Washington — Pyroxenite and Rornblendite in Brazil. 79 



Art. YII. — An Occurrence of Pyroxenite and Horiiblendite 

 in Bahia. Brazil; by Henry S. Washington. 



Introduction. 



In June, 1909, 1 had occasion to visit and examine a reported 

 copper mine near Maracas, in the State of Bahia, Brazil. 

 Although the so-called mine proved to be valueless as a 

 commercial source of copper, jet the igneous rocks encountered 

 were so interesting that they deserve description. 



The locality is about 300 km. west of the city of Bahia, two 

 days ride (about 100 km.) south of Tambury, a station on the 

 Paraguassu River, on the San Felix-Bandeira da Mello railroad, 

 215 km. from the former. The locality is in the complex of 

 plutonic rocks and crystalline schists, which extends in a north 

 and south zone through the state, between the Cretaceous 

 to the east and the (probably) Jurassic Salitre limestone and 

 Carboniferous Lavras sandstones to the west. The general 

 geology has been briefly sketched by Branner,* who considers 

 the igneous and metamorphic complex to be pre-Cambrian. 



The pyroxenite-hornblendite mass is found about 5 leagues 

 (20 miles) north-northwest of the small town of Maracas, 

 and forms a hill called San Gonsalvo Velho about one league 

 northwest of a cluster of houses known as Furnaca. 



Surrounding it are gneisses, none of the specimens of which 

 are now, unfortunately, available. They are, however, white 

 or pinkish, largely composed of feldspar and quartz, with little 

 mica, a decided schistose structure, and with apparently little 

 crumpling, the foliation being generally more or less horizontal. f 



It is assumed that the rock mass in question is an igneous 

 inclusion and not part of the metamorphic complex, as the 

 region shows many such instances of inclusions of various 

 plutonic bodies in the gneisses and other metamorphic rocks, 

 The microscopical examination also favors this view. There 

 is an apparently similar mass of pyroxenite near Sitio Novo. 

 This I could not visit, but I was shown specimens of the rock 

 which seems to be a coarse-grained hypers thenite, and samples 

 of a very long-flbered asbestos which occurs there. 



The igneous mass forms a north-south ridge, about 1 km. 

 long, 200 m. wide and 80 m. high. The upper, and apparently 

 the central, part is a coarsely granular hornblendite, while the 

 sides are formed of a finer grained pyroxenite, which is some- 



* J. C. Branner, Eng. Mining Jour., lxxxvii, p. 982, 1909 (Map). 



fin this the formation differs from the region to the southwest, where, 

 near Eio de Contas, I found that the foliations were often vertical, as well as 

 to the north, where vertical foliations were observed by Branner and myself. 



