GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 357 



spread out upon the then level surface of the Batatal sediments. 

 The iron oxide and silica deposits which now constitute the Itabira 

 iron formation were then laid in regular succession upon it. If 

 this basic flow were more widely extended throughout the iron 

 district and were of greater mass, one might be inclined to look 

 to it for a source of the iron now locked up in the great iron forma- 

 tion which followed so closely upon it. Such an explanation has 

 been advanced to explain the origin of much of the iron formation in 

 the Lake Superior region. In their great monograph on the geology 

 of the Lake Superior region, Van Hise and Leith have come to the 

 conclusion that the iron formations there are so intimately asso- 



3.S.K 



Caraca Quarlxiie Baialal JtabiraAvaFbrmalion 

 Scnisl 



Piracicdba Schist 

 Fig. 5. — Section through Fazenda da Alegria 



ciated with basic lavas that the source of the iron in the great ore 

 beds is to be sought in the igneous activity.^ 



In Minas Geraes, however, remnants of a basic lava flow near- 

 the iron formation in the stratigraphic column have been dis- 

 covered up to the present time only at the two adjacent localities 

 just mentioned. Though these probably belong to a single flow 

 which extended continuously from Morro da Mina nearly to Santa 

 Barbara, this flow after all covered but a comparatively limited area 

 which, taken together with its thinness, makes it quite insignificant 

 in size and extent in comparison with the tremendous iron forma- 

 tion above it (Fig. 5). Thus it seems more reasonable to account 

 for the deposition of the iron formation through some other agency. 



^ C. R. Van Hise and C. K. Leith, "The Geology of the Lake Superior Region," 

 Mon. 52, U.S. Geol. Surv. (191 1), pp. 409-570. 



