Page:Handbook for Boys.djvu/135

114 in layers closely resembling the beds of sedimentary rocks; still others, like slate, tend to split into thin plates.

The earth's surface is continually being changed; the out-cropping hard rock is worn away by wind and rain, and is broken up by frost, by solution of some minerals, etc. The loose material formed is blown away or washed away by rain and deposited elsewhere by streams in gravel bars, sand beds, and mud flats. The streams cut away their beds, aided by the sand and pebbles washed along. Thus the hills are being worn down and the valleys deepened and widened, and the materials

Wave-cut cliff with beach and spit built by waves and currents

of the land are slowly being moved toward the sea, again to be deposited in beds.

Along the coast the waves, with the pebbles washed about, are wearing away the land and spreading out its materials in new beds elsewhere. The shore is being cut back in some places and built out in others. Rivers bring down sand and mud and build deltas or bars at their mouths.

Volcanoes pour out melted rock on the surface, and much fine material is blown out in eruptions. Swamps are filled