BOCK-IXCRUSTATIOyS. 213 



R physical phenomenon lately observed in differ 3iit parts 

 of the globe, and not yet sufficiently explained. Among 

 the cataracts, and wherever the Orinoco, between, the Mis- 

 sions of Carichana and of Santa Barbara, periodically washes 

 the granitic rocks, they become smooth, black, and as il 

 coated with plumbago. The colouring matter does not 

 penetrate the stone, which is coarse-grained granite, con- 

 taining a few solitary crystals of hornblende. Taking a 

 general view of the primitive formation of Atures, we per- 

 ceive, that, like the granite of Syene in Egypt, it is a granite 

 with hornblende, and not a real syenite formation. Many 

 of the layers are entirely destitute of hornblende. The black 

 crust is 0'3 of a line in thickness; it is found chiefly on the 

 quartzose parts. The crystals of feldspar sometimes pre- 

 serve externally their reddish-white colour, and rise above 

 the black crust. On breaking the stone with a hammer, 

 the inside is found to be white, and without any trace of de- 

 composition. These enormous stony masses appear some- 

 times in rhombs, sometimes under those hemispheric forms, 

 peculiar to granitic rocks when they separate in blocks. 

 They give the landscape a singularly gloomy aspect; their 

 colour being in strong contrast with that of the foam of the 

 river which covers them, and of the vegetation by which 

 they are surrounded. The Indians say, that the rocks are 

 'burnt' (or carbonized) 'by the rays of the sun.' We saw 

 them not only in the bed of the Orinoco, but in some spots 

 as far as five hundred toises from its present shore, on 

 heights which the waters now never reach even in their 

 greatest swellings. 



What is this brownish black crust, which gives these 

 rocks, when they have a globular form, the appearance of 

 meteoric stones ? What idea can we form of the action of 

 the water, which produces a deposit, or a change of colour, so 

 extraordinary ? We must observe, in the first place, that 

 this phenomenon does not belong to the cataracts of the 

 Orinoco alone, but is found in both hemispheres. At my 

 return from Mexico in 1807, when I showed the granites of 

 Atures and Maypures to M. Koziere, who had travelled 

 over the valley ot Egypt, the coasts of the Bed Sea, and 

 Mount Sinai, this learned geologist pointed out to me tha* 

 the primitive rocka of the little cataracts )f Syene displq? 



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