258 Branner — Geology of the Serra do Mulato. 



dike is about 12 meters wide, but is not traceable any farther 

 toward the north ; it extends southwestward up Rio Salitre, 

 however, following the strike of the slates, and widening- some- 

 what in that direction. It was followed only for a distance of 

 200 meters or so. The following section shows the general 

 geology as it appears along the road from Joazeiroto the 

 fazenda Gamelleira. 



Fig. 3. 



\>** 

 Mdn 



TimiiX#i?l~^ 



Fig. 2. Generalized northeast-southwest section, showing the geology 

 between Joazeiro and Rio Salitre. ' 



To the right or north of the road, as the Serra do Mnlato is 

 approached, is a long, low and narrow range of hills called the 

 Serra da Batateira. With considerable difficulty one can 

 break and cut his way through the catinga forest and cactus to 

 this range. At its southern end it rises only abont twenty-five 

 or thirty meters above the level of the plain. " It is composed 

 of very compact quartzites, the beds are clearly defined and 

 stand nearly on end, the dip being 70° east, magnetic. This 

 ridge extends north and northeast some twenty kilometers 

 nearly to the Rio Sao Francisco. The flat plain about the 

 base of this ridge is strewn with fragments of this same quartz- 

 ite. About three kilometers to the south is another sharp hill 

 which was not visited, but which appears from a distance to 

 be of the same geologic structure as the Serra da Batateira, 

 In the flat ground between the two ridges the soil has the 

 peculiar snuff-color characteristic of the limestone soils here- 

 about, and the occasional loose lumps of limestone show" that 

 there is here a thin sheet of recent lime-rock, though it is 

 mostly decomposed. Two kilometers west of the Serra da 

 Batateira gneissoid granites are exposed on the plain, and sim- 

 ilar rocks crop out along the trail to the ranch house of the 

 fazenda Itumirim, which is about two kilometers north of the 

 base of the Serra do Mulato. 



elongate light-colored crystals set in a black matrix. The texture is that of a 

 coarse-grained diabase. 



The principal minerals seen in the thin sections are : (1) hornblende, pris- 

 matic crystals with the characteristic pleochroism ; (2) ilmenite altered to 

 leucoxene ; (3) epidote, light-colored irregular mineral with high relief ; (4) 

 zoisite, recognized by high relief and Bei'lin blue interference colors ; (5) sec- 

 ondary quartz in clear grains. Besides these there are elongate miich altered 

 crystals which probably represent feldspars. The rock is an altered diabase, 

 with scarcely any of the original minerals present. 



A. F. Rogers. 

 Stanford University, Feb. 9, 1909. 



