the day soaks in. But in the night-time this water freezes, and water has the peculiar property of expanding as it solidifies; therefore each little crevice is occupied by an expanded sheet of water, and the rock, just as a frozen waterpipe will do, bursts. The products of glacial weathering are usually angular blocks and splinters, and the soil resulting from the action is barren, as in the tundras of Northern Siberia.
Weathering by plants is either mechanical, when the roots penetrate into cracks and, as they enlarge, widen them and so split the rock effectually, or the weathering of the rock is brought about by the chemical action of the organic acids secreted by the roots. A familiar experiment to illustrate the latter is that of germinating a pea on a polished slab of marble. As the root grows it eats out a shallow groove on the polished surface, which is quite noticeable when the little plant has been removed. The corrosive action of lichens on rocks is another example. In ill-drained areas the organic acids produced by the roots of plants, grass, &c., accumulate and cause the soil to become sour. If limestone is put on this soil the acids attack it, and the soil becomes sweet as long as the supply of limestone lasts.
Weathering by animals is accomplished by worms, ants, moles, rabbits, meer-cats, and similar burrowing animals, which bring the rock from below the soil, where it is protected, to the surface, where it can undergo disintegration by heat and water. In Europe worms simply turn over the soil, as the constant moisture prevents the casts on the surface from drying; but in South Africa the casts are brought up from the wet soil and dry, pulverize, and are blown away, so that
