138 ORVILLE A. DERBY 



or injections, of eruptive origin, which, judging from evidence 

 elsewhere, are rather to be expected than otherwise in a cutting 

 like that of Sao Joao da Chapada. For the question of the 

 genesis of the diamond this hypothesis is of prime importance 

 and the evidence thus far available for or against it will now be 

 examined. 



Of the three diamond-bearing bodies described by Burton 

 and Gorceix only two were seen by me. The masses shown me 

 were displaced by landslides, but, as nearly as can be deter- 

 mined, they represent the middle and lower bodies of Burton. 



The mass supposed to represent the lower body of Burton 

 and the mottled clay of Gorciex consisted of a considerable 

 rectangular block of quartz, with plates of specular iron, and 

 with laminated clay representing the decomposed country rock 

 adhering to it on one side. On the other side was a mass of 

 friable structureless reddish clay, sharply defined on the side 

 opposite the quartz from the harder laminated clay of the 

 decomposed country rock, which is here also reddish, but of a 

 different tint and aspect. The whole appearance of the mass 

 was that of a vein with sharply defined walls, and it was so 

 described on account of the quartz, though, as the earthy por- 

 tion was referred to a decomposed rock of undetermined char- 

 acter, the term dike might have been employed with equal 

 propriety. The earthy diamond-bearing mass was shown to 

 consist of an argillaceous portion stained with iron oxide and, a 

 sandy portion with quartz and tourmaline. The heavy residue 

 which has since been separated and examined consists princi- 

 pally of aggregations of specular iron and of a micaceous mineral 

 representing some altered silicate with a great abundance of 

 microscopic brown tourmaline. Yellowish, burr-like aggregates 

 of anatase are also abundant, while rutile is comparatively rare, 

 as are also grains of disthene. All of these minerals are evi- 

 dently authigenic. The rare grains of zircon are in part 

 distinctly worn, in part with the fresh appearance of an authi- 

 genic element. A few grains of staurolite also occur, and these 

 are, for the most part, rounded, giving them a worn appearance, 



