124 OR VILLE A . DERB Y 



much is yet to be learned regarding this unique deposit. The 

 region is characterized by inclined strata of micaceous schists, 

 in part staurolitic, which are regarded as metamorphosed elas- 

 tics, with intercalations of amphibolites, which are almost cer- 

 tainly metamorphosed eruptives. This schist series is cut by 

 dikes of granite which, so far as observed, are characterized 

 by muscovite either alone or in association with biotite, and 

 which are generally tourmaliniferous. Quartz veins which fre- 

 quently carry a little mica are also common. Upon this group 

 of schists and granite rest horizontal beds of soft sandstones, 

 with intercalated layers, or sills, of trap — augite-porphyrite or 

 melaphyre — which are presumably of Triassic age. In the 

 same region, although not definitely known in the immediate 

 vicinity of the mine, there is another obscure eruptive group 

 which has furnished material characterized by grains of pyroxine, 

 perofskite, and magnetite, to beds of clay and impure limestone 

 that overlie the sandstone and trap, and, in places, present some- 

 thing of the aspect of ash-beds or volcanic breccias. This 

 group, though very imperfectly known, is certainly distinct from 

 the traps, and its probable relations will be discussed below. 



The diamond-bearing bed of Agua Suja is a thoroughly 

 decomposed conglomerate, or breccia, in which both matrix and 

 the included pebbles are transformed into clay. The original 

 angular outline of the pebbles (or rather bowlders, as they are 

 often of considerable size), can, however, be recognized, as also, 

 in many cases, the type of rock to which they belonged. The 

 various types of the schists and granites upon which the dia- 

 mantiferous bed rests in part (in part also on sandstone and 

 trap) are recognizable, as well as masses of the sedimentary 

 and later eruptive series. Fragments of opal, which may be of 

 secondary origin, constitute a peculiar feature when this mine is 

 compared with others of the same region (Bagagem) or of the 

 other diamantiferous regions of Brazil. Still more peculiar and 



1891. E. Hussak : In the above cited pamphlet and in Relatorio da Commissao 

 Exploradora do Planalto, Rio de Janeiro, 1894. J. P. Calogeras : Revue Universalle 

 des Mines, XXIX, 1895. 



