154 STUDIES IN GEOLOGY, No. 4 



ation and topography. Thus east of Cochabamba in the 

 Sierras, leaf fall is noticeable in the winter season, whereas 

 north of Cochabamba in the true Yungas the valleys were 

 humid and moist and largely evergreen during the winter 

 that I visited them. Under the last mentioned conditions 

 the prominent elements in the vegetation with the possible 

 exception of the palms, reached notably higher altitudes than 

 was the case east of Cochabamba. 



There is probably no region where detailed studies of the 

 relations of life to the climatic and topographic environment 

 would prove more interesting and instructive than east of 

 the Cordillera Real in Bolivia. With such a body of facts 

 available for comparison it would be possible to attain a 

 much greater degree of precision in the interpretation of the 

 fossil floras of the Andes. 



There can be no question but that the closest approach to 

 the botanical character of the present fossil flora and to the 

 climatic environment under which it existed are to be found 

 at the present time in the Yungas north of Cochabamba at 

 altitudes under about 5,000 feet. The annual precipitation is 

 something that cannot be precisely evaluated since the ques- 

 tion of humidity is a modifying factor of such great import- 

 ance. The bulk of the fossil plants represent modern forms 

 which extend from the moist tropical lowlands into the humid 

 mountain valleys where rain may not fall from April to Sep- 

 tember. I should say that those found fossil at Pisllypampa 

 indicate humidity and a consequent growing season through- 

 out at least nine months of the year, and an annual precip- 

 itation of between 40 and 80 inches. 



AGE OF THE FLORA 



There is no direct method of determining the precise age 

 of the Pisllypampa flora. The deposits in their stratigraphic 

 position are po>st Silurian and preglacial. The fact that the 

 deposits are largely volcanic ash stamps them rather defin- 

 itely as having been formed at some time in the late Tertiarv. 



