21 



made at Mexico, conjointly with Mr. del Rio, 

 had led me to think, that the rocks of Atures, 

 which blacken the paper* in which they are 

 wrapped, contain, beside oxyd of manganese, 

 carbon, and supercarburetted iron. At the 

 Oroonoko, granitic masses of forty or fifty feet 

 thick are uniformly coated with these oxyds ; 

 and, however thin these crusts may appear, 

 they must nevertheless contain pretty consider- 

 able quantities of iron and manganese, since they 

 occupy a space of above a league square. 



It must be observed, that all these pheno- 

 mena of coloration have hitherto appeared in 

 the torrid zone only, in rivers that have periodi- 

 cal overflowings, of which the habitual tempera- 

 ture is from twenty-four to twenty-eight cente- 

 simal degrees, and which flow, not over grit- 

 stone, or calcareous rocks, but over granite, 

 gneiss, and hornblende rocks-f-. Quartz and 

 feldspar scarcely contain five or six thousandths 

 of oxyd of iron and of manganese ; but in mica 

 and hornblende these oxyds, and particularly 

 that of iron, amount, according to Klaproth and 



* I remarked the same phenomenon from spongy grains of 

 platina one or two lines in length, collected at the stream- 

 works of Taddo, in the province of Choco. Wrapped up in 

 white paper during a journey of several months, they had ie'ft 

 a black stain, like that of plumbago or supercarburetted 

 iron. 



t Horneblendcgestein. 



