woodworth: geological expedition to brazil and chile. 91 



which has been advocated by ^Ir. Marsden Marson finds Httle support 

 in the geological evidence derived from the occurrence of rocks at a 

 high temperature over large areas beneath the surface. The great 

 Triassic trap outflows of Brazil were erupted but shortly after the 

 Permian glaciation in that field. During the epoch of glaciation the 

 magma was still confined within the crust but had no recognizable 

 effect in controlling surface temperatures or in preventing glaciation 

 of the Permian land surface. 



VI. THE TRIASSIC TRAP PLATEAU. 



Overlying the Palaeozoic strata of the southern Brazilian highlands 

 and either intruded in or interstratified with a group of red beds of 

 presumed Triassic age is a series of trap sheets, mainly lava-flows, of* 

 vast extent, comparable in age and geological position with the basic 

 igneous rocks of the so-called Newark group of the eastern coast of 

 North America, and rivalling in their present surficial extension the 

 Cretaceous lava-fields of the Deccan of peninsular India if not also the 

 more recent lava-flows of the Columbia and Snake River basins of 

 western North America. From the northwestern part of the state 

 of Sao Paulo and the borders of ^Nlinas Geraes where the trap rests 

 upon the Pre-Devonian schists, these sheets of trap form high plateaus 

 broken through by the tributaries of the Rio Parana as far south as 

 central Rio Grande do Sul. From the sea-border near Porto Alegro 

 in the latter state the trap formation extends westward according to 

 the report of Dr. M. A. Lisboa as far as the Serra do Maracajii in 

 Matto Grosso at a point 460 kilometers east of the Rio Paraguay or 

 nearly 12 degrees of longitude west from Rio de Janeiro. The trap 

 formation has been recognized over a breadth of country five degrees 

 in longitude and some eight degrees in latitude between the parallels 

 of 20 and 28 degrees south. The area in which these rocks dominate 

 the surface is in round numbers about 100,000 square miles or ap- 

 proximately a region as great as that of the state of Nevada in North 

 America. 



In describing the geological features of the trap plateau in the 

 Rio Pelotas basin, I shall use for the igneous rocks the non-committal 

 term traj) since it avoids the inexactness which might arise from the 

 general application of such terms as basalt, augite-porph^Ty, etc., to 

 which kinds of rocks certain parts of the trappean series have at one 

 time or another been referred. 



As is stated more explicitly in what follows I include in the area of 



