PETROGRAPHY— EHYOLITES AND TRACHYTES. 527 



The various rhyolites and trachytes fall into three groups, according 

 to their similarity to each other, as follows : 



EHYOLITES. 



[ 127] — IN" ear Deer Mountains. 

 [131]— Southeast of Terry Peak. 

 [134]— Top of Custer Peak. 

 [135]— Top of Custer Peak. 

 [137]— Top of Custer Peak. 

 [152]— Inyan Kara? 

 [147] — North Peak, Inyan Kara. 

 [166]— North Peak, Inyan Kara. 

 [141-142]— Crow Peak. 



SANIDLN-TRACHYTES 



(normal). 



1491— Center Peak, Inyan Kara. 



151] — North Peak, Inyan Kara. 



157] — Hill, west of Inyan Kara. 



178J — Top of Warren Peaks. 



144] — Between forks of Bear Butte Creek. 



SAXIDIN-TRACHYTES 



(with hornblende, microlites, &c). 



[123]— Hill, southeast of Terry Peak. 



[159-162]— Bear Lodge. 



[124]— Southeast of Terry Peak. 



[140]— Black Butte. 



[150] — Northeast of Inyan Kara. 



[171] — Warren Peaks. 



[173] — Southwest of Warren Peaks. 



[175] — Southwest of Warren Peaks. 



In some cases felsite-porphyry resembles trachytic rocks, especially 

 rhyolite, so closely as to make the distinction between them extremely 

 difficult, and the last group, which differs considerably from the normal 

 sanidin-trachytes, might have been named felsite-porphyry were it not for 

 their low silica percentage and occurrence with the trachytes. 



In conclusion, the writer desires to express his obligations to Mr. 

 Arnold Hague, of the Survey of the Fortieth Parallel, Dr. Egleston, Mr. 

 Julien and Dr. Waller, of the School of Mines, New York, and other 

 friends, who have given him valuable suggestions and assistance in the 

 examination of these interesting rocks. 



