TEE LAVA FIELD OF THE PARANA BASIN 77 



Niagara, but they have receded only about one-seventh as fast as 

 those of La Guayra. 



Sandstones intercalated in the basalt are less resistant, the 

 vesicular flows decompose and erode more readily than the denser 

 lavas, and local columnar structure aids erosion. The result has 

 been the formation of step and terrace profiles along the valleys 

 both longitudinally and transversely, and of rapids and waterfalls 

 numbered by the thousands, with intermediate stretches of sluggish 

 and ponded waters throughout the entire drainage area of the 

 Parana Basin. Rapids, waterfalls, and local base levels are also 

 abundant where dikes and sills are found in the sedimentary 

 rocks. 



The present topography within the lava area has, in the 

 principal interstream divides, flat meseti-form plateaus capped by 

 lava or by the supra-basalt formation. These plateaus have great 

 extension in all the Brazihan portion of the Parana Basin, except in 

 Rio Grande do Sul, where erosion has reached for the most part 

 the stage of maturity. The streams have dissected deep canyons 

 and gorges and have falls and rapids. The topography is, therefore, 

 still in the youthful stage of development but one can venture to 

 assert that it is one of the oldest known youthful topographies, 

 considering the absolute length of time it has persisted. On the 

 whole the lava field is a plateau with surfaces gently sloping toward 

 the trough of the geosynchne, partially dissected by youthful 

 canyons and gorges, and bounded for the most part by high reces- 

 sional erosion escarpments. Those portions of the Parana Basin 

 l5dng without the lava area have mature topography, very rugged 

 aside from some local base-leveled areas. 



The residual soils — ^products of decomposition of the basalt — are 

 confined in large part to the flat upland interstream surfaces, 

 except in Rio Grande do Sul. The sides and bottoms of the 

 canyons and gorges are largely bare rock surfaces. The rain may 

 pour down for a week, yet the swollen streams with their average 

 high gradients run fairly clear at the end of the deluge. The main 

 reasons for this appears to be the dense cover of arboreal and smaller 

 vegetation which inhibits to a very great extent sheet wash, and the 



