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in the great, we perceive, that, like the granite 

 of Syene in Egypt, it is a granite with horn- 

 blende, 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 crys- 

 tals of feldspar have sometimes preserved exter- 

 nally their reddish white colour, and rise above 

 the black crust. On breaking the stone with a 

 hammer, the inside is found to be in fact white, 

 and without any trace of decomposition. These 

 enormous stony masses appear sometimes in 

 rhombs, sometimes under those hemispheric 

 forms, which are peculiar to granitic rocks when 

 they separate in blocks. They give the land- 

 scape a singularly gloomy aspect ; their colour 

 being in strong contrast with that of the foam of 

 the river which covers them, and of the vegeta- 

 tion by which they are surrounded. The Indi- 

 ans say, that the rocks are " burnt, or carboniz- 

 ed by the rays of the sun." We saw them not 

 only in the bed of the Oroonoko, but in some 

 spots as far as five hundred toises from it's pre- 

 sent 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 



c 2 



