MOPUNG HILLS. 735 



flesh-colored rhyolites, and still later than these are others of a greenish- 

 gray pearlitic variety, the whole mass being penetrated by a powerful series 

 of north and south fissures, whose sides show the evidence of powerful 

 motion. 



A very interesting variety in the foot-hills directly to the south of the 

 outlet of the lake has reddish-brown groundmass, containing beautiful choc- 

 olate-colored sphserulites, having a light, earthy nucleus. In this rock were 

 crystals of both orthoclase and plagioclase, but no quartz. Some purple 

 varieties consist of round, half-pellucid grains, which have a concentric 

 structure, the globules being flattened so as to give the rock a stratified 

 appearance. Toward the extreme northern end of the foot-hills is a dark 

 chocolate-colored rhyolite, containing much biotite, whose feldspar crystals 

 are very largely decomposed, having a hollow nucleus partially filled with 

 the groundmass. Still another interesting rhyolite in this group is a pale- 

 red rock, with gray and purple bands. Crystals of sanidin pass through 

 these narrow dark bands and penetrate the groundmass on both sides, while 

 in other instances the groundmass diverges, surrounds the crystal, and con- 

 verges beyond it. 



In the most decomposed portions of these rhyolites is much carbonate 

 of lime and some infiltrated salts incrusting the narrow fissures and cavities. 

 In some places, a very considerable efflorescence of common salt covers the 

 surface of the earthy rhyolites. 



It is noticeable that in the breccias the entire mass of the included frag- 

 ments is made up purely of rhyolitic material. 



Two specimens of rhyolite from the Mopung Hills have been subjected 

 to analysis, with a view to determine, if possible, if there exist any marked 

 differences in chemical composition between these rocks and the granular, 

 gray, porphyritic varieties that form the outbursts along the Pah-Ute 

 Range. 



No. 1 of these rocks has a characteristic lithoidal base, with well-devel- 

 oped but small sanidins scattered through it. No. 2 is a red, brecciated 

 rock with few individualized minerals. 



