. Elementary physical geography . lso important agents of weathering, and theiraction is both chemical and mechanical. They act chemi-cally by furnishing to percolating water many of the sub-stances with which it is able to dissolve and alter theminerals ; and they also extract mineral matter from thesoil in water absorbed through the roots. The mechani-cal action of plants is mainly that of their roots. Theseenter the rock crevices, and upon growing, enlarge thesecavities, causing the rocks to crumble (Fig. 117). Thisaction may often be seen upon a ledge on which lichens are DENUDATION OF THE
. Elementary physical geography . lso important agents of weathering, and theiraction is both chemical and mechanical. They act chemi-cally by furnishing to percolating water many of the sub-stances with which it is able to dissolve and alter theminerals ; and they also extract mineral matter from thesoil in water absorbed through the roots. The mechani-cal action of plants is mainly that of their roots. Theseenter the rock crevices, and upon growing, enlarge thesecavities, causing the rocks to crumble (Fig. 117). Thisaction may often be seen upon a ledge on which lichens are DENUDATION OF THE LAND. 235 growing; and the roots of trees are doing a very importantwork of this nature, because they extend through the soilto the rock beneath. Even animals are aiding in this work, particularly thosethat burrow in the earth. Earthworms are of great impor-tance in this respect, for they are engaged in the constantwork of pulverizing the soil. The action of the agentsabove described, is not confined to the solid rock, but it is. Fig. of a tree breaking a rock into fragments. constantly in progress in the soil, the tendency alwaysbeing to make this finer in texture. The results of this action of weathering are most wide-spread. All over the land, in nearly every place, the rocksare being destroyed by these agents; and weathering is themost important single cause for the destruction of the strataand the melting down of the surface of the land. Weather-ing is more rapid in some places than in others. On thecold mountain tops, its action is rapid (Fig. 224), as it is alsoin regions of moisture. On the other hand, in arid regionswhere rain is uncommon, weathering is relatively slow, as 236 PHYSICAL GEOGRAPHY. it is also in regions where a deep soil covering protects therocks. Upon exposed ledges, weathering is rapid ; and thisis particularly true of cliffs, where the fragments drop tothe base in the form of a talus (Figs. 118, 122), leaving therock-face bare to future at
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