The basic breccias belonging to the main mass of the Crandall volcano, 

 and forming the mountains on Cache and Calfee creeks and the body of 

 Saddle Mountain, extend across Lamar River toward the west and south 

 and constitute the basal portion of Mirror Plateau and of the mountains 

 between Lamar River and the head of Stinkingwater River. 



Overlying this breccia in Mirror Plateau are numerous sheets of basalt, 



