216 Deroy — Schists of the Gold and Diamond Regions. 



A high proportion of iron oxide, more frequently in the state 

 of hematite than of magnetite, is also a very common feature 

 in the micaceous as well as in the quartzose and calcareous 

 rocks of the region. In the latter case and in that of the 

 actinolitic and a part at least of the quartzose iron schists 

 (itabirites), it may be presumed to come from original carbonates. 

 In other cases it may be attributed to iron sand in clastic 

 deposits, to original elements either oxides or silicates, in erup- 

 tives, or to a leaching of limonite into rocks of any character. 

 It is almost invariably accompanied by a certain proportion, 

 often high, of titanium oxide as rutile (more rarely as anatase) 

 which after quartz and the iron oxides is the most abundant 

 and constant mineral of the gold and diamond concentrates. 

 This, whether in elastics or eruptives, is probably indicative of 

 original ilmenite. Chrome mica occasionally appears in the 

 quartzites of the region and in one case (a pebble from the 

 conglomerate of the Cavallo Morto diamond mine near Diaman- 

 tina) this could be traced very satisfactorily to grains, evidently 

 clastic, of chromic iron contained in the same rock and which 

 probably indicate that a peridotitic rock has contributed to the 

 original sandy deposit. 



