SERRA DO ESPINHAQO, BRAZIL 381 



near the mouth of the river, rises at once with a heavy grade to the 

 top of a gneiss plateau with a general elevation of 200 to 300™, over 

 which it runs in a line parallel to the river and about 20 kilometers 

 to the southward of it to kil. 165, where it again meets the river and 

 follows its margin to the terminal station at kil. 259. The gneiss 

 plateau away from the river is remarkably level, but dotted with 

 isolated knobs and ridges that for the most part seem to be com- 

 posed of granite, which also appears in numerous bare ledges 



Fig. 3. — Profile near Lenfoes. 



that do not rise perceptibly above the general level of the plain. Ex- 

 cept in the eastern part, about the headwaters of a small river drain- 

 ing directly to the sea, and apparently in a zone of abundant rainfall, 

 the soil-cap is very thin, often lacking. In this respect, and in the 

 general evenness of the surface, the western part of this plain, like 

 those above described along the Sao Francisco road, seems to offer 

 a good example of the formation, under arid conditions, of plains 

 by the truncation of strata that normally would present a strong 

 relief.^ The section accompanying the river is bordered by gneiss 



I See discussion of subaerial denudation in an arid climate, by Professor W. M. 

 Davis, in Journal of Geology, No. 5, 1905. No reliable data regarding the mean 

 annual rainfall in the districts here considered could be obtained, but it is tolerably 

 certain that it is less than a meter, and that the minimum is often less than half a 

 meter. With the exception of the Paraguassii and the streams of the Serra do Espin- 

 hafo belt, all streams dry up, or become reduced to strings of pools during several 

 months of each year. 



