240 NOXIOUS EXHALATIONS FROM 
tion in 1748 several tribes had been assembled, which 
subsequently dispersed, and their places were sup- 
plied 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 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 falls of the Orinoco, 
the granite rocks, wherever they are periodically 
submersed, become smooth and seem as if coated 
with black lead. The crust is only 0°3 of a line in 
thickness, and occurs chiefly on the quartzy parts of 
the stone, which is coarse grained, and contains 
solitary crystals of hornblende. The same appear- 
ance is presented at the cataracts of Syene as well 
as those of the Congo. This black deposite, accord- 
ing 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 chemically dissolved 
in the water, and not from an efflorescence of mat- 
