126 UNIVERSITY OF COLORADO STUDIES 



of the hade which is sometimes about 45 . Faults of small throw, 

 abrupt bends, and forkings are characteristic features. Jointing in 

 slabs parallel to the walls is common; and flow-structure, best seen in 

 the parallelism of the mica flakes, is pronounced. The same rock occurs 

 in a sheet in the Fountain sandstone exposed in Gregory Canyon south- 

 west of Boulder, on Flagstaff Hill, on the west slope of Red Rock, and 

 east of Sunshine Canyon. 1 



North of Lefthand Canyon dacite of almost identical mineral compo- 

 sition occurs in a dike with a westerly course, which was not traced 

 beyond the limits of the map. Flow- structure, which is so common in 

 the dikes near Sunshine Canyon, is here lacking. 



Petrography 2 



Megascopically this rock shows, when fresh, abundant hexagonal 

 flakes of highly lustrous biotite, occasional flakes of white mica, white 

 to bluish-gray feldspars and less numerous quartz crystals in an apha- 

 nitic greenish-gray groundmass. While the biotite crystals vary in abun- 

 dance from place to place they are everywhere the most numerous of the 

 phenocrysts. In diameter the flakes are usually less than 2 mm. but 

 may reach twice this size. Although they frequently retain their high 

 luster until after there has been considerable weathering of the ground- 

 mass they are in most of the exposures chloritized, and have lost their 

 elasticity. The feldspars sometimes reach a diameter of 8 mm. and 

 only occasionally do they show striae under a hand lens. They often 

 appear greenish toward the center of the crystals while the border is 

 white; in an advanced stage of alteration they become pinkish or 

 yellowish, and effervesce readily with hydrochloric acid. The quartz 

 phenocrysts are usually rounded by corrosion, and rarely 1 cm. in diam- 

 eter. They will average in number from five to ten on one face of a 



1 See Bulletin 265, U. S. G. S., p. 37, map. 



* Owing probably to the difficulty in finding fresh material for microscopic study and to the apparent 

 rarity of striations on feldspar cleavages in a megascopic examination, this rock has hitherto been regarded as 

 rhyolite or as simply quartz-porphyry. Dr. Palmer and Major Fulton described it under the name 

 of quartz-porphyry (C S. Palmer, Proceedings of the Colorado Scientific Society, Vol. IU, pp. 233 £f., 

 and C S. Palmer and Henry Fulton, idem, pp. 351-358). Dr. Fenneman (Bulletin 265, V. S. G. S., 

 p. 36) briefly describes the Sunshine dike and the Flagstaff sheet without naming the rock, but it is designated 

 as rhyolite on the map which accompanies his description. 



