TEE LAVA FIELD OF THE PARANA BASIN 75 



of the supposed Gondwana land and the formation of the South 

 Atlantic Basin. 



Physiography. — Upon the deformation of the lava into the 

 Parana geosyncline the drainage became consequent to the structure. 

 The geosyncline plunged to the southwest, and the master stream, 

 the Parana and its headwaters tributary, the Paranahyba, occupied 

 the trough of the geosyncKne. These two streams follow the 

 structural trough in the lava for more than 1,000 miles; they appear 

 to have come into existence near the beginning of the Cretaceous 

 period, and to have persisted in their courses. The tributaries 

 either took the shortest distance and line of maximum gradient 

 down the flanks of the geosyncline, or assumed courses in the 

 troughs of cross- or subsidiary synclines. A number of the greatest 

 tributaries of the Parana River still flow in the troughs of the 

 cross-syncHnes. The basalt is both so thick and so resistant to 

 erosion that there appears to have been little important subsequent 

 drainage adjustment within the area still covered by lava. 



The intercalation of quartzose sandstones with the lava flows 

 and the great preponderance of quartzose and other detritus derived 

 from the basement complex in the supra-basalt formation indicate 

 that the surfaces of the basement complex stood higher than that 

 of the lavas on the north, northeast, and east, in which directions 

 the detritus becomes increasingly coarser. This is in contrast to 

 present topographic conditions in which erosion escarpments of 

 the lava stand at higher altitudes than the less resistant sedi- 

 mentaries, and a part of the basement complex crystallines. 



At the point where the Parana River crossed the southwestern 

 margin of the lava, falls or rapids were originally formed. The lava 

 surface there constituted a base level for all the drainage of the 

 Parana Basin above that point. On this local base level of the 

 then slightly denuded lava surface were deposited the supra- 

 basalt sands and gravels. Their original surfaces, still in large part 

 intact, sloped very gently southward in southern Goyez and central- 

 eastern Matto Grosso, southwestward along the Paranahyba 

 drainage basin in southeastern Goyaz and the northern part of 

 the western triangle of Minas Geraes, and west-northwestward in 

 western Sao Paulo. Along the valley of the Alto (upper) Parana 



