HASEMAN, GEOGRAPHICAL DISTRIB UTION IN SO UTH AMERICA 27 



by post-Paleozoic seas. Hence continental and not marine deposits are 

 the more extensive deposits of Brazil. 



The Permian inland basin is not uniformly symmetrical in reference 

 to the altitude of the basal rock in different sections, but its northern 

 and eastern sides are always higher than the southern and western. 

 There is also a considerable elevation of this basal formation between the 

 headwaters of Eio Paraguay (also seen in the iron and manganese de- 

 posits 600-800 meters above the level of Eio Paraguay in the Serra de 

 Urucum) and the Amazon, but this may be due to unequal early Paleo- 

 zoic erosion. This is especially true, if the streams flowed westward 

 before the Amazon was reversed. This difference in elevation of the pre- 

 Cambrian and early Paleozoic or basal formation of the Piano Alto was 

 of the utmost importance for the formation of the Piano Alto. The 

 overlaps near these various higher (?) Archean elevations indicate that 

 much of the basal sandstone of the Piano Alto was derived from place 

 •erosion of the higher points of the floor of the Piano Alto. Hence no 

 ■considerable extension to the east of present coast of South America was 

 necessary. The uppermost strata were deposited from the higher Archean 

 mountains on the east and northeast and hence dip toward the southwest. 



The outlines of the Piano Alto shown on Plate XII may be slightly 

 extended by future work from southeastern Brazil toward Argentina, far- 

 ther into Paraguay and Bolivia, and farther west from Brazil toward the 

 Cordillera Oriental of Colombia and Ecuador. So much of the Piano 

 Alto has been deeply eroded and some entirely washed away that its exact 

 limits cannot be given at the present time. 



It is evident from Plate XII that the Piano Alto proper is bounded by 

 parts of the basins of Eios Sao Francisco, Orinoco, Mamore and various 

 other smaller rivers as well as by waterfalls and other changes in the 

 geological structure. In this connection, it is interesting to note that all 

 of the rivers which rise in the Piano Alto have clear water at least as long 

 as they flow on its formations, while many of the bounding rivers which 

 ■flow from the surrounding mountains often have yellow muddy water. 

 Eio Bermejo of Argentina, Eio Sao Francisco of Bahia, Eio Mamore of 

 Bolivia, Eio Solomoes of Brazil, Eio Gurgueia of Piauhy and Eio Colo- 

 rado 13 of northern Patagonia are good examples of such rivers. 



The yellow mud carried by all of these rivers, excepting Eio Sao Fran- 

 cisco, is produced for the most part by the erosion of Mesozoic and Ter- 

 tiary marine deposits; and inasmuch as these formations are not known 



13 From the size of the lower valley of this river and the identity of its fishes with 

 those of Rio San Juan, I am convinced that Rio Colorado was formerly much larger and 

 must have had some headwater from southwest Bolivia. 



