BASEMAN, GEO GRAPHICAL D 1ST RIB UTION IN SO UTH AMERICA 3 5 



condition is replaced in the lower course of the river by a series of rapids, 

 after which it enters the Mamore. 



The Guapore has then two great regions where sediments of different 

 nature are deposited at different altitudes and two great regions where 

 the water runs rapidly and carries away the eroded product. 



It is the highland semicircular valley which, I believe, is roughly 

 comparable to the Serra de Erere. This region is flooded yearly during 

 the rainy season, at which time much sand is deposited. This deposi- 

 tion of sand and other material (laterite, etc.) will eventually make a 

 secondary deposit of considerable extent, while the regression of the 

 headwaters of Eio Guapore will eventually carry away all of the original 

 Piano Alto remaining in the surrounding Serras de Parecis, Eicardo 

 Franco and Agoaphey. 



In fact, the Guapore has already carried away more than one half of 

 the original highland formation from this region. The Eio Paraguay 

 is naturally assisting in this destruction of the "hrighest point." As soon 

 as these two large rivers have obliterated the above mentioned sierras, the- 

 semicircular highland valley will become one of the highest points (300 

 to 600 meters). After years of erosion on such a new high point, iso- 

 lated sections or mesas will be formed and secondary sierras like that of 

 Erere will be produced which never were associated with colossal lakes. 



I have observed near Corumba, Brazil, a similar deposition of leaves, 

 snails, etc., at two distinct levels. One of these levels is around the base 

 of the Serra de Urucum, including even deposits of limestone, and the 

 other is in the near-by pantanals (swamps) of Eio Paraguay. The same 

 process is going on near Sao Luis de Caceres, Brazil, and San Matias, 

 Bolivia. 



Hence there is no evidence for a huge Amazonian lake and no neces- 

 sity for assuming one, for, as the writer conceives the reversal of Eio 

 Amazonas, it was a gradual process. The Tertiary rise of the Andes 

 did not suddenly close the exit of the East Andean Sea. but the water 

 cut deeply at the exit, until by the time this exit was almost closed, 

 stream piracy in the region of the old. divide (near Obidos) had pre- 

 pared a new exit for the water of the shallow East Andean Sea. For 

 some time, then, there were two exits for the East Andean Sea. As the 

 Andes rose higher and higher, the southwestern exit became closed and 

 all of the water rushed eastward. The amount of this water flowing 

 eastward was sufficient to wash away the old divide in a comparatively 

 short time. 



Even the Eio Guapore has washed away the most of the Permian for- 

 mation of the Piano Alto from a strip 50 to 150 miles wide. Is it not 



