186 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



other openings are about 150 yards to the northeast diagonally across 

 the draw and consist of open cuts. 



The opal deposits are in whitish chalklike decomposed rhyolite 

 in which occur inclusions of blocks and rounded masses of gray to 

 brown glassy perlitic ryholite. The perlitic rhyolite appears to be 

 the same as the inclosing decomposed rhyolite but has not undergone 

 alteration. A weathered yellowish fine porphyry bed occurs over 

 the opal bearing rhyolite outcropping as a hard stratum. The for- 

 mations have a gentle northerly dip and the rhyolite can be recog- 

 nized at numerous places by its light-colored outcrop. The opal 

 occurs in seams and veinlets, filling cracks and joints, as a filling in a 

 brecciated fracture zone, and in nodular masses, both in the altered 

 rhyolite and in the perlite. White and milky opal was plentiful 

 around the dumps and a few small chips of precious opal were seen. 

 Judging from the extent of the work done it is probable that valuable 

 opal was found. 



At another opal deposit about 2 miles east of south of Sommer 

 camp and 4 miles west of Enterprise, about half a dozen pits have 

 been made within 200 yards of one another, which range in size from 

 4 to 20 feet in depth and about the same in width. They are in a 

 bed of partly decomposed whitish rnyolite interbedded with brownish 

 glassy rhyolite. The formations are gently folded and the rhyolite 

 outcrops at several places to the south along the road on the hill 

 above. The beds have an aggregate north dip. Other prospects 

 were opened on some of the upper outcrops of the rhyolite. Con- 

 siderable chalcedony and white, milky, and bluish opal were seen on 

 the dumps around the pits. White opal and translucent gray chal- 

 cedony are banded together in some specimens like onyx. Very 

 little precious opal was seen around the old openings. 



A number of specimens of opal labeled as from 3 miles west of 

 Enterprise are preserved in the National Museum. The opal occurs 

 in a black highly vesicular lava in which only a small proportion 

 of the cavities are filled with opal. Apparently a soft ferric silicate, 

 chloropal or a related mineral first lined or partly filled the cavities 

 after which the opal was deposited. In some cases it appears as 

 though a carbonate, since removed, preceded the opal. 



Fossil bones collected from sandstone near Opaline at an elevation 

 of 2,400 feet were encrusted with opal and some of their cavities con- 

 tained fire opal. The bones were identified by Prof. O. C. Marsh 

 as Protohippus of Pliocene age. 87 



" Waldemar Lindgren. U. S. Geol. Survey, 20th. Ann. Rept., pt. 3, p. 99, 1900. 



