THEIR ORIGIN*. 5Mt"> 



the granitic rock, under the double influence of humidity 

 and the tropical sun, how is it to be conceived that these 

 oxides are spread so uniformly over the whole surface of 

 the stony masses, and are not more abundant round a 

 crystal of mica or hornblende than on the feldspar and 

 milky quartz? The ferruginous sandstones, granites, and 

 marbles, that become cinereous and sometimes brown in 

 damp air, have an aspect altogether different. In reflecting 

 upon the lustre and equal thickness of the crusts, we are 

 rather inclined to think that this matter is deposited by the 

 Orinoco, and that the water has penetrated even into the 

 clefts of the rocks. Adopting this hypothesis, it may be 

 asked whether the river holds the oxides suspended like 

 sand and other earthy substances, or whether they are 

 found in a state of chemical solution. The first supposition 

 is less admissible, on account of the homogeneity of the 

 crusts, which contain neither grains of sand, nor spangles 

 of mica, mixed with the oxides. We must then recur to 

 the idea of a chemical solution ; and this idea is no way at 

 variance with the phenomena daily observable in our labo- 

 ratories. The waters of great rivers contain carbonic acid ; 

 and, were they even entirely pure, they would still bo 

 capable, in very great volumes, of dissolving some portions 

 of oxide, or those metallic hydrates which are regarded as 

 the least soluble. The mud of the Nile, which is the 

 sediment of the matters which the river holds suspended, 

 is destitute of manganese ; but it contains, according to the 

 analysis of M. Eegnault, six parts in a hundred of oxide 

 of iron ; and its colour, at first black, changes to yellowish 

 brown by desiccation and the contact of air. The mud 

 consequently is not the cause of the black crusts on the 

 rocks of Syene. Berzelius, who, at my request, examined 

 these crusts, recognized in them, as in those of the gra- 

 nites of the Orinoco and Eiver Congo, the union of iron and 

 manganese. That celebrated chemist was of opinion that 

 the rivers do not take up these oxides from the soil over 

 which they flow, but that they derive them from their sub- 

 terranean sources, and deposit them on the rocks in the 

 Manner of cementation, by the action of particular aflini- 

 ties, perhaps by that of the potash of the feldspar. A long 

 residence at the cataracts of the Orinoco, the >ale, and the 



