Branner — Geology of the Serra do Mulato. 259 



Altlioug;h tlie fazenda Itumirim is some tliirty kilometers 

 south of the Rio Sao Francisco, the plain that borders the river 

 extends to the south of the ranch house and ends abruptly 

 against the base of the Serra do Mulato. Hereabout this plain 

 is not alluvial, but is a flat floor of granites and old crystalline 

 roclvs cut down apparently by the ancient river itself. So flat 

 is this plain that it is but rarely that a more resisting point 

 rises three or four meters above its general level. In the 



Fig. 3. 



Fig. 3. Section across tlie south end of Serra da Batateira, showing the 

 nearly vertical quartzites. 



vicinity of the fazenda Itumirim the rocks exposed in place 

 are all banded or sheeted granitoid gneisses. The sheeting- 

 dips eastward at an angle of 80°, and the texture of the rocks is 

 remarkably even. Over the surface of these rocks are strewn 

 loose fragments of quartz, most of them sub-angular and some- 

 what worn by water. 



Within two hundred meters of the base of the Serra is a 

 boss-like outcrop of compact and very hard granite that rises 

 two or three meters above the general level of the plain. It 

 has exfoliated somewhat from exposure to the sun, but here 

 and there over this boss, and especially about its base, are 

 well-pi'eserved remnants of waterworn surfaces. These worn 

 siirfaces are not pits or depressions such as are often made by 

 aborigines in grinding food, but they are uneven, rounded, 

 smoothed, and polished like the worn hard rock in the bed of 

 the stream. As these bosses stand in the open plain and out 

 of I'each of any possible local wearing, it seems clear that they 

 were worn by the Sao Francisco river itself at a time when its 

 flood waters covered a much larger area than they do at pres- 

 ent. The altitude of this plain at the base of the Serra, as 

 determined by aneroid readings carried from the railway 

 station at Joazeiro, is about 400 meters above sea level, and 28 

 meters above the station at Joazeiro. 



Starting up the side of the Serra do Mulato on its north face, 

 the first rocks found in place are some dark-colored crystalline 

 schists. Examined under the microscope, these rocks are found 

 to be composed chiefly of hornblende and epidote. 



From this point up to the base of the bare cliffs that encircle 

 the north escarpment of the mountain there are no trails, and 



