342 E. C. HARDER AND R. T. CHAMBERLIN 



side was deformed and thrown into mountains before the close of the 

 Paleozoic. As in our own Appalachians this mountain-building 

 period of eastern South America was followed by long-continued 

 erosion which base-leveled the mountains. Later successive 

 widespread uplifts produced low plateaus, out of which rejuvenated 

 erosion has carved the present ranges. Aside from these gentle 

 uplifts, which have occurred at several periods during later geologic 

 history, the eastern part of the continent has exhibited comparative 

 stability for a long time. Since the Paleozoic the deformation of 

 South America has largely been on the Pacific side. During 

 the Mesozoic and Tertiary remarkable stressed conditions on the 

 Pacific border developed the great world-ridge of Cordilleras which 

 runs from Cape Horn to Bering Strait and which makes the western 

 portions of the two continents also geologically related. 



The area considered in the present paper lies in the eastern half 

 of the South American continent. It is that section of the state 

 of Minas Geraes, Brazil, which lies between Diamantina (latitude 

 i8°i7' S.) and Ouro Preto, the former capital of the state (latitude 

 2o°23' S.), which is situated east of the 45th meridian. In many 

 respects this area is geologically the most typical and complex 

 part of eastern Brazil. Running north and south across it is the 

 Serra do Espinhago, or Backbone Ridge of Brazil, which forms the 

 eastern rim of the Sao Francisco basin and divides the waters flowing 

 into the Rio Sao Francisco from those which flow directly into the 

 Atlantic Ocean. Structurally and historically this mountain range 

 gives the key to the geology of most of eastern Brazil, for either 

 within the range, or closely associated with it, are all the important 

 sedimentary formations of Minas Geraes. These comprise a great 

 series of quartzites, schists, and iron formations with some asso- 

 ciated limestones. East of the Serra do Espinhago is a great area 

 of dominantly igneous rocks, presumably Archean, which extends, 

 with the exception of small isolated patches of overlying sedi- 

 mentary rock, continuously to the sea. West of the Serra do 

 Espinhago is a plateau country underlain by rocks of both the 

 sedimentary series and the basement complex. 



Within this region structurally so significant, occur also the 

 most important mineral deposits in Brazil (Fig. i). Here is 



