SOILS AND FERTILIZERS 311 



duced to plains, and lakes, and even oceans, filled to the condition 

 of dry land. We may see these processes in operation by the road- 

 side after a shower of rain, or in the garden, as well as in mountain 

 parks. In the Alps Mountains, in Switzerland, the tops are capped 

 by snow and ice which slide down the gorges with a tremendous 

 grinding force; the ice melts and this great volume of water flows 

 away with such violence as to furrow the rock slopes and carry away 

 every bit of loose material. Trees and smaller plants pry their 

 roots into the fissures, and winds send blasts of sand and dust against 

 the ledges. Thus by degrees the mountain of rock becomes a plain 

 of soil. 



One may see these results wherever he is, and may observe the 

 processes that have given rise to them. They may differ in mag- 

 nitude but not in kind. Note how frost breaks up clods of clay and 

 even of stone; ho,w the rivulets after a rain gully the hillside and 

 leave a mass of gravel or mud where the water comes to rest. Then 

 note the plants that spring up, and observe their roots how 'they 

 thread their \vay about through the spaces in rock and soil, expand- 

 ing each a little by their growth and search for water and food. 



Soils are undoubtedly formed from the disintegration and de- 

 composition of rocks, but rocks may also be formed from the con- 

 solidation of soils. These may again break apart into soils and be 

 again reconsolidated into rocks. The most important fact in this 

 connection is that all of our common rocks contain practically all of 

 our common rock-forming minerals, though in widely varying 

 proportions. One has but to look at a specimen or common 

 sandstone or shale to see the plates of mica and other forms of min- 

 erals associated with the quartz. We find the same minerals in the 

 sandstones, limestones, and shales as in the granite, gneisses, and 

 schists. The differences, aside from variations in the relative pro- 

 portionSj appear to be that one set is formed by water deposition 

 with or without pressure under ordinary temperatures, while the 

 other set is formed under conditions of high degrees of pressure and 

 temperature. There are of course exceptions in the case of any pure 

 sandstone as there are in the case of exceptionally pure forms of 

 igneous rocks. There is a question among geologists whether what 

 we consider the oldest crystalline rocks in the United States have 

 not in fact been formed by consolidation under great pressure and 

 high temperature from assorted material worked over from still 

 older and more primitive rocks. 



It is a mistake to suppose that sandstones always give rise to 

 sandy soils. Some of the most tenacious clay soils come from sand- 

 stone, as, for instance, the Penn clay from the Triassic. red sand- 

 stone rock. 



One of the most potent factors in the formation of all residual 

 soils is the process of leaching. Thus, from the leaching of disin- 

 tegrated rock have soils been formed. Limestone soils were orig- 

 inally ^impure limestone rock from which a very large percentage of 

 the^ original rock material has been removed by leaching. No geo- 

 logical fact is better established or more universally recognized. 



