ATMOSPHERIC AGENCIES. 7 



insoluble dust or soil, saturated with, a solution of the iron salt. We 

 have given these only as illustrative examples. We now proceed to 

 give examples of the principal kinds of rocks, and of the soils formed 

 by their disintegration. 



Granite, Gneiss, Volcanic Rocks, etc. — Granite and gneiss are mainly 

 composed of three minerals, quartz, feldspar, and mica, aggregated to- 

 gether into a coherent mass. Quartz is unchangeable and insoluble in 

 atmospheric water. Mica is also very slowly atfected. Feldspar is, 

 therefore, the decomposable ingredient. But feldspar is, itself, a com- 

 plex substance, partly soluble and partly insoluble. It is essentially a 

 silicate of alumina, united with a silicate of potash or soda, although it 

 often contains also small quantities of iron and lime. Now, while the 

 silicate of alumina is perfectly insoluble, the other silicates are slowly 

 dissolved by atmospheric water, with the formation of carbonates, and 

 the silicate of alumina is left as kaolin or clay. But, since we may 

 regard the original rock as made up of quartz and mica, bound together 

 by a cement of feldspar, the disintegration of the latter causes the whole 

 rock to lose its coherence, and the final result of the process is a mass 

 of clay containing grains of sand and scales of mica, and moistened 

 with water containing a potash salt. If there be any iron in the feld- 

 spar, or if there be other decomposable ingredients in the rock contain- 

 ing iron, such, for example, as hornblende, the clay will be red. This 

 is precisely the nature of the soil in all our primary regions. Volcanic 

 rocks decompose into clay-soils often, though not always, deeply colored 

 with iron. 



Limestone. — Pure limestone may be regarded as composed of gran- 

 ules of carbonate of lime, cohering by a cement of the same. The dis- 

 solving of the cement by atmospheric water forms a lime-soil, moistened 

 with a solution of carbonate of lime (hard water). Impure limestone 

 is a carbonate of lime, more or less mixed with sand or clay ; by disin- 

 tegration it forms, therefore, a marly soil. 



Sandstones. — Sandstones consist of grains of sand cemented together 

 by carbonate of lime or peroxide of iron. Where peroxide of iron is 

 the cementing substance, the rock is almost indestructible, since this 

 substance is not changed by atmospheric water : hence the great value 

 of red sandstone as a building-material. But, when carbonate of lime 

 is the cementing material, this substance, being soluble in atmospheric 

 water, is easily washed out, and the rock rapidly disintegrates into a 

 sandy soil. 



Slate. — In a similar manner slate-rocks disintegrate into a pure clay 

 soil by the solution of their cementing material, which is often a small 

 quantity of carbonate of lime. 



There can be no doubt that all soils are formed in the manner above 

 indicated. We have given examples of soils formed in situ, but, as 



