324: PROCEEDINGS OP SECTION C. 



15.— OUTLINE OF THE GEOLOGY OF THE BLACK ' DIAMOND 

 REGION OF BAHIA, BRAZIL. 



By JOHN C. BRANNER, Ph.D., F.G.S., F.G.S.A., &c. 



There seems to be a disposition the world over to suppose that 

 diamonds are to be looked for only under those geological conditions 

 under which they are found in South Africa. The object of the 

 present paper is partly to point out to the geologists of Australasia 

 that valuable diamonds and carbonados occur in paleozoic sediments 

 in Brazil, and to suggest the possibility of similar deposits being 

 found in other parts of the world. 



Occurrence of Diamonds in BrxIzil. — Diamonds are found in 

 Brazil in three widely separated districts. The first, and formerly 

 the most important, is in the vicinity of the city of Diamantina, 

 State of Minas G-eraes; the second, and now the most important, 

 is in the interior of the State of Bahia, about the city of Len9oes; 

 the third is on the head waters of the Paraguay River, near the town 

 of Diamantino, State of Matto Grosso. 



The writer has visited all three of these districts, and what he 

 offers here is, therefore, derived from a personal knowledge of them. 

 Most of the diamond washings in all three districts are in stream 

 deposits, either ancient or modern. In the Matto Grosso deposits the 

 stones have never been found save in alluvial or stream deposits, and 

 nothing further is now known of their origin. In the State of Minas, 

 however, one place was seen where the diamonds evidently came 

 directly from disintegrated itacolumite. Professor Gorceix, who 

 visited the diamond mines at Grao Mogor, some 300 kilometres north 

 of Diamantina, says the stones at that place are derived directly from 

 palaeozoic conglomerates which he regards as a part of the series con- 

 taining the itacolumitesi — that is, in the Minas series of the table 

 below. In the State of Bahia both the ordinary diamonds and the 

 black diamonds, carbons, or carbonados are derived directly from 

 palaeozoic quartzites and quartzitic conglomerates, though many of the 

 washings are in stream deposits of recent date. 



The Rocks of the Diamond Region.— One of the chief difficulties 

 in a study of the geology of the Bahia diamond region lies in the 

 fact that none of the rocks of the several series represented contain 

 any recognisable fossils. It is, therefore, impossible to give the ages 

 of the rocks with certainty. There are, however, physical breaks and 

 lithologic characters, which, taken in connection with the structure 

 that has been worked out over large areas, have afforded satisfactory 

 evidence of the relative ages o f the various beds and throw much light 

 t Gorceix, Bui. Soc. Geol. de France, XIL, 538. Paris, 1884. 



