•_M4 
nOcK-TXCItCSTATIOIfS. 
like the rocks of the Orinoco, a glossy surface, of a blackish- 
grey, or almost leaden colour, and of which some ol 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 yellal< tS 
(rapids and shoals) that obstruct the river Congo or Zaire- 
Dr. Koenig has placed in the British Museum, beside the 
syenites of the Congo, the granites of Atures, taken from * 
series of rocks which were presented by M. Bonpland and 
myself to the illustrious president of the Royal Society o- 
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 
Sehor 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 fe et 
thick are uniformly coated with these oxides; and, liowevei 
thin these crusts may appear, they must nevertheless contain 1 
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 colon” 
tiou have hitherto appeared in the torrid zone only, in rive^ 
that have periodical overflowings, of which the habiting 
temperature is from twenty-four to twenty-eight centesii' 1 ^ 
degrees, and which flow, not over gritstone or ealcareod 
rocks, but over granite, gneiss, and hornblende roc^ 
Quartz and feldspar scarcely contain five or six thousand' ^ 
of oxide of Ron and of manganese; hut in mica and hoi ’ 
blende these oxides, and particularly that of iron, amoui' > 
according to Klaproth and Herrmann, to fifteen or twe» v 
parts in a hundred. The hornblende contains also s0 "p 
carbon, like the Lydian stone and kiesehcMefer. j 
these black crusts were formed by a slow decomposition 
* I remarked the same phenomenon from spongy grains of platina 
or two lines in length, collected at the stream-works of Taddo, in 
vinca of Clioco. Having been wrapped up in white paperdurmg ajour ^ 
of several months, they left a black stain, like that of plumbago or sul 
carburetted iron. 
