146 STUDIES IN GEOLOGY, No. 4 



divide the broken Andean ranges fan out and cover an area 

 of about twenty-four square degrees, reaching eastward to 

 Santa Cruz de la Sierra, about 250 miles from the western 

 border of the eastern ranges. Although the mountains are 

 somewhat less impressive in this last region than habit leads 

 one to expect in the Andes, many of the peaks rise above 

 15,000 feet and several go two or three thousand feet 

 higher. The enclosing rampart of this mountain mass is 

 formed by the Sierra de Cochabamba on the northeast and 

 the Sierra de Misiones on the east. 



Cochabamba the Sierra, and Cochabamba the second city 

 of Bolivia, both derive their name from the Cochabamba 

 basin, the name being a partial Spanish corruption of the 

 Indian Cocha or lake and pampa or flat, an interesting eto- 

 mology for what appears to have been a late Tertiary lake 

 now drained by the Rio Grande. Systematic exploration 

 should eventually bring to light some traces of a Tertiary 

 mammalian fauna in this region, which must have been a 

 garden spot in late Tertiary times. 



The rim of the basin north of Cochabamba over which the 

 route to the plant locality passes is made up almost exclu- 

 sively of rocks of Silurian age, although inconsiderable dark 

 unfossiliferous shales observed northeast of the pass may be 

 Devonian. The divide itself is a bold mass of Bilobites sand- 

 stone, reached by painfully slow mule-back over a poor trail 

 that crosses from the plain over the immense detrital fans 

 that skirt the mountains, and switchbacks up the lateral 

 spurs, necessitating a five hours ride to cover the four 

 leagues from the town to the pass. 



Some distance below the pass on the south side a compact 

 yellowish sandstone was found to be sparingly fossiliferous, 

 containing Silurian forms of Orthis and other brachiopods. 

 This observation is of importance as giving a clue to the age 

 of the identical sandstones north of the pass and underlying 

 the Tertiary. The geographical relations and something of 

 the geology are shown on the accompanying sketch map 



