THE VOYAGE OF THE BEAGLE 23 



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

 tered the sea, I observed a fact connected with a subject dis- 

 cussed by Humboldt. 7 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." Here the coat- 

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

 to be composed of ferruginous matter alone. Hand speci- 

 mens 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 cat- 

 aracts in the great rivers. In like manner, the rise and fall 

 of the tide probably answer to the periodical inundations; 

 and thus the same effects are produced under apparently dif- 

 ferent but really similar circumstances. The origin, how- 

 ever, of these coatings 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 remaining the 

 same. 



One day I was amused by watching the habits of the 

 Diodon antennatus, which was caught swimming near the 

 shore. This fish, with its flabby skin, is well known to pos- 

 sess the singular power of distending 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 

 quantity both of water and air is absorbed by the mouth, 

 and perhaps likewise by the branchial orifices. This process 

 is effected by two methods : the air is swallowed, and is then 

 forced into the cavity of the body, its return being prevented 

 by a muscular contraction which is externally visible: but 

 the water enters in a gentle stream through the mouth, 

 which is kept wide open and motionless; this latter action 

 must, therefore, depend on suction. The skin about the 



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



