IX.] CHEMISTRY OF NATURAL WATERS. 101 



It was found, moreover, that in the case of a rock apparently 

 composed of labradorite and pyroxene, the removal of the lime 

 and magnesia from the decomposed portion was much more 

 complete than that of the alkalies ; showing thus the compara- 

 tively greater stability of the feldspathic element. The decom- 

 position of the feldspar in these mixed rocks is however at 

 length effected, and the final result approximates to a hydrous 

 silicate of alumina or clay. This slow decomposition of sili- 

 cates of protoxide-bases appears to be due to the action of car- 

 bonic acid, which, removing the lime and magnesia as carbon- 

 ates, liberates the silica in a soluble form ; while the iron and 

 manganese, passing to a state of higher oxidation, remain 

 behind, unless the action of organic matters intervenes to give 

 them solubility. 



1 3. Notwithstanding the complete decomposition, resulting 

 in the production of kaolin, to which orthoclase, in common 

 with the triclinic feldspars (and some other feldspathides, such 

 as the scapolites, beryl, and leucite), is subject, it is to be noticed, 

 that under ordinary atmospheric conditions orthoclase appears 

 less liable to change than the lime-soda feldspars such as 

 albite, oligoclase, and labradorite. Weathered surfaces of these 

 become covered with a thin, soft, white, and opaque crust 

 from decomposition, while the surfaces of orthoclase under 

 similar conditions still preserve their hardness and translucency. 

 A gradual process of this kind is constantly going on in the 

 feldspathic matters which form a large proportion of the me- 

 chanical sediments of all formations ; and in deeply buried 

 strata is not improbably accelerated by the elevation of temper- 

 ature. The soluble alkaline silicate resulting from this process 

 is in most cases decomposed by carbonates of lime and magnesia 

 in the sediments, giving rise to silicates of these bases (which 

 are for the greater part separated in an insoluble state), and to 

 carbonate of soda. Only in rare cases does potash appear in 

 large proportion among the soluble salts thus liberated from 

 sediments, partly because soda-feldspars are more subject to 

 change, and partly from the fact that potash-salts would be 

 separated from the percolating waters in virtue of the reactions 



