132 ORVILLE A. DERBY 



the Sao Francisco. Two mines, the Barro (clay) and the Duro 

 (hard), opened on opposite sides of the ridge have come 

 together in the center (the footbridge in the middle ground of 

 the view marks the division) so as to form a continuous cutting 

 nearly 500™ long with about 40™ of maximum depth. The 

 Barro mine drains to the Sao Francisco through the Rio Pardo, 

 an affluent of the Rio das Velhas, the Duro to the Jequitinhonha 

 through the Caethe-merim. Both the Pardo and the Caethe- 

 merim, but particularly the latter, are famous diamond streams. 



The material exposed in the mines is a soft, soapy clay that 

 is graphically described by Burton 1 as follows. "The material 

 is a hardened paste of clay, whose regular and level stratifica- 

 tion argues it to have been deposited in shallow water. The 

 eastern side of the gap is the more ferruginous formation (terra 

 vermelha); on the west it is mixed with beds of white sand. 

 Below one foot of brown soil the argillaceous matter has the 

 usual staining and marbling, glaring white like fuller's earth, 

 with feldspar and kaolin, chocolate-brown or rape-colored with 

 organic matter, blue-green with traces of copper [?], pink and 

 rose purple, and dark yellow with various oxides of iron, espe- 

 cially hematite, and dark steel color with oxide of manganese[ ?] ." 



In the whole extension of this immense cutting, nothing 

 approaching the gravel, the usual characteristic of a diamond 

 mine, is to be seen and, with the exception of quartz veins, it 

 is difficult to find a specimen that will resist the pressure of the 

 fingers. The structure as exposed on the sides of the cutting, 

 is much obscured by slides and weather action, and Burton's 

 mistake of horizontal stratification and other indications of a 

 shallow water deposit (thus corresponding, except in the char- 

 acter of the material with the other diamond deposits of the 

 region and with the preconceived ideas of his informants, the 

 miners) was a natural one on the part of a non-geological 

 observer. The true nature of the clay as a decomposition prod- 

 uct of schistose rocks had already been clearly recognized by 

 Heusser and Claraz, who first described the mine and who iden- 



1 The Highlands of the Brazil, London, 1869, II, p. 129. 



