THE LAVA FIELD OF THE PARANA BASIN 69 



the Karroo of South Africa. Shales and sandstones, incHned, len- 

 ticular, and cross-bedded, and a few conglomerates make up the 

 formation. Overlying is the Sao Bento heavy and cross-bedded 

 sandstones with a few conglomerate beds. Volcanism began after 

 the deposition of an average of 650 feet of the Sao Bento sandstone. 

 We have so far outHned conditions in the eastern part of the 

 Parana Basin. On the north and west in eastern Paraguay, cen- 

 tral and southern Matto Grosso, southern Goyaz, and the western 

 triangle of Minas Geraes the deposits of the first cycle are not yet 

 known with certainty, but the red beds there lithologically resemble 

 the Rio do Rastro and Sao Bento of the eastern part of the basin. 



THE PARANA BASALTS AND DIABASES 



Extent of basalt area. — The sketch map (Fig. i) which accom- 

 panies this paper represents an attempt to outHne the present 

 minimum extent of the basalt flows in the Parana Basin as deter- 

 mined by former geologic explorers in Brazil, by Karl Walther in 

 Uruguay, and the writer in the western triangle of Minas Geraes, 

 southern Goyaz, central and southwesternmost Matto Grosso, west- 

 ern Parana, eastern Paraguay, northern Argentina, and northwest- 

 ernmost Uruguay. The boundaries are fairly well determined by 

 prominent erosional escarpments on all sides except in eastern 

 Paraguay and northern Argentine, where the mesopotamian alluvial 

 deposits of the River Plate lowlands, and the Pampean loess, 

 obscure or entirely conceal the bed rock. There is quite possibly 

 a larger area of basalt in Paraguay and Argentina than is represented 

 on the map. Any other possibly important errors, more particularly 

 on the north and northwest boundaries, can safely be attributed to 

 the imperfections in all existing maps. There is at least one 

 considerable outlying area of basalt in southern Rio Grande do 

 Sul not included in the forthcoming figures and not represented on 

 the map. A considerable dome-shaped area around Lages in south- 

 eastern Santa Catharina, now stripped of lava cover, is included in 

 the computations. 



The basalt flows, as tentatively mapped, now cover an area of 

 over 300,000 square miles (800,000 square kilometers). Assuming 

 the average thickness of the basalt flows as 1,000 feet, their total 



