As hard iron rusts and crumbles to powder when exposed to the weather; so will the minerals and the rocks decay and fall to bits; but rocks require a very much greater time for this than does iron. It happens that the soil of New York has not been produced by the decay of rock; and, therefore, although the soils in many parts of the world have been formed in this way, we will not delay longer in studying this subject now, nor in considering the exact way in which rocks are enabled to crumble. Another way in which rocks may be powdered may be seen in most parts of New York. The rains wash soil from the hillsides causing the streams to become muddy. In the streams there are also many pebbles, possibly the larger fragments that have fallen into the stream after having been broken from the ledges. The current carries these all along down the stream, and, as they go, one piece striking against another, or being dragged over the rocks in the stream bed, the pebbles are ground down and smoothed (Fig. 23), which means, of course, that more mud is supplied to the stream, as mud is furnished from a grindstone when a knife or scythe is being sharpened on it. On the pebbly beaches of the sea or lakeshore much the same thing may be seen; and here also the constant grinding of the rocks wears off the edges until the pebbles become smooth and round. [Illustration: _Fig. 24. Near view of a cut in glacial soil, gullied by the rains, and with numerous transported pebbles embedded in the rock flour._] Supplied with bits of rock from the soil, or from the grinding up of pebbles and rocks along its course, the stream carries its load onward, perhaps to a lake, which it commences to fill, forming a broad delta of level and fertile land, near where the stream enters the lake. Or, possibly, the stream enters the sea and builds a delta there, as the Mississippi river has done. [Illustration: _Fig. 25. A scratched limestone pebble taken from a glacial soil._] But much of the mud does not reach the sea. The greatest supply comes when the streams are so flooded by heavy rains or melting snows that the river channel is no longer able to hold the water, which then rises above the banks, overflowing the surrounding country. Then, since its current is checked where it is so shallow, the water drops some of its load of rock bits on the flood-plain, much as the muddy water in a gutter drops sand or mud on the sidewalk when, in time of heavy rains, it overflows the walk. Many of the most fertile lands of the world are flood-plains of this kind, where sediment, gathered by the streams farther up their courses, is dropped upon the flood-plains, enriching them by new layers of fertile soil. One does not need to go to the Nile, the Yellow, or the Mississippi for illustrations of this; they abound on every hand, and many thousands of illustrations, great and small, may be found in the State of New York. Doubtless you can find one.