268 J. C. BRANNEK — DECOMPOSITION OF ROCKS IN BRAZIL. 



for two weeks ; there were land-slides on the North line, several on the 

 D. Pedro II and on the Uniao Valenciana ; several on the Leopoldina; 

 one slip took place on the S. Paulo branch in decomposed gneiss/^ 



In general the land-slides so common through the Serra do Mar region 

 represent a considerable part of the denudation on account of the great 

 masses of earth moved short distances ; they also expose fresh surfaces 

 to the rain and running water and greatly hasten denudation in this 

 way. 



Along the coast from Rio de Janeiro to Bahia one sailing near the 

 shore may see here and there great red and yellow spots upon the land- 

 scape, caused b}^ the land-slides and barrancas or washes in the decom- 

 posed gneiss of that region. 



I see no reason for appealing to hydrostatic pressure for the explana- 

 tion of land-slides, as Burton has done,t or to earthquakes, as Castelnau 

 suggests. X "J^'hey are to be attributed to the profound decay of feldspathic 

 rocks which j^ield slippery clays and kaolins and to the concentration of 

 a large precipitation. 



TALUS AND ITS DECOMPOSITION. 



The formation of talus slopes is not so striking a feature of surface 

 geology in Brazil as it is in cold climates. This is probably due to the 

 fact that in cold regions talus is produced chiefly by spalling off and by 

 the freezing and thawing of water in the cracks of the cliffs. Such 

 agencies are wanting or but feeble in the tropics. Spalling off is caused 

 by changes of temperature, as will be pointed out later, but the rock 

 comes away less rapidly than in cold climates, and the tendency for it 

 to disintegrate is more pronounced than the tendency to flake off. 



The talus that does accumulate at the bases of cliffs, bluffs and ledges 

 decays rapidly and the slopes are therefore usually soil slopes instead of 

 rock slopes. The rapid decay of these fragments is due to the fact that 

 they expose a larger percentage of their surfaces to atmospheric agencies, 

 and, furthermore, they fall upon a soil supporting a rank vegetation and 

 abundant insect life, whose acids make quick work of their disintegra- 

 tion, while the heavy rainfall removes the residue rapidly. The talus 

 slopes in granite regions that most nearly resemble true rock talus are 

 those near the ocean, where the waves remove the soil at the base. There 

 is a slope of this kind at the south base of the Pao d'Assucar at Rio de 

 Janeiro. Typical talus slopes of Brazil occur along the whole length of 

 the Serra do Mar and of the Serra da Mantiqueira, in the mountains 



* Revista de Engenharia, ii, no. 2, FeVj. 15, 1880. The Rio News, Jan. 25, 1880. 

 t Highlands of Brazil, i, p. 73. 

 X Expedition, i, p. 202. 



