29-J: J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL, 



also well illustrated with certain limestones. The dark, compact lime- 

 stones have a very thin coat of decayed rock, often not more than a mil- 

 limeter, while the coarse grained marbles of Tennessee and Arkansas are 

 often affected to a depth of several feet. I suppose this to be due to the 

 greater plane surface of the larger crystals and to the uneven contraction 

 and expansion of these big crystals. 



Pumpell}'' has expressed the same view in regard to rock deca}'' in gen- 

 eral. ^ There are a few facts, however, that do not seem to be in accord 

 with this theor3\ The rock in the sunnnits of some of the high, l)old 

 peaks are very coarse grained. Such are the rocks in the summit of 

 the Corcovado and of the Sugar Loaf. 



The cause of fluting. — The fluting described on page 280 is produced 

 solely by surface disintegration and removal by rain water. It is com- 

 parable to the weathering we sometimes see on ver}" compact and homo- 

 geneous limestones. The changes of temperature tend to loosen the im- 

 mediate surface, and loosening and decomposition are aided by nitric 

 and carbonic acids falling upon the rocks in the frequent hot rains of the 

 countr3^ The water falling upon the rocks tends to flow straight down 

 the surface, and these little channels are simply produced b}^ tbe removal 

 of the surface as fast as it is loosened. 



ORGANIC CHEMICAL AGENCIES. 



General discussion. — In the decomposition of rocks in Brazil chemical 

 agencies are vastly more important, more varied and more active than 

 mechanical ones. This is due parti}'' to the enormous quantities of both 

 organic and inorganic acids washed over and into tbe rocks by the rains, 

 and partly to the high temperature of the waters and acids that attack 

 them. It is important to remember in this connection that the rains are 

 most abundant in that countiy during the hot season, at a time when the 

 rocks are exposed alternately to the direct rays of a blazing sun and the 

 rains, some of them drizzling and lasting for days ; others poured down 

 in torrents, f 



Whatever chemical agents fall in these rains or are picked u}) on the 

 rock surfaces are rendered more active by the water being heated by the 

 hot rocks upon which it falls and over which it flows. X 



*The relation of secular rock disintegration to loose glacial drift and rock basins. R. Pumpelly. 

 Amer. Jour. Sci., 3d ser., vol. xvii, 1879, p. 130. 



t Von Tscluidi says: "The severe nocturnal downpours followed by oppressively hot days dis- 

 integrate even the hard rocks in an extremely short time." Die Brasilianische Provinz Minas 

 Geraes. Gotha, 186-2, p. 5. 



I Agassiz: Journey in Brazil, p. 89. Poiseuille's Recherehes experimentales sur le mouvement 

 des liquides dans les tubes de tr6s petits diamfetres (Comptes Rend., 1840, xi, p. 1048; 1841, xii, pp. 

 112-115) show that liquids flow through capillaries three times as fast at 113 degrees Fahrenheit as 

 they do at 32 degrees. His later observations show, however, that most of the natural waters are 

 much retarded by the minerals they hold in solution (Compt. Rend., 1847, xxiv, pp. 107G, 1077). 



