28 ANNALS NEW YORE ACADEMY OF SCIENCES 



to exist in the Piano Alto, the yellow rivers assist in a most interesting 

 way to separate the Piano Alto from the rest of South America. 



The original surface of the Piano Alto, as well as its uppermost strata, 

 dipped gently as a whole toward the southwest. This broad conclusion is 

 based on the following facts, after due allowance is made for ages of 

 erosion, reversal of rivers and the Tertiary rise of the Andes. 14 



1. All of the streams and rivers which rise on the Piano Alto, i. e., on 

 the Permian sandstone, at first flow south, southwest or west, even though 

 they afterwards flow north and east, i. e., after they are eroded deeply 

 into the lower strata and flow over the older Paleozoic and Archean rocks. 

 For example, Eio Guapore flows at first about 200 miles south and then 

 makes an elbow bend and flows west, north and lastly northwest into Eio 

 Mamore. The streams of Jalapao in northern Goyaz, the headwaters of 

 Eios Parana and Paraguay and the streams of the Guiana highlands all 

 exhibit these same conditions in their headwaters. Even if stream piracy 

 is said to be responsible for these conditions, the general dip would still 

 be toward the southwest, because piracy could only be produced by the 

 more rapidly flowing northern and eastern streams robbing the head- 

 waters of the streams which flow toward the southwest, in order to explain 

 the existing conditions. The cases of stream piracy considered in the 

 following pages show that it has been produced in exactly this way. 



2. The second fact which supports the southwest dip of the Piano 

 Alto is that the north and east faces of isolated mesas or portions of the 

 original (not secondary) highlands are almost perpendicular, while the 

 south and west sides usually have gentle slopes. This is beautifully shown 

 by the Urucum Mountains near Corumba and by the many isolated mesas 

 in the Jalapao region of northern Goyaz. Also the west face of the Serra 

 de Parecis east of Villa de Matto Grosso is not nearly so perpendicular as 

 the east face of the Serra de Eicardo Franco, west of Villa de Matto 

 Grosso. The high Kaieteur Falls of an east-flowing river of British 

 Guiana and the Eio Branco flowing west from the same region having 

 only rapids support the same conclusion, i. e., the surface of the original 

 Piano Alto dipped as a whole toward the southwest. 



In view of all this, there appears to be no doubt that the Piano Alto 

 was previously much larger than generally considered. It has been very 

 stable since the Permian epoch. These facts may indicate a vast center of 

 evolution of plants and animals, but I hardly think so, for even at the 

 present time few plants and animals are able to thrive on this sandy ele- 

 vated region. In Permian and Mesozoic times, perhaps, this region was 



14 The dip is so gentle that it is difficult to detect in the strata at exposed surfaces. 



