NOXIOUS EXHALATIONS FROM THE ROCKS. 207 



forty-seven remained. At its foundation in 1748 

 several tribes had been assembled, which subse- 

 quently dispersed, and their places were supplied 

 by the Guahiboes, who belong to the lowest grade 

 of uncivilized society, and a few families of Macoes. 

 The epidemic fevers, which prevail here at the com- 

 mencement of the rainy season, contributed greatly 

 to the decay of the establishment. This distemper 

 is ascribed to the violent heats, excessive humidity 

 of the air, bad food, and, as the natives believe, to 

 the noxious exhalations that rise from the bare rocks 

 of the rapids. This last is a curious circumstance, 

 and, as Humboldt remarks, is the more worthy of 

 attention on account of its being connected with a 

 fact that has been observed in several parts of the 

 world, although it has not yet been sufficiently ex- 

 plained. 



Among the cataracts and falls of the Orinoco, the 

 granite rocks, wherever they are periodically sub- 

 mersed, become smooth, and seem as if coated with 

 black lead. The crust is only 0*3 of a line in thick- 

 ness, and occurs chiefly on the quartzy parts of the 

 stone, which is coarse-grained, and contains solitary 

 crystals of hornblende. The same appearance is 

 presented at the cataracts of Syene as well as those 

 of the Congo. This black deposite, according to 

 Mr. Children's analysis, consists of oxide of iron 

 and manganese, to which some experiments of 

 Humboldt induced him to add carbon and super- 

 carburetted iron. The phenomenon has hitherto 

 been, observed only in the torrid zone, in rivers that 

 overflow periodically and are bounded by primitive 

 rocks, and is supposed by our author to arise from 

 the precipitation of substances chymically dissolved 

 in the water, and not from an efflorescence of mat- 

 ters contained in the rocks themselves. The Indians 

 and missionaries assert, that the exhalations from 

 these rocks are unwholesome, and consider it dan- 

 gerous to sleep on granite near the river ; and our 



