ON THE GENESIS OF THE DIAMOND 139 



but as some of them are distinctly etched, it is thought that this 

 aspect may be due to etching rather than to attrition. Unfor- 

 tunately, it is not absolutely certain that the zircon and staurolite 

 may not have been introduced from a foreign source, as at the 

 time the washing was made the extreme care now found neces- 

 sary to avoid admixture was not observed. On the assumption 

 that the residue is a pure one (as it is believed to be), 1 the inter- 

 pretation would be that the original vein material contained 

 primary tourmaline and zircon with iron and titanium minerals 

 that have furnished material for secondary hematite, anatase, 

 and rutile, and that the accompanying schist contained clastic 

 zircons, staurolite that is authigenic if the rounding of the grains 

 can be attributed exclusively to etching, and disthene. The 

 hypothesis that best suits these conditions is that of a granitic 

 (pegmatitic) vein accompanied by phenomena of contact meta- 

 morphism. 



The mass that was shown to me as representing the Barro 

 Preto (black clay, middle body of Burton) had the character- 

 istic of a bed rather than of a vein. The clay is well laminated, 

 ribboned with fine regular alternating streaks of white and black, 

 the latter composed mainly of a fine powder of hematite. The 

 residue, freed from clay, shows a great abundance of black 

 hematite, so finely divided that much of it floats away in the 

 washing, a moderate amount of etched quartz, a small amount of 

 tourmaline in coarser grains than in the body above described, 

 and a comparative abundance of rolled zircons, which appear 

 also to have been somewhat malaconized. The titanium min- 

 erals, rutile and anatase, are absent, or extremely rare. A 

 sample subsequently received as representing the same body 

 agrees substantially with the above, except in the greater abun- 

 dance of quartz and the absence of tourmaline. All these indi- 



1 In the case of an admixture, rounded staurolites and fresh disthenes are not the 

 minerals that might be expected to be introduced in the residue through lack of care 

 in the preparation. Two or three grains of monazite were found that were certainly 

 introduced by accident, but this very circumstance gives confidence in the general 

 purity of the residue, as the much more abundant disthene and staurolite are not its 

 usual associates. 



