8 THE CANADIAN NATURALIST. [Fek 



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 comparatively greater stability of the feldspathic element. 

 The decomposition 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 silicates 

 of protoxyd-bases appears to be due to the action of carbonic acid, 

 which removing the lime and magnesia as carbonates, liberates 

 the silica in a soluble form ; while the iron and manganese passing 

 to a state of higher oxydation, remain behind, unless the action of 

 organic matters intervenes to give them solubility. 



§ 13. It is to be remarked that apart from the peculiar and 

 complete decomposition resulting in the production of kaolin, to 

 which orthoclase, oligoclase, and some other feldspathides, as leucite, 

 beryl, and perhaps also the scapolites and albite, are occasionally 

 subject, orthoclase is less liable to change than the soda-feldspars, 

 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. The de- 

 composition of feldspathides, and other aluminous double silicates, 

 whether rapid and complete, or slow and partial, apparently yields 

 the same results. A gradual process of this kind is constantly 

 going on in the feldspathic matters which form a large proportion 

 of the mechanical sediments of all formations ; and in deeply 

 buried strata is not improbably accelerated by the elevation of 

 temperature. 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 mentioned 

 in § 5. Hence it happens that apart from the neutral soda- 

 salts of extraneous origin, waters permeating sediments containing 

 alkaliferous silicates, generally bring to the surface little more 

 than soda combined with carbonic and sometimes with boric 

 acid, and carbonates of lime and magnesia with small portions 

 of silica. 



