AGENCIES OF DECOMPOSITION. 281 



Brazil are of two general classes, namely, mechanical and chemical. 

 These will be taken up in their order. 



MECHANICAL AGENCIES. 



General discussion. — The chief mechanical agencies are — 



1. Changes of temperature ; 



2. The penetration of the soil by plant roots ; 



3. The burrowing of animals. 



It has already been shown that over a large part of Brazil rock decay 

 has gone so far, especially in the parts of the country covered by a strong 

 vegetation, that the hard rocks are not now within reach of the roots of 

 plants, and where they are thus accessible the action of the roots is, so 

 far as I know, similar to that of plants in other countries. In any case, 

 the second and third of these are simply contributory in their operation 

 and will be considered under the head of chemical agencies. 



A fourth mechanical agency is that of water, as waves or running 

 streams; but while the waves along the coast and the running streams 

 do much mechanical geologic work they contribute comparatively little 

 to the mechanical destruction of the rocks in general. 



The immediate mechanical effects of changes of temperature upon a 

 rock are to produce expansion and contraction. I observe, also, that the 

 mica in the Brazilian gneisses (and the same is true when any other min- 

 eral constituent forms bands) is an element of weakness, tending both on 

 account of its low conductivity and its form to develop crevices along 

 which the rocks exfoliate more readily.* 



Before entering upon the discussion of the action of change of tempera- 

 ture I would call attention to the following facts : The unequal contraction 

 and expansion of the minerals composing the rock tend to disintegrate 

 the entire mass, while the even annual and diurnal changes and the ap- 

 proximately even penetration of these changes cause the rocks to exfoliate 

 or to spall off in layers of even thickness, like the coats of an onion, while 

 the crevices opened in the rocks admit acids and gases and set up a train 

 of reactions that sooner or later disintegrate and decompose the entire 

 rock mass. 



I have frequently observed that the decay of rock masses begins along 

 joints or cracks, from which it spreads in every direction. The cause of 

 these joints must therefore be of vital importance to the rock. 



The exfoliation of mountain masses and of boulders such as are here 

 described takes place and can take place only when the rocks are massive 

 and more or less homogeneous. Bedding planes and joints are lines of 



*It is usually supposed by those who have but little or no acquaintance with it tliat gneiss does 

 not make good building stone on account of its banded structure. This is a mistake. The cities 

 of Rio de Janeiro and Bahia are almost entirely built of gneiss, and a finer building stone, so far as- 

 durability is concerned, it would be impossible to find. 



