290 



J. C. BKANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



centric layers come away in large masses, which break up in sliding down 

 the mountain sides and exfoliate at the base, where they look like accu- 

 mulations of gigantic boulders. Where a single fragment remains be- 

 hind, as in the case of tlie southwest side of the Sugar Loaf mentioned, 

 held in place by its. upper extremit3% and being left free to expand on 

 these sides, it may remain thus suspended a long time, less liable to be 

 broken off than if it were surrounded by other portions of the same 

 layer, which might serve as levers to break it. 



It often happens that the concentric mass covering a hill of gneiss or 

 granite has its lower portions broken away, leaving suspended from the 

 upper part of the hill a square-faced mantle-like covering. This cover- 

 ^ ~g ing, in the ordinary process of weath- 

 ering, has its sharper angles first 

 removed by exfoliation, which is- 

 always deepest on these edges, and 

 then this same process gradually re- 

 moves the greater part of the promi- 

 nence. 



The rounding of the sharp angles 

 of rocks is due to their greater ab- 

 sorption of both heat and moisture 

 tluin any equal area of the surface 

 at any other part. 



Let A B C represent in section the 

 surface of an angular piece of stone- 

 If surfaces of a given size receive amounts of heat equal to a:, the surface 

 of R and P would each receive the x temperature, while that of would 

 receive 2x, or twice that amount. The resultant of the penetration of x 

 upon would be in the direction B N, and therefore tlie extra amount 

 of heat in would penetrate deepest in the direction B N:^ Except that 

 the process is reversed, cooling takes place exactly like heating — that is, 

 radiation from the angles is greater than from equal bulk at other parts 

 of the exposed fiices. It follows, also, that the more acute the angles of 

 the block the greater will be the absorption and radiation and hence the 

 deeper will the isotherm of a given temperature penetrate on the angle. 

 I endeavored to ascertain ^vhether the obliquity of the solar rays 

 against the surfaces of the granite hills about Rio has made any appre- 

 ciable difference in their topography. Rio de Janeiro being in south 

 latitude 23°, the sun is to the north during almost all the year. Its rays 

 therefore fall against the north slopes of these conical hills with but little 





'R 











P 



N 







Figure 6,—Diae:ya»i iUnstratin^ the exfoliat- 

 ing Effect of changing Tetnperatnre. 



*In his Geological Reconnoissanee in California, p. 146, W. P. Blake has a diagram illustrating 

 this process of exfoliation by decomposition of sandstone blocks. See also M. Tuomy : Report oa 

 the Geology of South Carolina. Columbia, 1848, p. G3. 



