So much for the dust that helps make raindrops; now for the raindrops that help make dust. This the raindrops do in several ways. Falling on big rocks or decaying pebbles, for example, they pound loose with their patter, patter, patter, any little bits of soil and grains of sand that have been made by the other soil makers--the sun, the wind, the lichens, the chemists of the air, and so on. This soil and these sand particles, if there is already any depth of earth there, they carry down into the ground. Some of this soil, with various stops and mixings with other soils on the way, finally reaches the sea, where it helps to make the rich limestone soils for the Kentuckies of millenniums yet to be, by supplying food for sea creatures and lime for their shells. For these shells become limestone when the shell-fish are through with them. Mother Nature, in addition to feeding her big, hungry families of to-day in the plant and animal world, is always laying by something for the future. But before it gets back to the sea, by far the greatest part of the ground-up soil the rivers carry is spread out in the lowlands in those "alluvial plains" your geography tells about and that make a large proportion of the fertile farms of the world. If the raindrops fall on comparatively barren rock--in the mountains, say--they carry some of this fresh soil to the mountain valleys below, and some of it they may spread in bottom-lands a thousand miles away, where the new soil helps feed the plants. The sand grains in it not only help the soil to get its breath by making little air spaces, but these sand grains themselves slowly decay and so make more soil. [Illustration: WHAT IRRIGATION DOES FOR DESERTS It is such land as this, in the arid regions of the West, that irrigation converts from a desert to a garden of abundance. The soil is rich in all the substances that plant life needs.] But it isn't alone that they carry away the soil already made and bury the sand grains. Some of the raindrops soak into cracks in stones and dissolve the material that binds the rock particles together, and so get them ready to give way under the fairy hammers of the next shower that comes along. After Nature finally gets an original waste of barren rock all nicely set with grass and flowers and trees and things, the raindrops help to make soil in still another way. Soaking through the decaying leaves, they pick up acids which are just the thing for eating into rock and crumbling it into soil. To be sure, the water soaking into the soil and coming out of springs carries some plant food away with it; but it takes it to lands farther down the river valleys, and more than makes up for what it carries away by the new soil made by its acids from the rocks, as it soaks into their pores and runs among the cracks. HOW RAINDROPS MANAGE TO GRIND UP THE ROCKS