22 



Herrmann, to fifteen or twenty parts in a hun- 

 dred. The hornblende contains also some car- 

 bon*, like the Lydian stone and kieselschiefer. 

 Now if these black crusts were formed by a slow 

 decomposition of the granitic rock, under the 

 double influence of humidity and the tropical 

 sun, how is it to be conceived, that these oxyds 

 are spread so uniformly over the whole surface 

 of the stony masses, and are not more abun- 

 dant round a crystal of mica or hornblende, 

 than on the feldspar and milky quartz 5 The 

 ferruginous sand-stones, granites, and marbles, 

 that become cinereous and sometimes brown in 

 damp air, display an aspect altogether different. 

 In reflecting upon the lustre and equal thick- 

 ness of the crusts, we are rather inclined to 

 think, that this matter is deposited by the Oroo- 

 noko, and that the water has penetrated even 

 into the clefts of the rocks. Adopting this hypo- 

 thesis, it may be asked, whether the river hold 

 the oxyds suspended like sand, and other earthy 

 substances, or they be found in a state of che- 

 mical solution. The first supposition is less 

 admissible, on account of the homogeneity of 

 the crusts, which contain neither grains of sand, 

 nor spangles of m ica, mixed with the oxyds . We 

 must then recur to the idea of a chemical solu- 



* Hoffmann und Breithaupt, Mineralogie, 1815, Bz. Abth. 

 % p. 120 and 151. 



