302 J. C. BRANNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



the surface waters.* Thej?- also help to keep the surface of the rock moist 

 and thus increase the radiation at nio-ht and enable the acids to attack 

 the rock. Besides, they color the surface of the rock black or dark brown, 

 and this color causes the rock to absorb more heat than it could other- 

 wise do. It is worthy of note in this connection that vegetation, except 

 upon the bare surfaces, while it hastens decomposition, retards denuda- 

 tion. Scouler says that " m the vicinity of the coast this rock is pro- 

 tected from the influences of the weather by the dense vegetation which, 

 covers the soil."t This. is true only in so far as it relates to denudation. 



It has been pointed out by Corenwinder that the sun shining upon, 

 decayed organic matter develops carbonic acid. X 



Decomposition of the rocks seems to take place most rapidly and ex- 

 tensively where drainage is sluggish, denudation but slight and the soil 

 supi:)orts a rank vegetation and where the decay of organic matter is most 

 rapid. Water falling upon such a surflxce is impeded by the vegetation,, 

 and, charged with organic acids, it soaks into the ground and performs 

 its work of rock destruction. Hartt was of the opinion that deep decom- 

 position was confined to regions which are or have been covered by 

 forests. He says : 



*'I believe that the remarkable decomposition of the rocks in Brazil has taken 

 place only in regions anciently or at present covered by forests." § 



Julien expresses the opinion that — 



"Both in Brazil and in onr sonthern states the main a^jents of disintegration 

 have been mainly (lerived . . . from neither atmospheric nor snbterranean 

 sources, but from the heavy forest-growth which preceded the inroads of her pres- 

 ent civilization." 



This is a theory which commends itself to ever}^ geological observer in 

 Brazil. There are so many instances of deep decay, however, in regions 

 now not forest-covered, and where it is impossible to say positively 

 whether they have ever been so covered, that one is obliged to own that 

 it cannot be either proven or disproven. 



It is generally understood that the more watery vegetable substances 

 decay more rapidly. This heavy, spongy and watery nature is charac- 

 teristic of the greater part of tropical plants, and especially of those form- 

 ing the undergrowth. 



*0n the geological action of the humus acids. Alexis A. Julien. Proc. Am. Assoc. Adv. Sci., 

 vol. 28, 1879, pp. 311-410. How Crops Feed. 8. W. Johnson. P. 138. 



tThe Edinburgh Journal of Science, vol. v, October, 1826, p. 201. Gilbert's statement of the 

 geological influence of vegetation is well illustrated by Brazilian rocks. Geology of the Henry 

 Mountains. G. K. Gilbert. Pp. 104, 105. 



I La production du gaz acide carbonique par le sol. M. B. Corenwinder. Comptes Rendus, 41,. 

 1855, pp. 149-151. 



g Hartt's Geology and Physical Geography of Brazil, pp. 25, 319. 



