78 CHARLES LAURENCE BAKER 



low gradients of the instream flats. Between the falls and rapids 

 there are long stretches of slack water where there is deposition of 

 detritus. 



The rolHng uplands of Rio Grande do Sul are grass-covered. 

 The Rio Jacuhy which drains the southeastern part of the basalt 

 area of that state has the advantages of relatively short course 

 and steep gradients to the Atlantic. The local base level of the 

 middle Uruguay, formed by the basalt falls near Salto, Uruguay, 

 and Concordia, Argentina, Hes at a relatively low level. These 

 factors, combined possibly with others, have operated to bring 

 about a more advanced stage of erosion over a large portion of the 

 lava region of Rio Grande do Sul. 



The five major tributaries of the Parana in the states of Sao 

 Paulo and Parana — namely, the Rio Grande, the Tiete, the Parana- 

 panema and its largest tributary, the Tibagy, the Ivahy, and the 

 Iguassu — have their sources east of the present lava area (all of 

 them except the Ivahy in the crystalHne area of the Serra do Mar, 

 very close to the Atlantic Coast) — flow downstream across the 

 sedimentary area, and cut through the eastern scarp of the basalt. 

 It is probable that these rivers have largely persisted in initial 

 courses determined by the westerly slope of the deformed land 

 surface. The only one of the five which shows marked subsequent 

 adjustment is the Ivahy which for a portion of its course flows at 

 the eastern foot of the lava escarpment, shifting its channel down 

 dip in accordance with Gilbert's Law. It is probable that the 

 original surface had greater gradient near its eastern margin than in 

 the vicinity of the geosyncKnal axis. This made possible more rapid 

 erosion in the headwater courses of these streams, and perhaps the 

 lava cap was thinner there than farther west. These streams, as 

 well as some original tributaries of the Parana in Goyaz, Matto 

 Grosso, and Minas Geraes later beheaded by tributaries of the 

 Amazons and Sao Francisco, originally drained areas of the base- 

 ment complex which were the sources of the supra-basalt formation. 

 The basalt Hes directly upon the basement complex in the head- 

 waters regions of the Rio Grande and Paranahyba. 



The Parana Basin has been land since the Trias and with the 

 possible exception of the Guianan Highlands and the eastern Brazil 



