clays. T\ 



O. A. Derby — Occurrence of the Diamond in Brazil. 4.1 



s layer of red soil above, with some coarse ferruginous gravel 

 at the base, resting on soapy parti -colored clays. The disposi- 

 tion of these clays is much obscured by slides, but in a number 

 ■of places it may be seen that they result from the decomposi- 

 tion in situ of unctuous (hydromica) schists underlying a bed 

 of itacolumite which is well exposed at the entrance of the 

 Barro mine. This bed strikes N. 5° W. and dips 40 3 E. The 

 direction of the cut is approximately K-S., showing thot the 

 diamond-bearing material has been followed along the strike of 

 the beds. 



The diamond-bearing clays are found in layers up to H 

 thickness intercalated in the midst of the barren 

 iree distinct layers have been described of which I 

 specimens of two in considerable masses that had been 

 dislocated by slides. One was a soft bluish black mass show- 

 ing on a fresh fracture thin alternating layers of white clay and 

 black powdery hematite. The second mass consisted of a 

 section of a quartz vein adhering to a mass of reddish mottled 

 earth about half a meter thick. Layers of red decomposed 

 schist adherent on one side to the quartz, and to the earth on 

 the other, prove beyond a doubt that this, as well as the quartz, 

 belongs to a vein. The third layer is described by Professor 

 Gorceix as consisting of lithomarge with crystals of quartz 

 presenting the same aspect as the topaz-bearing li1 

 veins of Ouro Preto. 



The reddish earth of the second mass is the diamond-bearing 

 material of that layer. It is evidently a decomposed rock con- 

 sisting of a clayey and a sandy portion. The sand consists, 

 according to Professor J. W. Mallet, who has kindly examined 

 >p"<:iinc!is for me, of quartz grains, microscopic tourmalines, 

 and another black silicate. The clayey portion consists largely 

 of iron. The original character of the rock from which this 

 material is derived can only be guessed at. The only rock 

 known to me in the province which might produce on decom- 

 position such a mixture of quartz, iron and tourmaline is that 

 of the veins of pyrites with quartz and tourmaline of some of 

 the gold mines, notably that of Passagcm near Ouro Preto, 



and iii the same geological series. The quartz portion of the 

 Sao Joao vein is much splintered and full of brilliant plates of 

 specular iron. 



Professor Gorceix states that the diamond-bearing layers or 

 veins accompany the stratilication of the enclosing 'beds, and 

 notes the great similarity of this diamond mine to the topaz 

 mines of the vicinity of Ouro Preto, as well as the marked 

 identity in the minerals which in one place accompany the 

 diamond and in the other the topaz. The most important 



