SHOSHCXNITE. 341 



olivine are like the phenocry sts of these minerals in absarokite ; the olivine 

 is partly serpentinized. 



Similar lava occurs at the north base of Specimen Ridge (1127). In 

 this the phenocrysts of feldspar are labradorite with about the composition 

 An 3 Ab 2 . The groundmass is partly globulitic and microlitic glass, contain- 

 ing many rectangular orthoclase crystals with lath-shaped lime-soda feld- 

 spars at their center, some of which prove to be labradorite (An 3 Ab 2 ) ; most 

 of them exhibit low angles, and may be andesine-oligoclase. Some small 

 rectangular plagioclases without orthoclase margins are labradorite (An 3 Ab 2 ). 

 There is nearly as much orthoclase as plagioclase in the groundmass. 

 There is, of course, augite, apatite, and magnetite, as in the other rocks. 

 Other modifications of this rock have a less highly crystallized groundmass, 

 in which orthoclase has not been developed. Such forms could not be 

 distinguished microscopically from some ordinary basalts. Other rocks 

 of this type with glassy and microlitic groundmasses occur at the creek 

 south of Opal Creek (1143 ), and in a cliff 3 miles up Soda Butte Creek (1137). 

 A holocrystalline rock of this character forms a dike on Timber Creek, 

 Crandall Basin (1328). Another forms a lava flow on the mountain east 

 of Pyramid Peak (1474, also 1457). A modification with small pheno- 

 crysts occurs as a surface flow at the falls of Timothy Creek on Mirror 

 Plateau (1153), and elsewhere on this plateau (1155), and is exposed in 

 association with andesitic breccia beneath rhyolite west of The Crags, 

 southwest of Gallatin Mountains (580). It has also been found in bowlders 

 east of the lower Geyser Basin. 



Another basaltic rock belonging to this class because of its chemical 

 composition occurs in an obscure exposure at the north base of Sepulchre 

 Mountain. It appears to have been a surficial flow of lava contemporaneous 

 with the acid andesitic breccias at that place. It is dark greenish gray 

 (379, 381), with few megascopic phenocrysts of augite and serpentinized 

 olivine, and without any of feldspar, and is vesicular, with amygdules of 

 agate and quartz. In thin section it is holocrystalline, with lath-shaped 

 plagioclase microlites surrounded by a zone of orthoclase, which is in con- 

 siderable amount. There are also prisms of augite and crystals of magnetite. 

 Apatite needles are abundant. Its chemical composition is shown in the 

 second analysis of the table just given. It resembles the first one closely 

 except in the oxidation of the iron. The lime-soda feldspars are confined 

 to the groundmass, in the form of microlites. 



