PETKtMJKAlMlV— IIUYOLITES AND T1{ACI1YTES. 527 



The various rliyolites and trachytes fall into three groups, according 

 to their similarity to each other, as follows : 



RHYOLITES. 



[127] — Near Deer Mountaius. 

 [131]— Southeast of Terry Peak. 

 [134]_Top of Custer Peak. 

 [135J— Top of Custer Peak. 

 [137]— Top of Custer Peak. 

 [152]_Tnyan Karaf 

 [147]— North Peak, Inyan Kara. 

 [166] — North Peak, Inyan Kara. 

 [141-142]— Crow Peak. 



SANLDIN-TRACHYTES 



(normal). 



149]— Center Peak, Inyan Kara. 



151] — North Peak, Inyan Kara. 



157] — Hill, west of Inyan Kara. 



178] — Top ot Warren Peaks. 



144] — Between forks of Bear Butte Creek. 



SANIDIN-TBACHYTES 



(with hornblende, mlcrolites, &g.). 



[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, jVIr. 

 Julien and Dr. Waller, of the School of ]\lines. New York, and other 

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

 examination of these interesting rocks. 



