^- 



2(U J. C. ERAXXEE OUTLIXES OF THE GEOLOGY OF BRAZIL 



Dr. John W. Evans^ are not supported, so far as Brazil is concerned, by 

 onr knowledge of tlie local details. 



Over nincli of the Archean area are scattered infanlted remnants of 

 old Paleozoic rocks that yield a peculiar and characteristic topography. 

 The infanlted masses are of all sizes, from areas of thousands of square 

 kilometers down to only a few square meters. These rocks, however, 

 belong with the next newer series, where they are spoken of more at 

 length. 



Economic geology of the Archean. — Our imperfect knowledge of the 

 stratigraphic limits of the Archean rocks of Brazil makes it difficult to 

 know precisely what rocks and minerals are to be credited to that division 

 of the geologic column. Furthermore, minerals that probably originated 

 in newer rocks are now often found in the Archean areas, where they 

 have been left by the denudation of later series to which they properly 

 belong. On the other hand, the Archean rocks are the original sources 

 of certain minerals that are now found in later deposits in which they 

 have been concentrated, as, for example, the monazite sands of the coasts. 

 It can confidently be stated that the Archean ' rocks of Brazil contain 

 many minerals which have not yet been develoi^ed economically in that 

 country. The entire list includes gold, copper, platinum, tungsten, mica, 

 marble, talc, apatite, graphite, potash-bearing rocks, precious stones, and 

 excellent building stones. This list, however, is not complete. 



EARLY PALEOZOIC 



General ohservations. — The separation of the rocks of the Brazilian 

 complex from the next newer series can not be made with confidence at 

 present, but for purposes of discussion the top of the Archean is placed 

 at the base of the quartzit^ called ^"itacohmiite.*' in Minas. 



The groups of rocks here called early Paleozoic can not be clearly de- 

 fined for lack of stratigraphic data. Xo fossils have been found in any 

 of them. Many of them have been profoundly metamorphosed : others 

 have been much less affected : over vast areas they have been almost com- 

 pletely removed by denuding agencies, but over these same areas remnants 

 of the old Paleozoic sediments stand out in bold relief as isolated knobs, 

 peaks, and even as bold and rugged mountain ranges. The rocks vary 

 from soft clay shales to the hardest of quartzites, the quartzites invariably 

 forming the ridges, while in the parallel shale beds intervening valleys 

 have been cut. Within limited areas the structural features are some- 

 times remarkably uniform, though not perfectly so. The structure shows 

 that these early Paleozoic sediments have been extensively dislocated by 

 thrust-faults and one end of the beds thrust down into the underlvins: 



Quart. Jour, Geol. Soc. vol. Ixii. p. 90. London, 1906. 



