8 JGKW'JLTURE. [CH. 



some. -Httle 'list \\nc.c be'ow the surface : sometimes it is a 

 difference of colour, sometimes a variation in the rough- 

 ness, but whatever the difference may be, it is clear to 

 the eye that there is a difference in the character of the 

 soil. The portion that so differs from the surface soil is 

 called the sub_-soil, or under-soil. In speaking of the 

 upper or surface soil we usually call it the soil, and that 

 portion which lies below it is known as the subsoil. 



5. The question naturally arises, how is it that 

 the land is thus covered by this earthy matter, and 

 'whence did the soil come ? Soils are produced 

 by the breaking up or crumbling of rocks. If a rock 

 were reduced into powder either by grinding, or by any 

 other mechanical means, that pulverized rock would be 

 a soil. But soils are not formed by rocks being pul- 

 verized by man's industry ; natural agencies carry out 

 this work very perfectly, sometimes with, and at other 

 times without, our co-operation. 



6. There are three agencies which thus turn rocks 

 into soil, and thereby produce for the farmer the earth 

 from which he makes his crops to grow. Water is one 

 of these agents. If water falls upon or soaks into a 

 piece of rock, it has a tendency to dissolve some por- 

 tion of the stone, and then pass away with its spoil as 

 soon as other water is ready to take its place. Thus, 

 rocjvs are softened by water and some portions dis- 

 solved out of them. 



7. Water also acts powerfully because it contains 

 some atmospheric air in it. Rain-water in falling 

 through the air takes into, and amongst its particles, 

 some portion of the air through which it passes, and 

 retains it. Thus water has generally some atmos- 

 pheric air in it. This air is a mixture of two gases 

 oxygen and nitrogen with some others in small pro- 

 portions, but of the latter we now only notice one, 

 carbonic acid. 



8. When water carries into a rock the oxygen which 

 it contains, this gas has a tendency to form chemical 



