TEACHYTIO EHYOLITE. 321 



not distinctly brecciated (1613). Microscopically it is the same as the 

 rock of Coulter Peak, the lithoidal portion being holocrystalline, with 

 abundant prismatic microlites of feldspar that exhibit a pronounced fiuidal 

 arrangement; The glassy portion consists of clear yellowish-brown glass, 

 with microlites, as in the. glass of the Coulter Peak rock. The few small 

 phenocrysts of brownish-green hornblende have margins of loosely aggre- 

 gated pyroxene and magnetite. The other mass has the same characters. 

 It is dense, gray, and lithoidal, and is partly vesicular with good crystals 

 of tridymite (1611, 1610). It also has the same microscopical characters. 

 A surficial flow of pyroxene-andesite occurs on the summit of the ridge 

 north of Middle Creek, already mentioned in another place. 



TRACHTTIC RHYOLITE. 



Closely connected with the early acid breccia on Yellowstone River 

 in the neighborhood of Crescent Hill and Junction Butte are remnants of 

 a lava stream of light-colored lithoidal rock whose most pronounced miner- 

 alogical feature is the presence of abundant phenocrysts of sanidine 

 without any of quartz. It passes into breccia in places, and appears to 

 have been contemporaneous with the earlier acid andesitic breccia of the 

 region. Whether the scattered occurrences of this rock in. the bottom of 

 one large valley were originally connected and were parts of one large 

 sheet, or whether they belong to several eruptions, is not definitely known. 



Areas of this rock occur along the south side of Yellowstone River, 

 forming a cliff at about 6,400 feet altitude from Greode Creek around the 

 north base of Crescent Hill to Yanceys. It occurs somewhat higher on 

 the north side of the Yellowstone River west of Hellroaring Creek, lying 

 between 6,600 and 7,200 feet altitude. It extends east around the north 

 end of Junction Valley and forms the base of Junction Butte east of the 

 river. It occurs on the north of Lamar River opposite the butte, and 500 

 feet above the river, and also near the mouth of Slough Creek. Patches 

 of it are found on the end of Specimen Ridge 1,500 feet above the river. 

 Lavas similar to this occur as intimately associated with the basic andesitic 

 breccias north of the Yellowstone Park, on Buffalo Plateau and on the 

 flanks of Sunset Peak. 



The rock is generally light colored, and varies from white and gray to 

 yellow, buff, brown, red or pink, purplish, and green. In most places it is 



MON XXXII, PT II 21 



