106 PROCEEDINGS OF THE NATIONAL AIU8EUM. vol.54. 



and Corocoro. The latter, according to Prof. R. S. Lull, probably 

 represents an Upper Triassic amphibian. Unfortunately the strati- 

 graphic relations between the two outcrops is unknown, but there is 

 no reason to doubt Professor Lull's determination or the possible 

 presence of rocks of Triassic age in the vicinity. 



The fossil plants are, however, of more immediate interest, since 

 they occur in the horizon of the "vetas," and hence fix the age of 

 the immediate country rock and mineralization as late Tertiary. 

 While the fossil plants are neither abundant nor well preserved, suffi- 

 cient species can be identified to fix the horizon as very nearly the 

 same as that at Potosi, and hence determine the age of the copper- 

 bearing rocks rather definitely as late Tertiary. 



The plants which I have identified are the following: Polystichum 

 holivianum, Acacia uninervifolia, Mimosa arcuatifolia, Mimosites en- 

 gelhardti, Cassia ligustrinafortnis, Copaifera corocoriana, Terminalia 

 singewaldi. 



All but the last two are present at Potosi. The Copaifera and 

 Terminalia are both represented by fruits. The latter is very similar 

 to the not uncommon fruits of Terminalia antiqua from Potosi, and the 

 leaflets of a species of Copaifera that might represent the same species 

 as the fruits from Corocoro are present at Potosi. Thus the paral- 

 lelism between the two floras is extremely close. 



When a meager flora like that found at Corocoro is compared with 

 a more extensive flora like that at Potosi, the age might differ appre- 

 ciably, and yet the more common species of the larger flora might be 

 expected to be present in the smaller. When, however, it is not only 

 the commonest species of the larger that are found in the smaller, but 

 also forms like the Polystichum found in a single specimen in both, 

 additional indication of contemporaneity is afforded. Furthermore 

 all of the Corocoro plants are represented in the modern flora by 

 closely related species east of the mountains and all of the genera are 

 still found in the same general region where cUmatic conditions differ 

 from those prevailing at the present time on the high plateau of 

 Boh via. The latter region, because of its altitude and consequent 

 coldness, and the aridity due to the interposition of the lofty eastern 

 Andes in the path of the prevailingly eastern tradewinds, is prac- 

 tically treeless and in striking contrast with the conditions at the time 

 the fossil flora was living in this region, all of the seven recorded 

 species except the Polystichum being arborescent and some of them 

 usually large trees. If the evidence for the PHocene age of the Potosi 

 flora is regarded as conclusive then there can be no doubt but that the 

 Corocoro flora is also Pliocene, and this age is thereby estabhshed for 

 the copper-bearing rocks of this mining district. 



