Study this soil with the eye and you may not see the tiny bits, though in sandy soils one may easily notice that there are bits of mineral. Even fine loamy and clay soils, when examined with a pocket lens or a microscope, will be found to be composed of tiny fragments of mineral. It is evident that in some way mineral has been powdered up to form the soil; and since the minerals come from rocks, it is the rocks that have been ground up. That powdered rock will make just such a substance as soil may be proved by pounding a pebble to bits, or by collecting some of the rock dust that is made when a hole is drilled in a rock. Much the same substance is ground from a grindstone when a knife is sharpened on it, making the water muddy like that in a mud hole. It will be an interesting experiment to reduce a pebble to powder and plant seeds in it to see whether they will grow as well as in soil; but in preparing it try to avoid using a sandstone pebble, because sandy soils are never very fertile. [Illustration: _Fig. 22. A glacial soil, containing numerous transported pebbles and boulders, resting on the bed rock._] Not only is soil made up of bits of powdered rock, but it everywhere rests upon rock (Fig. 25). Some consider soil to be only the surface layers in which plants grow; but really this is, in most places, essentially the same as the layers below, down even to the very rock, so that we might call it all soil; though, since a special name, _regolith_ (meaning stone blanket), has been proposed for all the soft, soil-like rock-cover, we may speak of it as regolith and reserve the word soil for the surface layers only. In some places there is no soil on the bare rocks; elsewhere the soil-cover is a foot or two in depth; but there are places where the regolith is several hundred feet deep. In such places, even the wells do not reach the bed rock; nor do the streams cut down to it; but even there, if one should dig deep enough, he would reach the solid rock beneath. How has the hard rock been changed to loose soil? One of the ways, of which there are several, may be easily studied whenever a rock has been exposed to the air. Let us go to a stone wall or among the pebbles in a field, for instance, and, chipping off the surface, notice how different the inside is from the outside. The outer crust is rusted and possibly quite soft, while the interior is harder and fresher. Many excellent examples of this may be found in any stony field or stone wall. [Illustration: _Fig. 23. The bed of a stream at low water, revealing the rounded pebbles that have been worn and smoothed by being rolled about, thus grinding off tiny bits which later are built into the flood-plains._]