THE LAVA FIELD OF THE PARANA BASIN, 

 SOUTH AMERICA 



CHARLES LAURENCE BAKER 

 Cordova, Illinois 



This paper is a summary of the most important results of the 

 writer's recent journeys of 8,500 miles, during seven months, in the 

 eastern River Plate region of South America. The late Dr. J. C. 

 Branner, in May, 192 1, expressed the opinion that the "trap" 

 field of southern Brazil and adjacent regions was possibly greater 

 in extent than any other. The writer's subsequent field studies 

 have demonstrated that the area covered by basaltic lavas is at 

 least nearly as great as the combined areas of the Deccan and 

 Columbia River basalt fields, heretofore supposed to be the two 

 largest in the world. 



Either basalt or diabase ("dolerite") is found almost everywhere 

 in the hydrographic basin of the Parana River, and extends beyond 

 into the southern part of the Amazons hydrographic basin 

 (Amazonia) and the eastern part of the Rio Paraguay drainage 

 basin. The drainage basin of the Uruguay River, with extensive 

 areas of basalt, probably was, up to very recent times, directly 

 tributary to the Parana. Structurally all this region is a single 

 vast downfold (geosyncHne), bounded on all sides by outcrops of 

 a pre-Cambrian or early Paleozoic basement complex of plutonic 

 and metamorphic rocks. In this complex the major structural 

 trends, so far as known, are northeast-southwest, parallel to the 

 southern Brazil and Uruguay coast line, and to the major axis of 

 the Parana downfold. Apparently this eastern region of present 

 hill ranges was one of lofty mountains in late pre-Cambrian or 

 early Paleozoic times. Today the highest mountains and ridges 

 are composed either of folded and faulted metamorphic quartzites 

 or of great bosshke masses of syenitic intrusives. 



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