Quartz And Feldspar Scarcely Contain Five Or Six Thousandths Of Oxide Of Iron And Of Manganese; But In Mica And Hornblende These Oxides, And Particularly That Of Iron, Amount, According To Klaproth And Herrmann, To Fifteen Or Twenty Parts In A Hundred.

The hornblende contains also some carbon, 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 oxides are spread so uniformly over the whole surface of the stony masses, and are not more abundant round a crystal of mica or hornblende than on the feldspar and milky quartz? The ferruginous sandstones, granites, and marbles, that become cinereous and sometimes brown in damp air, have an aspect altogether different. In reflecting upon the lustre and 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 be asked whether the river holds the oxides suspended like sand and other earthy substances, or whether they are found in a state of chemical solution. The first supposition is less admissible, on account of the homogeneity of the crusts, which contain neither grains of sand, nor spangles of mica, mixed with the oxides. We must then recur to the idea of a chemical solution; and this idea is no way at variance with the phenomena daily observable in our laboratories.

Enter page number Go  Previous Next
Page 368 of 777
Words from 99529 to 99791 of 211397