WARREN PEAKS. 199 



the red arenaceous clay is too nearly structureless to retain readily such 

 evidence. 



Warren Peaks are the crowning points of the "Bear Lodge range," 

 an elevated, broken plateau between the Redwater Valley and the Belle 

 Fourche. The peaks are not remarkably prominent, but their total ele- 

 vation above the sea, 6,830 feet, is equal that of some of the principal 

 peaks in the Hills, while their height above the Red Valley immedi- 

 ately south is about 1,800 feet. The trachyte area to which they belong 

 is the largest of the whole group, and covers fifteen or twenty square miles. 

 Around this the strata of the sedimentary series are uplifted and arranged 

 in concentric circling outcrops, so as to make a sort of miniature copy 

 of the Hills. The trachytic nucleus has an extension from northeast to 

 southwest of about eight miles, and a width of two or three miles. Its 

 surface consists of high, rounded, grass-covered hills, with little or no 

 timber, and from this rise the two or three more elevated points to which 

 the name Warren Peaks has been applied. These more central points 

 are surrounded by smaller and less prominent peaks, which are sepa- 

 rated by deep ravines or gulches forming the lines of drainage. Besides 

 this great nucleal mass of igneous rock, several outbursts, very local in 

 character, were observed in the zone of encircling strata. The rock is a 

 trachyte, dark gray in color, and containing frequently large and perfect 

 crystals of sanidin which give it a porphyritic character. Small crystals 

 of mica and hornblende are also prominent, and the rock yields more to 

 weathering than that of some of the other peaks. In different portions of 

 the district the rock varies somewhat in its character, though evidently of 

 the same general nature. As a rule it is more highly crystallized than the 

 trachytes farther south, the large, well formed feldspar crystals being more 

 common and also the small crystals of mica and hornblende. 



A short distance northeast of the central peaks an irregular, ill-defined 

 ledge was found, traversing the trachyte for some distance in a northwest 

 direction. It is varied in its composition, but consists mainly of a coarsely 

 crystallized feldspathic rock, with the crystals imbedded in a fine-grained 

 feldspathic matrix, the whole mass having a yellowish or reddish brown 

 color. Near the middle the mass is irregularly intermixed with compact 



