Waring: Geology of Northeastern Brazil. 197 



ten to eight kilometers south of Taua. This rock seems to be asso- 

 ciated with a more feldspathic, pegmatitic material. The strike of 

 the dark ledge may be nearly parallel with the road (north-south), 

 and hence it may form only a narrow zone. A prominent north- 

 south ridge five kilometers southwest of Taua has a core formed 

 by a vertical ledge of white rock. This may be quartzite, but as 

 seen from the town it appeared to be pegmatite. Several small 

 ledges of pegmatite are present in the lower land near the town. 



Between Taua and Cratheus the gneiss has fairly constant north- 

 south strike. The dip in the main is nearly vertical; but in a few 

 places, notably at thirteen and twenty-five kilometers north of 

 Vertentes, it has low dips. It exhibits the usual phases, from massive 

 granitic to finely laminated material. Near the divide between the 

 basins of the Rios Jaguaribe and Poty the rock is granitic rather than 

 gneissic, but near Vertentes (five kilometers north of the divide), the 

 more typical gneissic phase again is in evidence. For the last six or 

 seven kilometers to Cratheus the gneiss is replaced by pink porphyritic 

 granite. A short distance north of Cratheus the gneiss again appears, 

 and continues, with its usual variations in texture, to the canyon of 

 the Rio Poty. Along this stretch northward from Cratheus the dip 

 of the gneiss flattens, and local crumpling renders the direction of 

 strike indeterminable in some places. Observations in the vicinity 

 of Assis, Aguas Bellas, and Novilho, as plotted on the map (Plate IV), 

 indicate the extent of local variation in the structure. 



An escarpment two hundred meters or more in height here forms the 

 east face of the Serra Grande, which is composed of sandstone, which 

 is believed to be of the same Cretaceous series as the sedimentary 

 beds of Chapada do Araripe.^^ The gneiss however is exposed in the 

 channel of the Rio Poty for several kilometers below the head of the 

 canyon. The last exposures observed, before the gneiss disappears 

 beneath the sandstone, exhibit northwest dips. The Cretaceous 

 sandstone thence continues westward, with slight west dip where 



13 Small (Publ. No. 32 of the Inspectoria, p. 63 and figs. 15, 16), regards the rocks 

 of Serra Grande as lower Permian or pre-Permian in age, because of certain uncon- 

 formities he observed between them and Permian beds farther west. Crandall 

 (Manuscript report "Gewtfj-aJ Notes on Norlhsastern Brazil"), and Miguel Arrojado 

 R. Lisboa {The Permian Geology of Northern Brazil, Am. Jour. Set., May, 1914, 

 fig. 2), consider them to be Cretaceous. Dr. Branner accepts Small's classification 

 on account of the structural relations of the beds, but he recognizes the lack of 

 paleontologic evidence. 



