24 



to examine these crusts, and recognized in them, 

 as in those of the granites of the Oroonoko and 

 Rio Congo, the union of iron and manganese. 

 This celebrated chemist thinks, that the rivers 

 do not take up these oxyds from the soil over 

 which they flow, but derive them from their 

 subterraneous sources, and deposit them on the 

 rocks in the manner of cementation, by the 

 action of particular affinities, perhaps by that 

 of the potash of the feldspar. A long residence 

 at the cataracts of the Oroonoko, the Nile, and 

 the Rio Congo, and an examination of the cir- 

 cumstances that accompany this phenomenon 

 of coloration, could alone lead to the complete 

 solution of the problem we have discussed. Is 

 this phenomenon independent of the nature of 

 the rocks ? I shall content myself with observ- 

 ing in general, that neither the granitic masses 

 remote from the ancient bed of the Oroonoko^ 

 but exposed during the rainy season to the al- 

 ternations of heat and moisture, nor the grani- 

 tic rocks bathed by the brownish waters of the 

 Rio Negro, assume the appearance of meteoric 

 stones. The Indians say, " that the rocks are 

 black only where the waters are white." They 

 ought perhaps to add, " where the waters ac- 

 quire great swiftness, and strike with force 

 against the rocks of the banks." Cementation 

 seems to explain why the crusts augment so 

 little in thickness. 



