280 J. C. BRAXNER — DECOMPOSITION OF ROCKS IN BRAZIL. 



There is " a confusion of rounded blocks " of diorite produced by con- 

 centric decomposition at Praia Grande on the lower Tocantins.* 



At Erere boulders of diorite are abundant.f These were mistaken b}'' 

 Agassiz for glacial boulders. J The sandstone blocks at the Serra de 

 Erere are also rounded b}^ decomposition. One great block, 50 feet high^ 

 caps the summit of the mountain on the north side.§ 



FL UTING. 



Fluting is a peculiar method of surface decay by which granites or 

 gneisses are left with a corrugated or fluted surface. I have seen several 

 examples of this kind of weathering, but the onl}^ ones of which I have 

 notes are in the state of Pernambuco. One of these is in the southwest- 

 ern 2)art of the state, on Engenho Trapiche, between the mouth of Rio 

 Formozo and the village of Serenhaem. This is a large subangular 

 fragment of granite. One side contains a dozen of these little channels, 

 from 1 to 4 inches deep and from 3 to 10 inches apart from center to 

 center. These channels run straight down the face of the rock. 



The other example is on the public road leading from the railway 

 station at Palmares to Bonito, state of Pernambuco. The granite block 

 in which it is best shown is about 40 feet high. This block is fluted in 

 the same way as that near Serenhaem, but the grooves are somewhat wider. 

 In both cases the fluted boulders are openly exposed to the sun.|| 



There is another form of disintegration somewhat resembling fluting, 

 yet decidedl}^ diff'erent from it. In this case the massive rocks are cut 

 by straight-sided channels or ravines varying in depth from a few inches 

 to more than 100 feet. These grooves run straight down the mountain 

 faces. Good examples of them are shown in figure 5, page 275. 



Agencies of Decomposition.^ 

 The agencies which produce the widespread rock decomposition in 



* A contribution lo the geology of the Lower Amazonas. O. A. Derby. Proc. Amer. Phil. Soc , 

 vol. xviii, 1879, p. 164. 



tAmazonian drift. Ch. Fred. Hartt, Amer. Jour. Sci., 3d ser., vol. i, 1871, pp. 294-296. 



X Tertiary basin of the Marailon. Ch. Fred. Hartt. Amer. Jour. Sci., 3d sen, vol. iv, 1872, p. 58. 



g Contributions to the geology and physical geography of the Lower Amazonas. Ch. Fred. 

 Hartt. Bui. Buffalo Soc. Nat. Hist., 1874, p. 216. 



II Professor Derby has told me in private conversation of having seen boulders similar to these 

 at and about the Serra de Itatiaia. 



% I have not undertaken to discuss the suggestion of Mr Darwin that the decomposition of the 

 rocks in Brazil occurred under the sea. (Geological Observations. 2d ed , London, 1876, p. 428.) 

 The agencies evidently in operation are competent to account for the work done. Besides, it is 

 not clear to my mind by what process such extensive decay could take place beneath the sea. 

 The zeolites and other minerals dredged up from the ocean are supposed to indicate such action 

 by sea water, but I know of no reason why such minerals might not be dissolved from organic re- 

 mains as they sink to the bottom. 



