0. A. Derby — Magnetite Ore Districts in .Brazil. 311 



Central New York gave a very characteristic residue of zircon 

 with yellow grains that look exceedingly like monazite, though 

 as I have not as yet been able to subject them to the necessary 

 optical and chemical tests, the identification is not complete. 

 A specimen of granite from Westerly. Rhode Island, kindly 

 furnished by Prof. J. F. Kemp, gives an extremely abundant 

 heavy yellow residue similar in appearance to the Brazilian 

 monazite and, like it, showing the absorption band of didy- 

 mium when examined under the microscope with the spectro- 

 scopic eye-piece.* An experiment with the latter apparatus 

 made for me by Prof. Simon Gage of Cornell University proves 

 that the characteristic absorption bands of didymium and of 

 erbium can be readily perceived on microscopic grains (pre- 

 sumably also in thin rock section, though unfortunately none 

 were at hand for testing) of monazite and xenotime, and thus 

 a very simple and rapid method of detecting minerals contain- 

 ing the cerium and yttrium group of elements is afforded. 



Art. XXXYII. — On the Magnetite Ore Districts of \Tacupi- 

 ranga and lpa?iema, Sao Paulo, Brazil ;f by Orville A. 

 Derby. 



Two ore districts that have lately been investigated by the 

 Sao Paulo Geological Survey afford instances of what appears 

 to be a hitherto unnoticed mode of occurrence and association 

 of magnetic iron ores. The districts in question, which agree 

 so closely that they serve to mutually explain each other, are 

 situated about 150 kilometers apart on opposite sides of the 

 Serra do Mar. That of Jacupiranga is in a region of compara- 

 tively low hills at the extremity of a high ridge of the Serra 

 do Mar system, rising from the low plains of the coast and of 

 the lower course of the Ribeira (or Iguape) river. The 

 Ipanema ore deposits are in the center of an isolated mountain 

 block, bounded by fault planes that rise some 300 meters 

 above a plain that is essentially horizontal with a mean eleva- 

 tion of about 700 m . The basement rocks in both districts are 



* An examination, not yet concluded, kindly made by Dr. G-. H. Williams, 

 seems to prove that this cannot be monazite. Whatever it may prove to be, it 

 illustrates the value of the batea in petrographical study, as by its use an abund- 

 ance of a rare and interesting: accessory was obtained with about ten minutes' 

 work. 



f This paper is an English abstract of two reports prepared in the Portuguese 

 language for publication in the Boletina da Commissao Geographica e Geologica 

 de Sao Paulo, by the present writer, with, the cooperation, in the case of the 

 Ipanema district of Dr. Luiz Gonzaga de Campos. The petrographical work has 

 been carefully revised by Dr. Eugen Hussak while Mr. Henry K. Bauer, an able 

 amateur geologist and volunteer assistant, has iurnished much valuable material 

 and information regarding the Jacupiranga district. 



