EARLY ACID BRECCIA. 219 



After erosion had reduced the breccia countrv to very near its present 

 configuration, the region to the southwest was flooded by an immense body 

 of rhyolitic lava. One edge of this rhyolite covered a portion of the 

 breccias in a thin sheet, remnants of which are found in the southwestern 

 part of the district. The}* occur in the most unlooked-for places, as, for 

 instance, at an elevation of 10,000 feet on a narrow spur near the summit 

 of Saddle Mountain and at various altitudes over its slopes; also near the 

 bottom of Cache Creek Valley, where they form a bench between the 

 levels of 7,500 and 8,000 feet. The rhyolite was erupted from a distinctly 

 different center and after the volcano of Crandall Basin had long become 

 extinct, so that it need not be considered in connection with the history of 

 this volcano. 



After the rhyolite had been in turn eroded and the valley of Lamar 

 River had been cut 500 feet below the surface on which the rhyolite had 

 flowed, basalt was again erupted and filled the bottom of the valley. The 

 source of this eruption has not been discovered. The period of volcanic 

 activity may be considered as not yet ended, though it is at present con- 

 fined to such manifestations as are found in geysers, hot springs, and fuma- 

 roles, some of which occur within the district under discussion. 



The general features of the region having been sketched, the geology of 

 the volcanic rocks within the area of the map may be taken up and described 

 in greater detail. 



EARLY ACID BRECCIA. 



The breccias and lavas covering the country are essentially basaltic, 

 but there are remnants of an older breccia which is acid. The term brec- 

 cia is used for subaerial accumulations of tuff and scoria and fragments 

 of rock derived from explosive action, and it will be seen how great a pro- 

 portion of all of the material lias been subjected to this kind of action. 

 The acid breccia is found underlying the basaltic lavas in several locali- 

 ties in the district. It is exposed about Republic Creek, where it is rudely 

 bedded, the bedding being produced by layers of different mixtures of tuff 

 and angular fragments. They pitch steeply to the south and pass under 

 basaltic breccia, which also is rudely bedded at the bottom, but becomes 

 well bedded higher up. The lower layers dip steeply to the south and 

 southwest, but the upper layers are nearly horizontal. The acid breccia 

 is light colored and variegated, and consists of hornblende-mica-andesite 



