COROCORO COPPER DISTRICT OF BOLIVIA 41 



constitute 13 of the 20 known dicotyledonous species, and 

 they are also the most abundant individually, whole surfaces 

 of the bedding planes being frequently covered with the 

 leaflets of Calliandra. 



Nineteen of the species recorded from Corocoro are repre- 

 senjted in the Potosi tuffs and the two deposits are hence 

 regarded as the same age. Since the Potosi flora is much 

 more extensive than that found in the coarser matrix at 

 Corocoro the conclusive evidence of the Pliocene age of the 

 latter rests partially on the more complete analysis of the 

 Potosi flora, which was given as fully as the evidence in hand 

 permitted in 1917 and which has been much augmented in 

 the field studies and from a study of the collections made by 

 the authors around Potosi in 1919. This evidence will 

 appear in connection with the authors' report on the Potosi 

 district and need not be repeated in the present connection. 

 Taken by itself the Corocoro flora offers intrinsic evidence 

 of its late Tertiary age. Thus all of the plants, so far as 

 they can be accurately determined, are of still existing 

 genera which still survive at lower levels east of the Andes 

 where they are represented by closely allied species. Most 

 of them are arborescent forms that could not exist in the 

 present-day altitude and semi-aridity of Corocoro. There 

 are 5 fossil forms found at Corocoro whose modern repre- 

 sentatives sometimes reach high altitudes, namely, Poly- 

 stichum, Podocarpus, Osteomeles, Amicia, and Vaccinium. 

 These reach variable elevations, greatest perhaps in the case 

 of the Osteomeles, but they are overwhelmingly offset by 

 the presence of such warm low-altitude types as Terminalia, 

 Copaifera, Dalbergia, Machaerium, Enterolobium, Calliandra, 

 etc., and by the many additional types associated with these 

 forms in the Potosi tuffs. 



There is no doubt but that the Corocoro flora indicates a 

 much lower altitude and a greater rainfall at the time it 

 flourished, than obtains at the present time at Corocoro. 

 This also has a bearing on' the tectonic history of the 

 mountain masses that bound the high plateau on the east 



