woodworth: geological expedition to brazil and chile. 107 



stage of the river presumably was developed on the surface of the 

 Tertiary beds. 



The divide between the Tiete at IMog}' das Cruzes and the great 

 bend is occupied by rock-hills of low relief rising about 200 feet above 

 the weakly developed drainage lines of the district. The natural 

 course of the Parahytinga would appear to be westward into conflu- 

 ence with the Rio Tiete of which it may be regarded as a beheaded 

 portion, captured by the Rio Parahyba, which, pushing its head 

 southwestwards along the easily eroded Tertiary beds, diverted the 

 stream before erosion had swept away the Tertiary beds between the 

 Parahyba basin and that of the Tertiary beds at Sao Paulo. 



The Pleistocene and Recent Formations. — The discrimination of the 

 Post-Tertiary changes in extra-glacial regions into Pleistocene and 

 Recent is attended with difficulties. In Brazil the surface deposits 

 are prevailingly residual clays or clays, sands, and pebble beds derived 

 from the secular washing and transportation of weathered Pre- 

 Pleistocene formations. Great differences exist as to the depth of the 

 decayed rock even on the same formation. Where the rainfall is 

 heavy at certain seasons of the j'ear, the slope of the ground steep, 

 and the run-off effective, the decayed materials are removed nearly as 

 fast as their disintegration or decomposition is accomplished and thus 

 nearly fresh rock occurs at the surface. I was frequently surprised 

 in the valleys of the Rio Negro and the Rio Tuberao by the apparent 

 freshness of carbonaceous shales at a depth of a few centimeters be- 

 low the surface but in these situations erosion has been and still is 

 actively in progress. 



The new cuts of the railway in construction from Bury in Sao 

 Paulo via Sao Pedro de Itarare and Jaguariahyva to Ponta Grossa 

 in Parana gave at the time of my visit an unusual opportiniity to see 

 many excavations in the mantle rock. Along the banks of the Rio 

 Jaguaricatu in the Permian tillite beds these cuttings were often from 

 5 to 10 meters deep. At these depths most of the pebbles were still 

 undecomposed. 



At numerous localities along the railway line across the mature 

 topography of southern Sao Paulo and Paranji the rounded swells 

 between streams display traces of ancient gravel beds usually with 

 concave lower limits as if occupying old stream channels long since 

 abandoned. The same phenomenon is observable in a pronounced 

 manner where the railway from Ponta Grossa to Serrinha Station in 

 Parana skirts the lower westward slopes of the sandstones of the 

 Devonian cuesta. In all these cases the history of the surface appears 



