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



reduced to a common level. Upon this plain, material washed 

 down from the iron-formation monadnocks developed a covering 

 sheet of canga. The canga formation, by obliterating some of the 

 remaining irregularities in the erosion surface, has produced a 

 remarkably level plain, much of which still remains. This canga 

 plain is now at an average altitude of about 900 meters above the 

 sea. Judging from the occurrences at Fonseca it appears to be of 

 later age than the lignite deposits, which have been referred to the 

 Miocene or Pliocene, and would seem to belong to the late Tertiary, 

 Over considerable areas it has scarcely been trenched at all. 



Conglomerate formations. — Benches, and other remnants, corre- 

 sponding to the canga plains in the neighborhood of the iron 

 formation, are also to be seen in the Diamantina region. These 

 remnants occur in the neighborhood of the streams which have 

 worked back and partially dissected the plateau-like Backbone 

 Range. As the formation of the range is quartzite, the gravels 

 developed on these plains are quartzose, though possibly to be 

 correlated with the iron-formation conglomerate, or canga, of the 

 iron-ore district. As these gravels and conglomerates are composed 

 of concentrated residual material derived from diamond-bearing 

 formations, they have been extensively worked for diamonds. 



PRESENT STATUS OE EROSION 



: The formation of these several Tertiary plains was terminated 

 supposedly by uplifts which accelerated stream action and started 

 their dissection. Portions of the canga plain near Agua Quente 

 have suffered considerable erosion since its formation. Toward 

 the northeast, in the region between the Serra do Caraga and the 

 Piracicaba iron-ore district, the plain has been much carved. This 

 area being for the most part farther from the hills of iron formation, 

 the thick capping of tough canga which has preserved the plain 

 intact at Santa Rita Durao and Agua Quente was much less devel- 

 oped. As a result, erosion progressed more readily. The easily 

 weathered underlying granites and gneisses which almost exclu- 

 sively make up this region have been cut into a state of mature 

 erosion, though the level of the former peneplain is well preserved in 

 the tops of these low hills. 



