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WOODWORTH: GEOLOGICAL EXPEDITION TO BRAZIL AND CHILE. 91 
which has been advocated by Mr. Marsden Marson finds little 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 Minas 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 trap since it avoids the inexactness which might arise from the 
general application of such terms as basalt, augite-porphyry, etce., 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 
