112 I'KOCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



floristically from the western or leeward slopes and the plateau lands 

 behind them. The first has almost daily rains and is forested to the 

 timber line. The second are so dry that they permit the growth of 

 only drought-resisting grasses and low scrub, and over very large 

 areas there is no vegetation whatever. Consequently it might seem 

 that a moderate reduction in elevation would have permitted some 

 of the moisture-laden winds to pass and made possible the Potosi 

 and Corocoro floras. When, however, it was found that the former 

 was associated with a marine form, also of very modern aspect, it 

 was realized that the change of elevation involved had actually 

 amounted to about 2^ miles. 



The admirable physiographic studies of Bowman in the Peruvian 

 Andes,^ as well as less detailed studies farther south, furnish distinct 

 evidence of glaciation thought to be late Pleistocene because of the 

 freshness of the deposits and the related topographic forms. More- 

 over his evidence of the profound erosion as indicated by the mature 

 topography below the present rough summit topography leads him 

 to regard the Andes as having undergone progressive elevation 

 throughout the Tertiary, and he concludes that there has been a 

 change of elevation in the late Tertiary amounting to about 5,000 

 feet. 



It would seem then, that if the physiographic history of the region 

 is correct, in even its broader outlines, the fossil floras are pre-Pleisto- 

 cene in age. The Bahia Pliocene flora, previously mentioned com- 

 prises about 70 forms, none of which have been adequately described 

 and none at all have been figured so that comparisons with the 

 Potosi flora rest entirely upon names. Notwithstanding this diffi- 

 culty it may be noted that the following genera, all of which should 

 be determinable with reasonable certainty, are common to the Potosi 

 flora and the Pliocene flora of Bahia: Inga, Cassia, Copaifera, Dal- 

 hergia, Terminalia, and Weinmannia. All are, of course, typical 

 members of the tropical flora of the Amazon Basin and hence this 

 agreement may be without any special significance. At the same 

 time it is worth noting that if this resemblance is worth anything it 

 tends to confirm the evidence independently reached by a comparison 

 of the Potosi flora with the existing flora of tropical South America 

 east of the Andes. That comparison may now be briefly sketched. 



I have assembled in the accompanying table the fossil species in 

 one column, the most closely related existing species in another 

 column, and the range of the latter in a third column. Where the 

 resemblance of fossil to Uving species was not extremely close or 

 where I lacked material for adequate comparison I have named no 

 existing species so that the resemblances are underestimated rather 

 than adequately emphasized and the table is therefore much more 



1 Bowman, Isaiah, The Andes of Southern Peru. Amer. Geogr. Soc, 1916. 



