INFLUENCES AFFECTING EXFOLIATION. 289 



eventually destroy the entire mass — plants and animals, acidulated waters 

 and gases. ^ 



It must be, too. that the slight changes of temperature which peiietrate 

 to a depth of 15 feet are more potent than similar changes would be at 

 the surface, for the rocks are there confined and are not at liberty to seek 

 relief in several directions ; but whether these changes of temperature 

 seem competent to produce exfoliation there are certain cases which can- 

 not, in my opinion, be explained in any other way. 



Where large flat surfaces of gneiss are openly exposed to the sun's rays 

 great flal?:es or shells are sometimes bulged and lifted away from the 

 cooler mass below. Where roads pass over such places they give forth 

 a hollow sound to the horses' hoofs. 



In the stone quarries of Rio de Janeiro these sheets of rocks are some- 

 times 15 feet thick and are utilized by the quarrymen in breaking out 

 blocks of convenient sizes.* These sheets, however, are more commonly 

 from 2 to 10 feet thick, and they are often as thin as a knife blade. 



The thicker masses on peaks and mountain sides generally seem to 

 come to a feather edge on the down-hill side and to cover the upper por- 

 tion of the hill like a close-fitting cap. The breaking off of the lower 

 edge of these flakes gives rise to a rock form quite common in the gneiss 

 region along the coast. It is shown in figure 1, page 270. 



I have said elsewhere that exfoliation is not produced by changes of 

 temperature alone, and the idea that these great layers may have been 

 produced by other agencies has not been overlooked. But the other 

 agencies producing exfoliation are chemical, and I know of no way in 

 which water can get into many of these cracks, and there is certainly no 

 evidence of decay along their edges where they are freshly exposed. 

 Except in this respect, the exfoliation of mountain masses takes place 

 in precisely the same way as that of boulders, large and small. 



The process of exfoliation in Brazil is not essentially different from 

 that common in other parts of the world, but it is doubtless hastened 

 and given certain characteristics by some of the peculiarities of a tropical 

 climate. One of the finest examples of this exfoliation is to be seen upon 

 the southeastern face of the Pao d'Assucar (Sugar Loaf) (figure 2, page 

 271), the majestic peak which stands at the entrance of the harbor of 

 Rio de Janeiro. The last disintegrating layer that has fallen away from 

 this peak has left behind and clinging to its sides a few remnants of its 

 original thickness, the lower surface and sides being broken off squarely. 



In cases of exfoliation of large steep-faced masses, such as the moun- 

 tains about Rio de Janeiro, Theresopolis and Nova Friburgo, the con- 



*One of the best illustrations of this kind of exfoliation I have seen is given in the Twelfth 

 Annual Report of the State Mineralogist of California, 1894, p, 384. The exfoliated sheet of granite 

 worked in the Raymond quarry is from 5 to 15 feet thick. 



