12 



BAHIA— BRAZIL. 



Feb. 1832. 



absorbed or evaporated before it reached the ground. I 

 will not at present attempt to describe the gaudy scenery of 

 this noble bay, because, in our homeward voyage, we called 

 here a second time, and I shall then have occasion to re- 

 mark on it. 



The geology of the surrounding country possesses little 

 interest. Throughout the coast of Brazil, and certainly for 

 a considerable space inland, from the Rio Plata to Cape 

 St. Hoque, lat. 5° S., a distance of more than 2000 geo- 

 graphical miles, wherever solid rock occurs, it belongs 

 to a granitic formation. The circumstance of this enor- 

 mous area being thus constituted of materials, which almost 

 every geologist believes to have been crystallized by the 

 action of heat under pressure, gives rise to many curious 

 reflections. Was this effect produced beneath the depths 

 of a profound ocean ? or did a covering of strata formerly 

 extend over it, which has since been removed? Can we 

 believe that any power, acting for a time short of infinity, 

 could have denuded the granite over so many thousand 

 square leagues ? 



On a point not far from the city, where a rivulet en- 

 tered the sea, 1 observed a fact connected with a subject 

 discussed by Humboldt.* At the cataracts of the great 

 rivers Orinoco, Nile, and Congo, the syenitic rocks are 

 coated by a black substance, appearing as if they had been 

 polished with plumbago. The layer is of extreme thinness; 

 and on analysis by Berzelius it was found to consist of 

 the oxides of manganese and iron. In the Orinoco it 

 occurs on the rocks periodically washed by the floods, and 

 in those parts alone, where the stream is rapid ; or, as the 

 Indians say, the rocks are black, where the waters are 

 white." The coating is here of a rich brown instead of 

 a black colour, and seems to be composed of ferrugineous 

 matter alone. Hand specimens fail to give a just idea of 

 these brown, burnished, stones which glitter in the sun^srays. 

 ft 



* Pers. Narr,, vol. v., pt.i., p. 18. 



