THEIR OEI&IJT. 
IMA 
*be granitic rock, under the double influence of humidity 
ar >d the tropical sun, how is it to he conceived that these 
°xides are spread so uniformly over the whole surface of 
stony masses, and are not more abundant round a 
Cr ystal of mica or hornblende than on the feldspar and 
milky quartz? The ferruginous sandstoues, granites, and 
garbles, that become cinereous and sometimes brown in 
camp air, have an aspect altogether different. In reflecting 
Cpon the lustre ancl equal thickness of the crusts, we are 
rather inclined to think that this matter is deposited by the 
'Orinoco, and that the water has penetrated even into the 
clefts of the rocks. Adopting this hypothesis, it may he 
asked whether the river holds the oxides suspended like 
®and and other earthy substances, or whether they are 
° u nd in a state of chemical solution. The first supposition 
18 less admissible, on account of the homogeneity of the 
'-r, usts, which contain neither grains of sand, nor spangles 
°1 mica, mixed with the oxides. We must then recur to 
lle idea of a chemical solution ; and this idea is no way at 
ariance with the phenomena daily observable in our labo- 
ra tories. The waters of great rivers contain carbonic acid ; 
ri! ‘> were they even entirely pure, they would still be 
capable, in very great volumes, of dissolving some portions 
°xide, or those metallic hydrates which are regarded as 
11 c least soluble. The mud of the Nile, which is the 
cdinient of the matters which the river holds suspended, 
8 destitute of manganese ; but it contains, according to the 
‘naly s j s 0 f Regnault, six parts in a hundred of oxide 
^ lr on ; and its colour, at first black, changes to yellowish 
c r °ivn by desiccation and the contact of air. The mud 
^nsequently is not the cause of the black crusts on the 
tli ! S Syene. Berzelius, who, at my request, examined 
cse crusts, recognized in them, as in those of the gra- 
e » of the Orinoco and River Congo, the union of iron and 
(.jC-’Aanesc. That celebrated chemist was of opinion that 
e , l ivers do not take up these oxides from the soil over 
ter - ^ e y d° w ) but that they derive them from their sub- 
''Uiean sources, and deposit them on the rocks in the 
ti e auer of cementation, by the action of particular affini- 
perhaps by that of the potash of the feldspar. A long 
•Pence at th'e cataracts of the Orinoco, the Nile, and the 
