woodworth: geological expedition to brazil and chile. 105 



the headwaters of the Rio Paranapanema drain almost entirely the 

 western slopes of the Devonian beds, as does the Ribeira de Iguape the 

 crystalline terrane on the east of the Devonian sandstones. 



The Permian tract is bounded on the west by the Triassic escarp- 

 ment of sandstones crowned by trap sheets. The westward dip of the 

 formation combined with the flow of the streams in that general 

 direction has caused the trap to retreat far to the west along the axis of 

 the main drainage lines, such as that of the Paranapanema and the 

 Iguassil. In the western part of the south Brazilian states of Sao 

 Paulo, Parana, and Santa Catharina the rivers flow over the trap 

 sheets whose resistance to erosion holds up to their local baselevel 

 the entire drainage area of the planalto. The Rio Parana on the 

 confines of Brazil and Uruguay is gnawing back the southern edge of 

 the trap sheets. Below the cascades and falls the river joins the 

 drowned valleys of the La Plata system. 



The rate at which the falls of the Parana and Iguassil are receding 

 has not I believe been determined, liut it is evident that the rivers 

 have cut back from the southern edge of the trap sheets since the land 

 had something like its present elevation above the sea. If the land 

 at the confluence of the Iguassil and the Parana had been as long above 

 sea-level as it has in the upper valley of the Iguassil, where the trap 

 has been swept away over a large tract of the Permian, it is incon- 

 ceivable that the youthful characteristic of falls and cascades should 

 still persist. We are therefore compelled to conclude that the country 

 immediately adjacent to the Parana and Paraguay rivers has recently 

 been uplifted in relation to the sea. As on the south of the trappean 

 country in Banda Oriental there are marine Tertiary beds now above 

 sea-level (Darwin, 1846, p. 1-3) and as along the coast of Brazil 

 from north of Rio de Janeiro to the Amazon (Derby, 1907, p. 218-237) 

 there are evidences of uplift since the Tertiary beds were there laid 

 down, it seems a valid hypothesis that the excavation of the Parana 

 channel in the traps began in later Tertiary time through an uplift 

 of the whole planalto of Brazil. 



The upper courses of small streams in eastern Sao Paulo and 

 northern Parana generally flow in narrow gorges so recently cut that 

 many side streams particularly of the wet-weather type enter by a fall 

 over the brink of the gorge. Without a thorough understanding of 

 the local baselevels of the Parana system it seems out of the question 

 to infer the cause of this revived stream-action. ^ 



A remarkable example of one of these streams is the Rio Itarare 

 flowing north into the Paranapanema along the boundary between 



