158 STUDIES IN GEOLOGY, No. 4 



lowland warm humid regions. Since all of the other mem- 

 bers of this flora belong to the last type of plant association, 

 it is to be presumed that the same is true of the cassias. 



There are then no montane types represented, although 

 most of the forms are what a modern botanist would call 

 sub-andean meaning thereby the belt of country with warm 

 temperatures and a heavy rainfall that is characteristic of 

 the eastern slopes of the Cordillera Oriental from central 

 Bolivia northward, and which goes by the name of the mon- 

 tana, of which geographic belt the Bolivian Yungas are a 

 special province. 



All of the Pisllypampa species are types that it would be 

 perfectly proper to designate as tropical. There is, however, 

 nothing to prevent such an assemblage of tropical types from 

 being perfectly at home in the sub-tropical altitudinal zone. 

 The determination of the probable upper limit of the latter 

 zone and the difference between it and 1 1,800 feet will give the 

 minimum amount of change of level that has occurred at 

 Pisllypampa since the fossil flora was buried by the volcanic 

 ash. 



Commencing with Humboldt's classic zones of vegetation 

 I think that botanists have rather generally underestimated 

 the ability of most plants to live at high altitudes where tem- 

 peratures were not absolutely prohibitive and where mois- 

 ture or wind become the important limiting factors. 8 

 Certainly in the Bolivian Yungas such types as Heliconia, 

 Melastomatacese, Piperaceae, etc., extend much higher than 

 I expected to find them. The only conspicuous forms that 

 seemed to me to fail to surge upward were the palms, and 

 I would be inclined to attach considerable weight to the 

 presence of the latter at Pisllypampa as indicative of an al- 



8 1 have found the discussion of zones in Chapman's "The Distri- 

 bution of Bird-Life in Colombia" the most satisfactory and instruc- 

 tive of any contributions, either general or special, that I have con- 

 sulted. Bull. Am. Mus. Nat. Hist, vol. 36, 1917. 



