Page:Handbook for Boys.djvu/136

Rh by dead vegetable matter and by sand and mud washed in.

These materials from new rocks and build up the surface. Thus the two processes, the

wearing down in some places and the building up in others, are tending to bring the surface to a uniform level. Another process, so slow that it can be observed only through long periods of time, tends to deform the earth's crust and to make the surface more irregular. In times past, layers of rock once horizontal have been bent and folded into great arches and troughs, and large areas of earth's surface have been raised high above sea-level.

At almost any rock outcrop the result of