STRATIGRAPHIC GEOLOGY 207 



place just as other sedimentary series do, and that the disagreements of 

 geologists are due to their efforts to generalize from their observations. 

 The scientific and economic importance of this series of rocks in Brazil 

 entitles it to the careful attention of geologists. 



Economic geology of the early Paleozoic. — Economically, the early 

 Paleozoic rocks of Brazil are of the greatest importance, for they include 

 not only the gold veins from which existing rock mines obtain their gold, 

 but they are the indirect sources of the gold obtained from the old placer 

 washings. This series also includes the great iron and manganese de- 

 posits of Brazil. The diamonds and other precious stones of Brazil are 

 also supposed by some geologists to have originated in these early Paleo- 

 zoic beds, though there is difference of opinion on this subject. Without 

 going into details, it is evident that this series of rocks is of the greatest 

 economic importance and of the highest scientific interest. 



SILURIAN 



The oldest known fossils found in Brazil are from the Silurian, in the 

 States of Amazonas and Para. (See Clarke and Katzer under those 

 states.) 



The Silurian rocks, where they are recognized as such, are marine sedi- 

 ments, mostly in the form of thin-bedded sandstones. They are exposed 

 where crossed by the Trombetas, Curua, and Maecuru rivers on the north 

 side of the Amazon Eiver, and dip gently southward. On the Trombetas 

 the zone is from six to eight kilometers wide. The fossils show them to 

 be equivalent to the Niagara beds of New York State. Although the 

 rocks that overlie the Silurian north of the Amazon appear again on the 

 south side of that stream, no rocks known, from paleontologic evidence, 

 to be Silurian appear immediately south of the Amazon.^ 



It is highly probable that there are rocks of Silurian age in many other 

 places in Brazil, notably in Bahia and Minas,''' but the absence of fossils 

 has thus far prevented their confident identification.^ 



DEVONIAN 



General ohservations. — The Devonian is found in the States of Ama- 

 zonas and Para in the north, in Sao Paulo and Parana in the south, and 

 in Matto G-rosso in the west. For the fossils found in the Brazilian De- 



^ F. Katzer : Geologie von iinteren Amazonas Gebietes, p. 216, 



^ See Branner : Am. Jour. Sci., vol. xxx, November, 1910, p. 343, and vol. xxxl, June, 

 1911, p. 481. 



s On the Rio Sao Francisco, at Bom Jesus da Lapa, State of Bahia, are limestones in 

 which fossil corals (Favosites and Chetetes) are said to be found. Derby believed them 

 to be Silurian, but both forms may well be Carboniferous. 



