SOIL-Foh'MING ROCKS 183 



"temporary" hardness, because they decompose easily on 

 aeration or boiling and precipitate the carbonates. The 



sulphates of calcium and magnesium give water "permanent" 

 hardness, since they are not so easily precipitated. 



Of course in the case of drainage and river waters there is 

 a very wide variation in the content of dissolved material, 

 just as there is in the film water. A granitic or sandy country 

 will yield water that is very "soft" or almost pure, whereas 

 a limestone country gives just the opposite. Take two 

 fresh water lakes in Wisconsin, for example. One — Devil's 

 Lake— receives the drainage from a granitic region, the 

 other — Lake Mendota — receives that from a limestone 

 country. Table X gives the analyses of both lakes. 



Table X. — -Composition of Lakes. 

 (Expressed in parts per million.) 



Lake Mendota 

 Devil's Lake . 



143. Soil-forming Rocks. — To obtain some idea of the 

 composition of a soil, it is necessary to know something of 

 the rocks from which the soil is derived. Rocks are mineral 

 aggregates, that is, they are composed of two or more minerals 

 welded together either by heat or pressure. They are classi- 

 fied as igneous, formed by the cooling of molten masses; 

 sedimentary, laid down by water; and metamorphic, changed 

 by heat and pressure from their original forms. Knowing 

 the rocks in a general way will give an idea of the minerals 

 to be expected in a soil derived from any given rock, and also 

 the character of the compounds resulting therefrom. This 

 applies, of course, only to soils in place. Soils that have been 

 transported by water have been sorted more or less accord- 

 ing to the specific gravity of their various constituents and 

 therefore do not contain all of their original minerals. It is 

 impossible to describe all of the rocks or even the common 

 rocks from which soils are derived, but it may suffice to 

 mention a few characteristic soil-forming rocks. 



