THE LAVA FIELD OF THE PARANA BASIN 73 



Many of the flows are vesicular, and some even cavernous. 

 Others are very dense, in which case there is some columnar struc- 

 ture. The amygdaloidal fillings are quartz (both common and 

 amethystine), agate, chalcedony, calcite, zeolites, native copper, 

 copper minerals, and green iron siHcate. There has nowhere 

 been found any pyroclastic material, or any indication of true 

 volcanoes, although some of the rocks are so greatly decomposed 

 that the original nature is not known. The great number of 

 dikes in the sedimentary area east of the basalt escarpment shows 

 how widespread were the fissures by which the lavas reached the 

 surface, and since the general thickness is maintained far beyond 

 regions where these fissures are visible, we may assume that fissure 

 eruptions were prevalent throughout the entire Parana Basin. The 

 fissuring, so far as known, was not accompanied by great faulting. 



In composition the basalts and diabases ("dolerites") range 

 from andesite and olivine-free augite-porphyrites to typical hmbur- 

 gites with abundant oHvine. 



Later sedimentation. — Eruptions began during the time of deposi- 

 tion of the upper part of the Sao Bento sandstone. At first there 

 appears to have been alternation of lava flows and beds of sandstone; 

 later on came the epoch of greatly predominant volcanism. The 

 lava flows were later deformed into the Parana geosyncHne as we 

 find that structure today. The deformation renewed erosive 

 activity on the bounding crystalKne areas to the north, northeast, 

 and east, and a supra-basalt formation of sands and conglomerates, 

 called in Sao Paulo the Bauru formation, was deposited on the rather 

 slightly eroded lava surfaces. Today a vast surface of the supra- 

 basalt formation constituting a depositional plain is found in western 

 Sao Paulo, southern Goyaz, and the western triangle of Minas 

 Geraes; remnants of smaller extent exist in southeastern Matto 

 Grosso, and on the great divide between the Jacuhy and Ibicuhy 

 rivers in north-central Rio Grande do Sul. The Bauru formation 

 of western Sao Paulo contains large dinosaurs of supposed Wealden 

 (late Jurassic or early Cretaceous) age. The surface of the supra- 

 basalt formation slopes very gently toward the Alto (upper) 

 Parana and Paranahyba rivers in the northeastern part of the 



