2 14 OcK-I2f CKUSTATION8. 



like the rocks of the Orinoco, a glossy surface, of a blackish* 

 \?rey, or almost leaden colour, and of which some of the 

 fragments seem coated with tar. Recently, in the un* 

 fortunate expedition of Captain Tuckey, the English natu- 

 ralists were struck with the same appearance in the yellalas 

 (rapids and shoals) that obstruct the river Congo or Zaire. 

 J)r. Koanig has placed in the British Museum, beside the 

 syenites of the Congo, the granites of Atures, taken from a 

 series of rocks which were presented by M. Bonpland and 

 myself to the illustrious president of the Royal Society of 

 London. "These fragments," says Mr. Koenig, "alike re- 

 semble meteoric stones ; in both rocks, those of the Orinoco 

 and of Africa, the black crust is composed, according to the 

 analysis of Mr. Children, of the oxide of iron and man- 

 ganese." Some experiments made at Mexico, conjointly with 

 Senor del Rio, led me to think that the rocks of Atures, 

 which blacken the paper in which they are wrapped,* contain, 

 besides oxide of manganese, carbon, and supercarburetted 

 iron. At the Orinoco, granitic masses of forty or fifty feet 

 thick are uniformly coated with these oxides ; and, howevei 

 thin these crusts may appear, they must nevertheless contain 

 pretty considerable quantities of iron and manganese, since 

 they occupy a space of above a league square. 



It must be observed that all these phenomena of colora- 

 tion have hitherto appeared in the torrid zone only, in rivers 

 that have periodical overflowings, of which the habitual 

 temperature is from twenty-four to twenty-eight centesimal 

 degrees, and which flow, not over gritstone or calcareous 

 rocks, but over granite, gneiss, and hornblende rocks. 

 Quartz and feldspar scarcely contain five or six thousandths 

 of oxide of iron and of manganese ; but in mica and horn- 

 blende these oxides, and particularly that of iron, amount, 

 according to Klaproth and Herrmann, to fifteen or twenty 

 parts in a hundred. The hornblende contains also some 

 carbon, like the Lydian stone and kieselschiefer. Now, if 

 these black crusts were formed by a slow decomposition of 



* 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 pro- 

 vince of Choco. Having been wrapped up in white paper during a journey 

 of several months, they left a black stain, like that of plumbago or super- 

 carburetted iron. 



