RATE OF DECOMPOSITION. 313 



ened by the decay of the feldspar. It is doubtful, however, whether 

 these cases of the decay of building stones can be taken in evidence. 

 The walls and buildings first constructed of gneiss in Brazil were in all 

 probability made of rock from the surface or from near the surface, and 

 possibly already more or less affected. 



The most marked evidences of decay are along the joints between sep- 

 arate blocks in some of the old buildings. Here there is often a round- 

 ing off of the corners and a crumbling of the mortar. Doorposts and 

 pillars of stone are sometimes affected at the bottom and not at the top, 

 due, no doubt, to the greater amount of organic acids and moisture 

 reaching these lower points. Such cases, however, are not to be taken 

 into consideration in the discussion of rocks under natural conditions. 

 It is certainly true that the agents of rock decay are much more active in 

 Brazil than in cooler climates. Caldcleugh expressed the opinion years 

 ago that in Europe " the agents of destruction are feeble compared with 

 those of a tropical country."* 



It is also to be noted that waters falling upon and flowing over these 

 bare rocks are, to begin with, unsaturated and therefore have greater 

 dissolving power than spring or stream water. 



Kesume. 



1. Decomposition is widespread and deep ; depths of 100 feet are com- 

 mon ; some of more than 300 feet are known. 



2. Land-slides caused by deep decay are abundant. 



3. Decomposition is not universal, and its absence is especially to be 

 noted in the Cretaceous and Tertiary areas. 



4. Talus slopes are rare. 



5. Mountains and peaks of gneiss and granite exfoliate like enormous 

 boulders of decomposition, producing a characteristic topography which 

 often resembles glaciated surfaces and roches mountonees. 



6. The fragments of nearly all the massive homogeneous rocks tend to 

 exfoliate. 



7. Openly exposed blocks of massive crystalline rocks sometimes 

 weather in trenches or in fluted boulders. 



8. Changes of temperature cause the openly exposed rocks to crack 

 and to exfoliate. But little decomposition is caused by the direct action 

 of changes of temperature in Brazil, but they open crevices in the rocks 

 which admit moisture and acids — the principal agencies of rock decay. 



9. The daily range of temperature sometimes amounts to more than 

 100 degrees Fahrenheit. 



* On the geology of Rio de Janeiro. Alexander Caldcleugh. Trans. Geol. Soc, 2d ser., vol. ii, 

 1S29, p. G9. 



