ORIGIN OF THE ANDES 
22 . 
from the Pacific (Negritos and Lobitos formations of Peru, and 
unnamed formations in Colombia) on the flanks of the Andean 
land-mass. Similar marine transgressions of Early and Mid 
Miocene times are recorded in the Zorritos and Talara formations 
of Peru and Ecuador, the Navidad beds of Chile, the Patagonian 
beds of Argentina, and unnamed formations in Colombia and 
Venezuela. There is no paleontologic evidence for the remainder 
of the Miocene period but there is a considerable amount of 
physiographic evidence for predicating a considerable uplift dur¬ 
ing the late Miocene, and possibly also the early Pliocene times,- 
with great volcanic activity, the formation of lava-plateaus, and a 
subsequent mature erosion of the upland with valley filling. Everv 
competent observer who has visited the Andes has been impressed 
with the mature topography of the Andes in the cycle of erosion 
that preceded the most recent one and the only differences of 
opinion have been as to the relative dates at which these events 
took place. 
The Williams Expedition, which the Johns Hopkins University 
sent out in 1919, collected abundant data bearing on this very 
question, all of which are still unpublished, so that I can only 
give it in very brief form in the present paper. Extensive fossil 
floras of unquestionable Pliocene age were found at a number of 
localities in the Eastern Andes and on the altaplanicie of Bolivia, 
which show that both these regions were at that time much nearer 
sea-level than now, and that there were no elevations sufficient to 
reach the snow-line, nor to interfere with the tradewind circula¬ 
tion. The maximum elevation could not have been over 6,500 
feet and it may have been considerably less than that figure. A 
preliminary account of one of these floras (that from Potosi) I 
published a few years ago.^^ 
Following this stage of widespread lowland mesophytic floras 
over the now high and arid uplands of Bolivia, differential move¬ 
ments are recorded by slight folding, warping and faulting of these 
Pliocene deposits, followed by late Pliocene and probably pre- 
Glacial Pleistocene uplift in the Eastern and Western Ranges, with 
relative or positive sinking of the inter-Andine plateaus and some 
at least of the trans-Andine plateaus and valleys, and the extinc¬ 
tion of the so-called Pleistocene faunas found in the high Andes 
12 Proc. U. S. Natl. Mus., Vol. LIV, pp. 103-164, 1917. 
