Carbonados in the State of Bahia, Brazil. 487 



One other mineral which is not mentioned in the table is 

 found in the diamond-bearing beds in Bahia. This mineral 

 is a jet black, opaque, hydrocarbon having a lustrous conchoi- 

 dal fracture, a hardness of 2*2, specific gravity of 1'51, and is 

 related to asphaltum, but is not certainly identifiable with any 

 known form. It was thought at first that this mineral might 

 be especially interesting on account of its association with the 

 diamonds and carbonados, but it occurs in large lumps and 

 seems to have no apparent relation to them. 



Comparison of the minerals of Bahia and Minus. — In 

 order to compare the minerals found in different districts I have 

 brought together here the Bahia lists published by Damour, 

 Gorceix, and Hussak,* and I have made up from various sources 

 a list of all minerals reported from the diamond washings of 

 Minas Geraes. 



In comparing the minerals from the Minas diamond wash 

 ings with those at Bahia, it should be kept in mind that in 

 Minas the diamonds are found for the most part in river beds, 

 either actual or abandoned, and that the minerals associated 

 with them are the result of a long and high degree of concen- 

 tration. In one instance to my own knowledge the Minas dia- 

 monds were obtained from a metamorphosed rock that was 

 broken up by weathering. In the Bahia region this long and 

 complete concentration has occurred in some instances, but 

 in others there has been practically no natural concentration 

 since the minerals were set free from the quartzite matrix. 



Conclusion regarding the relations of the Bahia and Minas 

 districts. — Taking the Bahia list of minerals as a whole and 

 comparing it with the Minas list, we must conclude that the 

 only difference between them that seems worthy of note is the 

 presence of the carbonado in Bahia and its absence from Minas. 

 The other differences, such as the finding of ilmenite, monazite, 

 topaz, and spinel in the Bahia deposits, and the occurrence of 

 tantalite, euclase, fibrolite, titanite, chrysoberyl, chromite, and 

 klaprothine in the Minas gravels I regard as purely accidental, 

 and liable to disappear with further search for those minerals 

 in the regions from which they have not yet been reported. 

 The talc and graphite in the Minas deposits may be regarded 

 as purely local, as is that of asphaltum in the Bahia deposits. 



The diagnostic minerals. — A study of this table at once 

 shows that most of the minerals mentioned have no diagnostic 

 value. Quartz, for example, may occur in rocks of any age, 

 and in sedimentary, eruptive, deepseated, or metamorphic rocks, 

 in pegmatites, or in ordinary veins. It canrtot therefore be re- 



*Eugen Hussak. — Ein Beitrag zurKenntnis der sogenannten "Favas" der 

 brasilianischen Diamantsande. Tschermak's Min. u. Petr. Mitteilungen, 

 xviii, 334-359, Vienna, 1899. 



