Branner — Geology of the Serra do Mulato. 257 



distant isolated peak or mountain cluster rising above the other- 

 wise even horizon line made by the low forest grovi^th. The 

 most prominent to be seen from Joazeiro is the Serra do 

 Mulato, which lies about fifty-one kilometers to the southwest. 

 The other peaks or i-anges visible from Joazeiro have tlie 

 rounded or irregular outlines that Brazilian geologists are 

 accustomed to see in mountains of granite or other crystalline 

 rocks, while the top of the Seri-a do Mulato is fiat and the 

 sides are precipitous, suggesting hard, horizontal beds at the 

 top resting upon more yielding ones. 



Although the Serra do Mulato is only thirty-two kilometers 

 south of the Kio Sao Francisco at the village of Tatuhy, it was 

 never visited by a geologist so far as can be learned, until Sept. 

 1907, when the writer made a trip to its summit in company 

 with Dr. Alfredo de Carvalho, a Brazilian engineer of Per- 

 nambuco. 



The geology of the mountain is quite simple on the whole, 

 but taken in connection with what was subsequently learned of 

 the region to the south and southwest, it has helped greatly to 

 clear up our conceptions of the geology of a large area in the 

 interior of Bahm regarding which there has long been much 

 doubt and uncertainty. 



The region between Joazeiro and the Serra do Mulato is 

 quite flat, and, with the exception of a few small glades and a 

 few cultivated fields, is covered with the short scrubby catinga 

 forests characteristic of the semi-arid portions of the interior 

 of Bahia. Near the Rio Sao Francisco there are some wide 

 alluvial belts, but for the most part, although the surface is 

 flat, the rocks are either granites, or gneisses, or they are lime- 

 stones of recent freshwater origin, spread in a thin layer over 

 an old planed down granite surface. These limestones are 

 characteristic of the high-flood plains of the Rio S. Francisco, 

 and will be described in a later paper. 



The road from Joazeiro to the Serra do Mulato crosses into 

 Salitre at fazenda Gamelleira, forty kilometers from Joazeiro 

 and thirty-eight from where the Salitre enters Rio S. Fran- 

 cisco. The rocks exposed at that place are slippery green 

 talcose slates standing nearly on end and traversed by many 

 small quartz veins. On the southwest side of the river these 

 slates strike S. 50° W. magnetic ; at the fazenda G-amelleira 

 residence the strike of the slates is S. 15° W. magnetic. 



About thirty meters west of Rio Salitre at Gamelleira cross- 

 ing a dike of dark greenish diabase traverses the slates.''^ This 



* Dr. A. F. Eogers has examined specimens of this rock and kindly fur- 

 nishes the following note in regard to it : 



The specimen is a dark-colored basic igneous rock and is referred to a 

 diabase. As seen with a hand lens, it consists of two principal constituents, 



