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JENKINS— GEOLOGY OF THE 



[May 29, 



Decomposition has had a marked effect in leveHng down this 

 region, so that the natural exposures are only in the shape of flat 

 bosses and exfoliated boulders. Mound-shaped hills, as that of 

 Torreao Peak, which is about seventy meters high, lying three or 

 four kilometers northwest of Baixa Verde, are composed of granitic 

 rocks. On their surface are scattered great boulders of exfolia- 

 tion, while at their base are bare, flat exposures of other crystal- 

 line rocks, giving to the whole the appearance of glaciation. 



In all the railway cuts it was noticed that dikes of granites and 

 pegmatites cut through micaceous schists. These dikes vary in 

 width from one to thirty meters, sometimes following the plane of 

 schistosity and sometimes cutting across it. Often one dike inter- 



h- 



FiG. 9. Diagram of railway cut near Baixa Verde, Rio Grande do Norte, 

 showing how the dikes intersect the schists and how the topography is un- 

 affected by these. 



sects another. These dikes and schists do not show in the topog- 

 raphy; all are eroded and decomposed to the same surface level. 



The following are descriptions of the rocks collected. 



Qttartsitic Arkose. — This specimen was collected near Taipu, 

 kilometer 53. It outcrops in the region of dikes and schists. The 

 rock is a medium-grained quartzitic arkose containing some minute 

 cavities. Under the microscope the grains show that they are irreg- 

 ular in size and angular to subangular in shape. They are principally 

 fragments of quartz, plagioclase, microcline, and orthoclase, all 

 cemented firmly by opal. Chalcedony occurs as a secondary mineral, 

 filling the minute cavities. 



