BOCJC-INCEUSTATIOXS. 
213 
a physical phenomenon lately observed in differ nit parts 
ot the globe, and not yet sufficiently explained. Among 
e cataracts, and wherever the Orinoco, between the Mis- 
sions of Carichaua and of Santa Barbara, periodically washes 
le granitic rocks, they become smooth, black, and as if 
coated with plumbago. The colouring matter does not 
Penetrate the stone, which is coarse-grained granite, con- 
fining a. few solitary crystals of hornblende. Taking a 
general view of the primitive formation of Atures, we per- 
_ eive that, like the granite of Syene in Egypt, it is a granite 
hornblende, and not a real syenite formation. Many 
Ule layers are entirely destitute of hornblende. The black 
Ust is 0-3 of a lme in thickness; it is found chiefly on the 
IL: ,rtz °s e parts. The crystals of feldspar sometimes pre- 
' c externally their reddish-white colour, and rise above 
t j e , bl aok crust. On breaking the stone with a hammer, 
con 1DSld ® 1S f ° Un<i t0 be white > and without any trace of de- 
tim P 08 '^ 011- L'i^se enormous stony masses appear some- 
lesm rhombs, sometimes under those hemispheric forms 
fj,f ttbar to granitic rocks when they separate in blocks.' 
Col e - v gwe the landscape a singularly gloomy aspect; their 
ri ; 0ur being in strong contrast with that of the foam of the 
ti® 1 '"'hich covers them, and of the vegetation by which 
‘lm J ''^ 0 surrou nded. The Indians say, that the rocks are 
the Ut> ( or carbonized) ‘by the rays of the sun.? We saw 
as ? not onl T tbe bed of the Orinoco, but in some spots 
heirrh' as 11 vc hundred toises from its present shore, on 
£nvfY ts '"'bich the waters now never reach even in their 
W, USt s wellings. 
r oeK lat . is this brownia b hlack crust, which gives these 
w P ’ • en th ®y have a globular form, the appearance of 
th e ' Jlac stones '■ "What idea can we form of the action of 
e *ta» a r ’ wbicb P roduces a deposit, or a change of colour, so 
thi s °f dmar . v ? We must observe, in the first place, that 
len °inenon does not belong to the cataracts of the 
1 ' e turi C< ? ‘d° ne ’ bu l' I s found in both hemispheres. At my 
Atm.„ 10 U1 . Mexico in 1807, when I showed the granites of 
° v er +i aud blay pores to M. Koziere, who had travelled 
. ol E ?ypt, the coasts of the Bed Sea, and 
the . na h ’•'bis learned geologist pointed out to me that 
t ittutive rocks of the little cataracts )f Syene disphv 
K 2 
