396 ORVILLE A. DERBY 



As Eschwege has already remarked, gneiss and micaschist only 

 appear in the lower levels of the Serra do Espinhafo zone, while, 

 with their associated eruptives, they form the main mass of the 

 mountains to the eastward and southward belonging to the Serra 

 do Mar and Serra da Mantiqueira systems. These rocks have been 

 but little studied in the region here discussed, but, so far as known, 

 they correspond with those of the other parts of Brazil, and can 

 safely be considered as of Archean age. It also seems tolerably safe 

 to consider the great majority of the Brazilian gneisses (at least those 

 that from their resistance to decay are most in evidence) as sheared 

 eruptives, the original types being granites, norites, diorites, etc. 

 A part of the micaschists are also of eruptive origin, but presum- 

 ably the larger part will prove to be metamorphosed sedimentaries. 

 In the part of Brazil here considered these rocks form a great shield- 

 shaped area in eastern Minas Geraes and Bahia (with the adjacent 

 states of Espirito Santo and Rio de Janeiro), which for the most part 

 has been dry land since Archean times. About its borders, particu- 

 larly on the west, and over its lower-lying portions the later sedimen- 

 tary deposits have been laid down. 



The schistose series of the Serra do Espinhafo and adjacent 

 regions, which may conveniently be denominated the Minas series, 

 consists of a great complex of predominantly argillaceous schists, 

 with subordinate masses of ordinary quartzites, ferruginous quartz- 

 ites (itabirites passing to pure iron ores), and limestones. All of 

 these rocks are greatly sheared, and characterized by a greater or 

 less development of a mica-like rnineral (biotite, sericite, micaceous 

 hemetite, chlorite, talc, etc.) ; and as in general they are much decom- 

 posed, there is great difficulty in distinguishing the different mem- 

 bers (except the quartzose and ferruginous ones), and thus far no 

 successful attempt to work out the order of succession in them has 

 been made. It is tolerably certain that the quartzose, ferruginose, 

 and calcareous members are repeated at various horizons, and even- 

 tually they will serve as reference lines for establishing the subdi- 

 visions of the series; but before this can be done the repetition due 

 to folding and faulting must be determined and taken into account. 

 Apparently the whole series has been bent into closely appressed 

 overturned folds, and doubtless it has been much faulted. Until 



