!8 SOILS. 



Granite and the rocks related to it are rather quickly acted upon, 

 because of the presence of the feldspar minerals containing 

 potash, soda and lime as bases l together with alumina. 



The results of this action are highly important; one being 

 the formation of clay, so essential as a physical ingredient of 

 soils; the other the setting-free of potash, one of the most 

 essential nutrients of plants. Hornblende and the related 

 minerals are similarly acted upon so far as they contain the 

 same substances. In all cases, of course, the silica (silicic acid) 

 set free by the carbonic acid remains partially or wholly in the 

 resulting soils, as such. Lime also at first mostly remains 

 behind in the form of the carbonate ; but potash and especially 

 soda compounds, being mostly readily soluble in water, are 

 largely carried away by the latter. 



The effect of carbonated water upon silicate minerals is 

 greatly increased by the presence of ammonia (ammonic car- 

 bonate), which always exists in atmospheric water to a greater 

 or less extent. This effect may readily be noted on the win- 

 dows of stables, or other places where animal offal decays, 

 by the dimming of the glass surfaces; also in glass bottles 

 containing solution of ammonic carbonate. 



Action of Oxygen. The effects of atmospheric oxygen on 

 rocks are of course confined to those containing substances 

 capable of farther oxidation. Chief among these are ferrous 

 (iron monoxid) and ferroso- ferric oxid the latter imparting 

 bottle-green, bluish and black tints to so many minerals and 

 rocks that these colors may usually be taken as indicating its 

 presence. By taking up more oxygen the ferrous and ferroso- 

 ferric oxids are converted into ferric oxid or its hydrate (rust), 

 the tints mentioned passing thereby into brick-red or rust color, 

 according as the former or the latter (or sometimes their in- 

 termixtures) is formed. In either case there is an increase in 

 bulk; and this when taking place in the cracks or crevices of 

 minerals or rocks, tends, like the freezing of water, to widen 



1 The increase of solvent power on feldspar when carhonated instead of distilled 

 water is used, was well exemplified in an experiment made by Headden (Bull. 

 , 65, Color. Exp't Sta., p. 29), who allowed pure distilled and carbonated water 

 respectively to act on fresh but finely pulverized feldspar, with frequent shaking, 

 for five days. The distilled water disolved .0081 gram, the carbonated water, 

 .0723 gram of solids, or nearly nine times as much as the distilled water. Both 

 residues gave strong reactions for potash with platinic chlorid. 



