16 SOPER— GEOLOGY OF PARAHYBA 



fossils have been found, but it is not improbable that this sandstone 

 was also once connected with that nearer the coast. The bases of the 

 Serras Martins and Porto Alegre, are formed of old metamorphic 

 schists. There is a mass of isolated limestone near the base of the 

 Serra Martins. So far as I have been able to ascertain, these sand- 

 stone capped serras, the sandstone basin of the Rio do Peixe, and 

 the coastal belt of sediments, form the total of sedimentary areas in 

 the states of Parahvba and Rio Grande do Norte. 





Fig. 5. Section across the Serra Martins, Rio Grande do Norte. 



One of the most interesting things in this connection is the rela- 

 tion of the geology to the topography. The crystalline area may 

 usually be recognized by scattered serrotes of hard granite, by occa- 

 sional mountains which rise abruptly from the surrounding plain, by 

 great, gently undulating stretches, covered with a scattered growth, 

 and by low ridges, steep-sided and rocky. On the other hand the 

 sedimentary area is notable first of all for its vegetation, which as- 

 sumes the form of an impenetrable forest : often there is a thick 

 undergrowth from 3 to 4 meters in height. The only prominent topo- 

 graphic features are a few low ridges and the broad Chapadas of 

 Apody and Sao Sebastiao, which on the south, end in a low escarp- 

 ment, but which on the north, slope gently toward the sea. Also, 

 very noticeable are the short streams with their steep banks which 

 cut cleanly through the soft sediments. These hard rocks and undu- 

 lating plains of the crystalline complex which give rise to a rapid 

 drainage, and the long, almost level stretches on the sedimentary 

 areas, play an important part in the conservation of the water supply. 



Summary of Observations on the Geology of Parahyba and 

 Rio Grande Do Norte. 

 I. The two great groups of rocks in the region covered are 

 crystalline and sedimentary. 



