460 Action of the Atmospkei-e upon newly-deepened Soil. 



in this manner it is likely assimilated by plants. Way has 

 pointed out that ammonia displaces the other bases that may be 

 in combination with silica, thus forming; a silicate of ammonia, 

 which is gradually soluble in water, and by whose decomposition 

 the silica would be yielded in a state suitable for the organs of 

 plants. 



Silica, therefore, exists in the soil in several forms and com- 

 binations ; under some of these it is little affected by the atmos- 

 pheric influences, but under others it is susceptible of a gradual 

 dissolution, slow it is true, but still perhaps sufficient in man}'' 

 cases for the wants of the cultivated plants. By depriving silica 

 of its oxygen, through the agency of potassium, or of some 

 powerful deoxidiser, its remaining element, silicon, is obtained — 

 a dark brown substance, incapable as yet of further decomposi- 

 tion, and therefore reckoned amongst the simple or elementary 

 bodies. 



Flint, one of the varieties of quartz, is abundant in many 

 English soils, being derived principally from the chalk formation ; 

 it often encloses organic remains, such as shells, sponges, and 

 infusoria. The exterior coat of it is often white, containing^ 

 according to Vauquelin, 5 to 10 per cent, of carbonate of lime. 

 7^he interior is silica, mixed with a varying proportion of lime, 

 peroxide of iron, alumina, potasli, and carbonaceous matter. 

 Flint may be dissolved by steam of a high temperature. 



Felspar is another of the most abundant ingredients of the 

 crust of the earth, and as it is frequently one of the most rapidly 

 decomposing minerals, the atmospheric modifications of the soil 

 are often due in a great measure to its changes. There are four 

 principal kinds of it, the orthoclase or potash felspar being the 

 most common in the granitic and plutonic rocks, while the soda 

 and lime species, albite, labradorite, and oligoclase, are more 

 prevalent in the volcanic products ; labradorite appearing to cha- 

 racterize the basaltic division of the volcanic rocks, and glassy 

 felspar the trachytic division. 



Orthoclase, or common felspar, may be looked upon as a 

 combination of silicate of alumina witli silicate of potash. These 

 silicates, as they occur in natui'e, differ widely in the facility 

 with which they are decomposed. " The granite of Corsica and 

 the felspar of Carlsbad," says Liebig, " crumble into dust in a 

 space of time during which the polished granite of the Berg- 

 strasse does not even lose its lustre." In many places here* the 

 granite may be seen for several feet down so decomposed that it 

 may be dug with a spade, whilst the features of the colossal 

 monuments of Egypt remain unchanged, although the storms of 



* In the neighbourhood of Ellon, Aberdeenshire. 



