GENERAL AND ECONOMIC GEOLOGY 



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and unimportant isolated masses, but again they are hmidreds of kilo- 

 meters in length, while in area they cover thousands of square kilometers. 

 On accomit of lack of knowledge of the details of their distribution, 

 many of the smaller areas are included in the area of the Archean rocks 

 given in the preceding paragraph. As a rule, they form belts or zones 

 and they frequently appear as parallel ridges or mountain ranges. The 

 rocks thus infaulted are generally metamorphosed, and, as they are more 

 resistant than the surrounding granites and gneisses, they usually form 

 the prominent features in the topography of the region. 



The age of these old sediments is not known, and they are therefore 

 represented on the map simply as early Paleozoic. They are probably 

 older than the Carboniferous. Their structural relations to the Archean 

 rocks are well shown in the accompanying section from the paper by 

 Harder and Chamberlin, page 368. 



/tali/ral' jn/crma/ion ^ ^PJrQcJcaba 3ch?jl 



C/^anl/e amj/ Gndss 



Caraoa 

 Quar/zile 



Cramfe and Gneiss 



CaraQa FJradcaba 

 Quar/zJle ^cMjf 



Figure 13. — Slice-faulted Section 



This section is between Villa Rio Piracicaba and Morro do Seahara. — Harder and 



Cbamberlin. 



The next newer rocks are probably a series of sandstones or quartzites 

 corresponding to and continuous with the diamond-bearing quartzites of 

 the State of Bahia. These rocks are exposed at Grao Mogol, in the north 

 of Minas, where they are diamond-bearing just as they are in Bahia, and 

 like those in Bahia they are referred provisionally to the Carboniferous. 



The structural relations of these quartzites are fairly well shown in the 

 section published by Helmreichen in 1846, and by Gorceix in 1884. 



Eocks of this series are probably exposed at many places about the 

 edges of the Permian and even inside of it. They can not be clearly dis- 

 criminated with our present knowledge, for much confusion has been 

 caused by the use of the word "itacolumite" for sandstones and quartzites 

 of many kinds and of several different geologic horizons. 



In view of all the data available, it seems probable that the diamonds 

 of Minas Geraes have come directly from these Carboniferous quartzites, 



