266 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



had penetrated over 600 feet of hard rock.^ Mr Chalmers, the present 

 superintendent, writes me that while the rocks in these mines are de- 

 composed in places to a depth of 180 feet, he has cut " the mineral at 

 one point only a very few feet below the surface, and it was not in the 

 least affected." 



Hunt is of the opinion that where the rocks are deeply decomposed it 

 is simply because the soft materials have not been removed, and " that 

 present climatic differences have nothing to do with the fact that similar 

 rocks are in one area covered with a thick la3^er of the products of decay, 

 and in another are wholly destitute of it." j 



It is not my purpose to discuss this question. It certainly goes with- 

 out saying that rock decay has been going on ever since land and water 

 existed and that the Paleozoic and other rocks overlying the crystalline 

 rocks of Brazil were formed from the residua of these original foundation 

 rocks of that part of the world. Where the Tertiary beds are seen rest- 

 ing against the granites, near the mouth of the Rio Formoso, state of 

 Pernambuco, the granites are decayed, but it cannot be said positively 

 whether that decay took place before or after the deposition of the 

 Tertiary beds. At the base of the Serra d'ltabaiana the Paleozoic (?) 

 beds rest upon gneiss, which is softened in some places and in others it 

 is not, while the overlying sediments are quite hard. But it is to be 

 expected that in passing through the beach condition the soft clays pro- 

 duced by decay of the underlying gneisses would be pretty completely 

 removed. 



Land-slides. — An evidence and result of the widespread and profound 

 decay of rocks in the mountainous regions of Brazil is the prevalence 

 of land-slides. Land-slides are much more common in that country than 

 in the temperate regions. These slides are common all over the country, 

 but they are especially so in the regions of cr3'stalline rocks of the Serra 

 do Mar. They are more frequent along railways and about cities, where 

 the removal of earth at the foot of the slopes has disturbed somewhat 

 the natural equilibrium. They are by no means confined to such places, 

 however, but occur also in the remote forests. Such land-slides were 

 formerly of more frequent occurrence in and about cities than they are 

 nowadays, for the following reasons : The decomposed rock, when dry 

 or not unusually wet, has sufficient cohesion to stand in vertical cuts 

 20 or 30 feet or more in depth. Before experience was had of the un- 

 trustworthy nature of such excavations they were frequently made in the 

 sides of decomposed hills and houses were built in or near them. 



* Forty-second Annual Report Saint John del Rey Mining Company, pp. 5 and 9. 

 t The decay of rocks geologically considered. T. Sterry Hunt. Amer. Jour. Sci., vol. xxvi, 1883, 

 p. 190. 



