312 O. A. Derhy — Magnetite Ore Districts in Brazil. 



the same and identical with the characteristic formation of the 

 intervening position of the Serra do Mar, which in this region 

 is made up principally of a series of more or less completely 

 metamorphosed schists cut by numerous and extensive erup- 

 tions of granite. These schists are mainly clay-slates, but 

 more highly metamorphosed argillaceous strata appear and 

 more rarely quartzites. One or more beds of limestone, gen- 

 erally black and amorphous but frequently altered to marble, 

 form a very characteristic and presistent member of the series. 

 As no fossils have yet appeared the age of this series cannot 

 be made out, but it is certainly pre-Devonian and presumably 

 Cambrian. The plain surrounding the Ipanema mass, as well 

 as a portion of the block itself, is of late Carboniferous or Per- 

 mian age. The eruptive rocks described below while in the 

 main confined to the Cambrian (?) area have been found at a 

 few points cutting the Carboniferous strata. At Jacupiranga 

 on the contrary nothing with the exception of eruptive rocks 

 has been recognized between the ancient formation and the 

 modern alluvial deposits, so that there is no criterion for deter- 

 mining the age of the eruptions. 



For purposes of study the Jacupiranga district far excels 

 that of Ipanema not only in the variety of its rocks, but also 

 in their state of preservation.* The one district proper in- 

 cludes an area of 30-40 square kilometers of nearly rectangu- 

 lar form lying to the west of the small river Jacupiranga, and 

 extending well over towards the great parallel stream, the 

 Bibeira, which below its great bend, receives the Jacupiranga. 

 The divide between the two streams consists of gneiss and 

 granite flanked on each side by Cambrian (?) schists cut by a 

 great variety of eruptive rocks. These last include various 

 types of orthoclase-pyroxene, orthoclase-nepheline, and plagio- 

 clase-nepheline rocks, nephelinites, teschinites, vosgesites, basalts,, 

 etc. In the study of the field relations of these various groups, 

 made in great part by Mr. Bauer, in part by the writer with 

 the assistance of Drs. Campos and Hussak, an intimate connec- 

 tion between the most of them and with the iron ore, has been 

 directly proven in many cases and is strongly suspected in others. 



The ore district proper is for the most part heavily wooded 

 and covered with a rich dark soil containing everywhere a 

 greater or less amount of magnetite in grains or blocks rang- 

 ing in size from the finest sand up to masses of the size of a 

 man's head. The predominant rock, as seen in the deeper 

 parts of the cuttings, is a dark- brown or black schist in inclined 

 layers with an abundance of mica flakes along the division 



* Unusual facilities for examination were afforded by the operations (since sus- 

 pended), for the erection of iron works. These included several large excavations 

 lor foundations and the grading of six kilometers of tramway affording a nearly 

 continuous section across a large portion of the district. 



