SOILS AND MANURES. 353 



weathering, which is necessary in preparing the soil for 

 vegetation. This consists in the action of chemical forces, 

 in connection with heat and moisture, which not only aid 

 in the pulverization, but also render some of the materials 

 soluble, and therefore available for vegetation. 



492. The Process Seen in Volcanic Countries. The forma- 

 tion of soil is continually going on in all parts of the earth 

 in the manner indicated. It can be best seen in the neigh- 

 borhood of volcanoes. The lava that has issued from a 

 volcano lies barren for years; but the varying tempera- 

 ture, the water, and the oxygen of the air at length pro- 

 duce sufficient disintegration and chemical change to make 

 a soil for lichens. These succeed each other year after year 

 for generations, from which there is a gradual accumula- 

 tion of humus. This by its decay assists the other agencies 

 of disintegration, and so there is a yearly addition to the 

 soil on the rocky lava. Thus is preparation made for other 

 plants, and so the accumulation goes on till at length, per- 

 haps- in the lapse of centuries, the soil becomes deep enough 

 for shrubs and afterward trees. The various steps of this 

 process, thus briefly described, may often be observed in 

 deposits of lava of different ages in the neighborhood of 

 volcanoes. 



493. Different Kinds of Soil. In the agitations by which 

 rocks were broken up there was so wide a scattering of 

 the broken materials that the varied mineral ingredients 

 of soil are well mixed up in all parts of the earth. Still 

 there are peculiarities in soils here and there, owing to the 

 predominance sometimes of one and sometimes of another 

 mineral ingredient. For example, in the regions of lime- 

 stone formations there is apt to be a predominance of lime 

 in the soil. The three chief mineral ingredients of soil are 

 these: 1, Silica, in the shape of sand; 2, Alumina, mixed 

 or combined with sand, as clay ; and 3, Lime, in the form 



