386 ORVILLE A. DERBY 



the main tectonic features of the region have been produced by fold- 

 ing and denudation. 



The sandstone sheet, estimated to be from 400 to 500"^ thick, 

 consists of a lower flaggy, reddish portion, well seen in the blocklike 

 hills about Santa Isabel, succeeded by a coarse conglomerate that 

 passes to a whitish sandstone with scattered pebbles and patches, 

 and layers of conglomerate, and finally to argillaceous sandstones 

 and sandy shales. The conglomerate, where seen in contact with 

 the lower red sandstone, contains large fragments of this rock that 

 indicate a time-break and unconformability, at least of overlap, 



Fig. 7. — The Tombador (Tumble Down) near Jacobina, by Mr. J. A. Allen. 



between the two. Some of the sandstone layers next to the con- 

 glomerate are very hard and quartzitic, but in general both members 

 of the series are unmetamorphosed. The lower member may take 

 the name of the Paraguassii group, as it is especially well developed 

 in the vicinity of Santa Isabel (or Sao Joao) do Paraguassu. The 

 upper member may appropriately be denominated the Lavras group, 

 as its conglomerate member is undoubtedly the principal, if not the 

 only, repository of the diamonds that have given the popular name 

 of Lavras ("Washings") to the whole district. The thickness of 

 the Paraguassu group can be estimated with approximate accuracy 

 at about 250"^; that of the Lavras group cannot be so closely calcu- 

 lated, but it is probably about the same. 



