EPIDEMIC FEVERS. 207 



forty-seven remained. At its foundation m 1748 several chap.xviii 



tribes had been assembled, which subsequently dispersed, j,jss~ ~ 



and their places were supplied by the Guahiboes, who Btation. 



belong to the lowest grade of uncivilized society, and a 



few families of Macocs. The epidemic fevers, which 



prevail here at the commencement 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 explained. 



Among the cataracts and ftills of the Orinoco, the Cataracts of 

 granitic rocks, wherever they are periodically submersed, '^^ Orinoco. 

 become smooth, and seem as if coated with black lead. 

 The crust is only 0"8 of a line in thickness, and occurs 

 chiefly on the quartzy parts of the stone, which is 

 coarse-grained, and contains solitary crystals of horn- 

 blende. The same appearance is presented at the cata- 

 racts 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 supercar- 

 buretted iron. The phenomenon has hitherto been singular 

 observed only in the torrid zone, in rivers that overflow piienomenon. 

 periodically and are bounded by primitive rocks, and i=! 

 supposed by our author to arise from the precipitation 

 of substances chemically dissolved in the water, and not 

 from an efflorescence of matters contained in the rocks 

 themselves. The Indians and missionaries assert, that 

 the exhalations from these rocks are unwholesome, and 

 consider it dangerous to sleep on granite near the river ; 

 and our travellers, without entirely crediting this asser- 

 tion, usually took care to avoid the black rocks at night. 

 But the danger of reposing on them, Humboldt thinks, 

 may rather be owing to the very great degree of warmtli 



