GEOLOGY OF CENTRAL MINAS GERAES, BRAZIL 421 



common, than of those particular clayey portions which suggest a 

 pegmatitic source. Because of this the Sao Joao da Chapada occur- 

 rence does not seem essentially different from the other diamond- 

 bearing conglomerate deposits. A pegmatitic dike, being less 

 resistant than the surrounding quartzite, may have determined the 

 location of the drainage channel in the first place, and because of its 

 ready yielding gave rise to the steep-sided trench which would 



Fig. 23. — -The famous diamond mine of Sao Joao da Chapada. The conglomerate- 

 filled trench in the quartzite has now been largely re-excavated, the undisturbed 

 quartzite appearing at various points in both walls and in the bottom of the excava- 

 tion. This mine has produced some of the finest of the Brazilian diamonds. 



otherwise seem peculiar in a peneplained surface. But we believe 

 that if such pegmatitic material was present it was only a contribut- 

 ing condition, and that the diamonds came with the material that 

 was washed into the trench filling it up. 



Since their deposition with the residual base-level gravels which 

 now constitute the chapada or upland deposits, several reconcentra- 

 tions of the diamonds have occurred, as the region has suffered 

 various changes of attitude. Upon the rejuvenation of the drainage 



