This is a salt marsh at mid-tide. How the sea itself adds such regions to the dominion of the land, and how human engineers, taking the hint, have put the sea to work, you will learn in this chapter. ] The sea also works with the busy little corals in building reefs and islands. Corals can only live and build where the water is kept in constant and vigorous motion by current and wave. From the air imprisoned in the bubbles by the stirring and turmoil of the waves and particularly from the air in the white foam of the crests these little people get their oxygen. At the same time they absorb out of the water the food on which they grow. The sea not only feeds these little wards of its bounty during their busy lives, but extends their usefulness after death, either by cementing to the reef the coral, ground up by the waves, or in storms scattering it over wide areas, to be made later into the finest of limestone; and still later into the best of soils. [Illustration: FATHER NEPTUNE FEEDING THE CORAL PEOPLE See that line of breakers just below the horizon? That shows where Father Neptune is serving the little coral people with food and fresh air, as explained in the text. ] We know also that the sea makes coal as well as stone in its rock mills; that the pressure of the overlying rock was in large part the source of the heat that changed the vegetation of the swamps, first into charcoal and then into coal. The subject of what the sea has done and is doing for us is almost as endless as the seas themselves; and no doubt the reason the sea is never still is because it has so much to do. Nothing in earth's animate or inanimate nature exercises an influence to be compared in importance to that of the sea, not only upon the land, but upon the whole life which land and sea support; and even in what seem to be the most aimless of its movements it in reality acts with the precision of a machine. III. The Artist And in the making of the rock in its presses under the water, as well as in the grinding which takes place along the shores, the sea evidently has an eye to beauty as well as use. As originally formed, the conglomerates or "pudding-stones" are always laid nearest the shore because there the retiring waves and the rivers emptying into the sea drop the heaviest part of their load, including the pebbles. Next is dropped the sand which is pressed into sandstone and beyond this the finest particles of all, the ground-up soil, which becomes slate rock. Still beyond the zone of slate is deposited the lime from the shells of sea creatures who can live only in this clearer water, away from the muddy waters nearer the shore. These deposits make limestone. The result of this natural sorting process is that all the four kinds of sedimentary rock are always laid down in just this 1, 2, 3, 4 order and no other: (1) pudding-stone; (2) sandstone; (3) slate; (4) limestone.