If the soil is sour, the acids elaborated by the plant roots collect. Now, organic acids have a powerful action on iron, and hence this material is dissolved out of the rocks on which the soil rests. As the water evaporates in the dry season the iron is deposited in the form of ironstone gravel, or concretionary limonite. This troublesome material forms under the soil, and is called in England moor-bed-stone, as it forms under the sour soil of the moors. In South Africa it has been given the name Ou klip. It is sometimes called laterite, but this only forms in tropical climates, and though its origin is similar it contains as much alumina as iron.
Fig. 13. — Section in Railway Cutting south of George Town, showing the formation of sour soilA, Decomposed granite. B, Quartz vein, C, Ironstone gravel. D, Grey soil. E, Humus.
In Bechuanaland and in Natal, where there is an excess of lime in the soil, hard, platy limestone or calcareous tufa forms beneath in the same way as the ironstone gravel does. As it prevents the roots of plants from penetrating to the moister parts beneath, it is extremely troublesome in many cases. It is called in India kankar. All these substances, kankar, laterite, ironstone gravel, freshwater quartzite, silicified sand, are instances of rocks being formed by weathering.
Erosion is the wearing away of rocks by the agency of wind, water, or ice; the tools of erosion are the sand grains, gravel, or boulders carried by these agents. In wind erosion the sand is carried with considerable force,
