GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 419 



in residual accumulations subject to removal by streams and other 

 subaerial agencies. If in part transported to the sea during the 

 deposition of the Caraga formation, they should be found probably 

 in association with quartz sand and such pebbles of vein quartz as 

 resisted the abrasian of stream action. Being the hardest of all 

 minerals and the most resistant to decomposition, it is not surprising 

 that the diamonds still retain their crystalline faces when most 

 other minerals originally present, except quartz, have disappeared. 



When in pre-Devonian times the sedimentary series was ele- 

 vated, consolidated, and metamorphosed, and the processes of 

 erosion had commenced, the diamonds began gradually one by one 

 to be freed from the quartzite and collected in the stream deposits 

 along with other minerals from the same source. This continued 

 for long ages, and while other minerals were disintegrating and 

 decomposing, the diamonds remained intact and became more 

 and more concentrated. Even after a general peneplain level was 

 reached, this process probably continued and was only interrupted, 

 by renewed elevation resulting in the present gorges. 



The foregoing discussion is based on the hypothesis that the 

 diamonds at one stage in their history were incorporated in the 

 Caraga quartzite. The alternative hypothesis is that the majority 

 of them never have been deposited in the quartzite, but that they 

 were brought in by streams directly from igneous areas some dis- 

 tance from the present diamond fields. An objection to this 

 hypothesis is that so few igneous pebbles occur in the conglomerate, 

 and that over large areas pebbles of igneous rocks are entirely 

 absent. However, it is possible that, on account of their predis- 

 position to rapid decay, they might have disappeared in the slow, 

 shifting process on the peneplain surface. But one wonders why, 

 if such diamond-bearing igneous-rock areas exist, not concealed by 

 the sedimentary rocks, they have not been discovered up to the 

 present time. If the diamonds were brought in from a long dis- 

 tance the chapada diamond-bearing gravels should necessarily 

 occur along a few main drainage courses arid not be irregularly 

 scattered over a wide area, while if they came from igneous areas 

 close by, the diamonds should have been discovered in streams 

 flowing from these areas. But in the present state of knowledge it 



