1. The action of frost. When water lodges in the crevices of rocks, and _freezes_, it expands, and bursts the rock, on the same principle as causes it to break a pitcher in winter. This power is very great, and by its assistance, large cannon may be burst. Of course the action of frost is the same on a small scale as when applied to large masses of matter, and, therefore, we find that when water freezes in the _pores_[M] of rocks or stones, it separates their particles and causes them to crumble. The same rule holds true with regard to stiff clay soils. If they are _ridged_ in autumn, and left with a rough surface exposed to the frosts of winter, they will become much lighter, and can afterwards be worked with less difficulty. [What is the effect of water on certain rocks? How are some rocks affected by exposure to the atmosphere? Give an instance of this.] 2. The action of water. Many kinds of rock become so soft on being soaked with water, that they readily crumble. 3. The chemical changes of the constituents of the rock. Many kinds of rock are affected by exposure to the atmosphere, in such a manner, that changes take place in their chemical character, and cause them to fall to pieces. The red kellis of New Jersey (a species of sandstone), is, when first quarried, a very hard stone, but on exposure to the influences of the atmosphere, it becomes so soft that it may be easily crushed between the thumb and finger. [What is the similarity between the composition of soils and the rocks from which they were formed? What does feldspar rock yield? Talcose slate? Marls? Does a soil formed entirely from rock contain organic matter? How is it affected by the growth of plants?] Other actions, of a less simple kind, exert an influence on the stubbornness of rocks, and cause them to be resolved into soils.[N] Of course, the composition of the soil must be similar to that of the rock from which it was formed; and, consequently, if we know the chemical character of the rock, we can tell whether the soil formed from it can be brought under profitable cultivation. Thus feldspar, on being pulverized, yields potash; talcose slate yields magnesia; marls yield lime, etc. The soil formed entirely from rock, contains, of course, no organic matter.[O] Still it is capable of bearing plants of a certain class, and when these die, they are deposited in the soil, and thus form its organic portions, rendering it capable of supporting those plants which furnish food for animals. Thousands of years must have been occupied in preparing the earth for habitation by man. As the inorganic or mineral part of the soil is usually the largest, we will consider it first. As we have stated that this portion is formed from rocks, we will examine their character, with a view to showing the different qualities of soils. [What is the general rule concerning the composition of rocks? Do these distinctions affect the fertility of soils formed from them?