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Show 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 occas10n 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. Roque, lat. 5° S., a distance of more than 2000 geographical miles, wherever solid . rock occurs, it .belongs to a granitic formation. The Circumstance of this enormous 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 entered the sea, I 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's rays. • Pers. Narr., vol. v., pt. i., p. 18. Feb. 1832. BURNISHED ROCKS. 13 They occur on1y within the limits of tidal action; and as the rivulet slowly trickles down, the surf must supply the polishing power of the cataracts in the great rivers. In the same manner, the rise and fall of the tide probably answers to the periodical inundations ; and thus the same causes are present under apparently very different circumstances. The real origin, however, 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 constant. One day I was amused by watching the habits of a Diodon, which was caught swimming near the shore. This fish is well known to possess the singular power of distending itself into a nearly s'pherical 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 was absorbed by the mouth, and perhaps likewise by the branchial apertures. 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, I observed, entered in a stream through the mouth, which was wide open and motionless : this latter action must, therefore, depend on suction. The skin about the abdomen is much looser than that of the back; hence, during the inflation, the lower surface becomes far more distended than the upper; and the fish, in consequence, floats with its back downwards. Cuvier doubts whether the Diodon, in this position, is able to swim; but not only can it thus move forward in a straight line, but likewise it can turn round to either side. This latter movement is effected solely by the aid of the pectoral fins ; the tail being collapsed, and not used. From the body being buoyed up with so much air, the branchial openings were out of water ; but a stream drawn in by the mouth, constantly flowed through them. The fish, having remained in this distended state for a |