The following outline is based on the generally accepted theories respecting the formation of soils.[7] The forces ordinarily concerned in the decay of rocks are: 1. Changes of temperature, including the ordinary daily and monthly changes, and the conditions of freezing and thawing. 2. Moving water or ice. 3. Chemical action of water and air. 4. Influence of vegetable and animal life: (a) Shades the rock or soil surface. (b) Penetrates the rock or soil material with its roots, thus admitting air. (c) Solvent action of roots. (d) Chemical action of decaying organic matter. 5. Earth worms. 6. Bacteria. =39. The Action of Freezing and Thawing.=—In those parts of a rock stratum exposed near the surface of the earth the processes of freezing and thawing have perhaps had considerable influence in rock decay. The expansive force of freezing water is well known. Ice occupies a larger volume than the water from which it was formed. The force with which this expansion takes place is almost irresistible. The phenomenon of bursted water pipes which have been exposed to a freezing temperature is not an uncommon one. While the increase in volume is not large, yet it is entirely sufficient to produce serious results. The way in which freezing affects exposed rock is easily understood. The effect is unnoticeable if the rock be dry. If, on the other hand, it be saturated with water, the disintegrating effect of a freeze must be of considerable magnitude. This effect becomes more pronounced if the intervals of freezing and thawing be of short duration. The whole affected portion of the rock may thus become thoroughly decayed. But even in the most favorable conditions this form of disintegration must be confined to a thin superficial area. Even in very cold climates the frost only penetrates a few feet below the surface, and therefore the action of ice cannot in any way be connected with those changes at great depths, to which attention has already been called. Nevertheless, certain building stones seem very sensitive to this sort of weathering, and the crumbling of the stone in the Houses of Parliament is due chiefly to this cause. On the whole it appears that the action of ice in producing rock decay has been somewhat overrated, although its power must not by any means be denied. But on the other hand a freeze extending over a long time tends to preserve the rocks, and it therefore appears that the entire absence of frost would promote the process of rock decay. At best it must be admitted that frost has affected the earth’s crust only to an insignificant depth, but its influence in modifying the arable part of the soil is of the utmost importance to agriculture.