6 SOPER— GEOLOGY OF PARAHYBA 



Of the various streams the rivers Apody (or IMossoro), Assu, 

 Ceara Mirim, and Parahyba, are the most important. The Rio 

 Jaguaribe in the southern part of Ceara might also be included, for 

 the territory drained by it is physically the same and has passed 

 through the same history as the adjoining region in Rio Grande do 

 Norte. Not a single stream in the entire territory under considera- 

 tion is perennial. The reason for this is partly the scarcity and 

 irregularity of the rainfall, partly the hard, impervious nature of 

 the rocks, and partly the hot, dry climate. 



Geology. 



For the purposes of the present study the geological formations 

 represented in the states of Parahyba and Rio Grande do Norte may 

 be classed in two groups as follows: first, the ancient (Archean to 

 early Paleozoic) rocks which have been called the Brazilian complex 

 by Dr. Branner;* second, the comparatively modern (Mesozoic to 

 Recent) rocks which we call the sedimentary series. 



The Brazilian complex consists of crystalline rocks (mainly 

 gneiss) and schists disposed in inclined beds which are quite thickly 

 threaded with dikes and bosses of various eruptives, among which 

 granites predominate. 



The rocks of the second group consist mainly of sandstones, 

 limestones, and incoherent sand and clay deposits (including dunes), 

 all of them disposed in horizontal or approximately horizontal beds. 



In general terms the two groups as shown on the accompany- 

 ing map are evenly distributed. With the exception of a com- 

 paratively narrow strip along the coast and a few isolated spots in 

 the interior, the whole of the two states is made up of the crystalline 

 rocks. No attempt at a systematic classification of this division will 

 be made here. I have not had the time nor the opportunity to do 

 more than to observe some of the broader features. However, to 

 intelligently understand the geologic conditions in this region, it is 

 necessary to know that the rocks of the Brazilian complex cover 

 practically all of the interior ; and that the more recent sedimentary 

 beds along the coast have been formed by the deposition of sedi- 



4 " Geologia elementar," 2(i ed., pp. 286-289. Rio de Janeiro, 1915. 



