16 BAHIA BRAZIL. 



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

 entered the sea, I observed a fact connected with 

 a subject discussed by Humboldt* At the cata- 

 racts 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 plum- 

 bago. 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." Here the coating is 

 of a rich brown instead of a black colour, and seems 

 to be composed of ferruginous matter alone. Hand 

 specimens fail to give a just idea of these brown 

 burnished stones which glitter in the sun's rays. 

 They occur only within the limits of the tidal 

 waves ; and as the rivulet slowly trickles down, the 

 surf must supply the polishing power of the cata- 

 racts in the gi'eat rivers. In like manner, the rise 

 and fall of the tide probably answer to the period- 

 ical inundations; and thus the same effects are pro- 

 duced under apparently different but really similar 

 circumstances. The origin, however, of these coat- 

 ings of metallic oxides, which seem as if cemented 

 to the rocks, is not understood ; and no reason, I 

 believe, can be assigned for their thickness remain- 

 ing the same. 



One day I was amused by watching the habits 

 of the Diodon antennatus, which was caught swim- 

 ming near the shore. This fish, with its flabby skin, 

 is well known to possess the singular power of dis- 

 tending itself into a nearly spherical form. After 

 having been taken out of water for a short time, 

 and then again immersed in it, a considerable quan- 

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



