CHAPTER II. 



YOLCANIC AEEA. 



By far the greater area of the district surveyed during the summer 

 of 1874 is covered by volcanic rocks. jSTeaily 1,800 square miles of 

 volcanic^ joift on to the region of 1,400 square miles which were reported 

 upon the year previous (Eeport United States Geological and Geo- 

 graphical Survey, 1873). It is evident that, where so large a mass of 

 volcanic material was ejected and spread over the country, that innu- 

 merable varieties will be found, and considerable difficulty will be 

 encountered in the attempt to reduce all the observed occurrences to 

 features already well known. 



Orographically this area may properly be divided into three definite 

 systems — into the — ' 



Plateau country, 



jB/2<^ country, and 



Mountain country. 



The first, the Plateau country, rises to considerable elevations, 

 averaging about 12,700 feet above sea-level for the summit. Geognosti- 

 cally the latter are mostly composed of basalt, although trachyte 

 plateaus are not wanting. Stations 3, 4, and 5 are located on plateaus 

 of this character, and may serve as types. Sloping off to the east, 

 veering around in that direction to the norih and south, they present 

 very steep sides on the west and toward the south: Owing to the high 

 elevation, their summits are barren, only the rocky debris covering 

 them. Analogous in aj^pearance, but varying from tbem in every other 

 respect, is the — 



BLUFF COUNTRY. 



As such, I mean to designate all those sloping ridges that have been, 

 formed by very extensive flows of the volcanic material, show a plateau- 

 like summit, but rarely reach the elevation of those before described, 

 nor the horizontal extent. In contradistinction to them they are inva- 

 riably formed by trachytic flows. Deep and precipitous canons cut 

 through them, partly the result of separation by strain, partly by 

 erosion. To this class all the lower regions of our district belong. 

 Frequently they are densely wooded, or show grassy flats. As a type, 

 the regions on either side of the Eio Grande, between Lost Trail Creek 

 and Anteloi^e Park, might serve, varying in elevation from 10,000 to 

 12,000 feet. 



THE MOUNTAIN COUNTRY 



comprises all the western and northwestern portion of the district, 

 where the highest elevations were found, upward of 14,300 feet, and 

 which present the most rugged aspect. It is here that the volcanic 

 rocks reach their highest development, tbeir greatest thickness. Eegu- 

 larly stratified, the mountains are separated by narrow but deep canons, 

 containing swift mountain- streams. This section, too, will comprise all 

 13 H 193 



