Waring: Geology of Northeastern Brazil. 219 



there are numerous patches of quartz-gravel, which seem to be 

 derived from near-by, or perhaps underlying, beds of quartzite or 

 dikes of pegmatite. 



In the places where the material is merely scattered over the surface 

 as subangular fragments in localities, where quartz-bearing beds, 

 veins, or dikes are present, the source of the gra^'el seems to be evident. 

 But where broad bands of more or less rounded gravel occur, as in 

 the vicinity of Jardim de Serido, with no evident connection either 

 with immediate sources of the material or with stream courses, their 

 origin must be sought either in an ancient sedimentary series, or in 

 ancient drainage-systems. 



Basic Igneous Rocks. The Brazilian Complex, which forms the 

 basement-rock of most of the region, and is exposed at the surface 

 over large areas, consists of numerous varieties of plutonic, meta- 

 morphic, and dike-rocks. Little petrographic study of them has 

 yet been carried on, but Jenkins'*'^ describes from the vicinity of Baixa 

 Verde in Rio Grande do Norte, a quartzitic arkose, a quartz-biotite 

 schist, two biotite schists, a granite haplite, granite pegmatites, and 

 dikes of granite. A petrographic slide of rock from near the city of 

 Bahia M^as examined for the writer by E. S. Larsen, Jr., who found 

 it to be probably a granodiorite, somewhat crushed and banded. 

 The rock contains plentiful quartz, much plagioclase feldspar (ande- 

 sine), and some orthoclase, with much biotite, usual amounts of mag- 

 netite and augite, and pyroxene as alteration material. The rock also 

 contains veinlets of sericitized material. An unusual feature is the 

 presence of abundant, large, prominent zircons. It will be noted 

 that all these petrographic descriptions are of acid rocks. 



Haplites, pegmatites, and other acid dike-rocks seem to be common, 

 but the more basic rocks are believed by the writer to be relatively 

 rare in the region under discussion. Farther west, in the State of 

 Piauhy, basic rocks are less uncommon. Dr. M. A. R. Lisboa, found 

 in that region two types of material, a compact diabase, and a trap.^^ 

 The former is a normal diabase, exposed in the bottoms of valleys, 

 where it has been uncovered by erosion. In some places it forms dikes 

 or sheets; in others it is found as great rounded blocks scattered over 



*'' Geology of the Region about Natal, Rio Grande do Norte, Brazil, by Olaf Pitt 

 Jenkins, Proc. Am. Phil. Soc, Sept. -Oct., 1913, pp. 12-14. 



^* The Permian Geology of Northern Brazil, by Miguel Arrojado R. Lisboa, Am. 

 Jozir. .Sc!., May, 1914, pp. 4.^2-433. 



