Differing in their extent, and in the nature of their surface, in every few miles of the coast of a maritime country like Great Britain, the zones have their peculiar animals and plants, and waifs and strays--the wreckage of the sea, of its floor, and of the coast-line. When the whole of the shore slopes very rapidly to the sea, the third and fourth zones are small in extent, but when the slope is gradual, they are large. And when the tide rises much and falls correspondingly, the third zone is usually uncovered but for a short time. The tide usually moves along the shore, and does not simply come in on to the land and recede; for one tide moves in one direction and the next in the opposite. Thus floating substances are carried along the coast for miles by the rising tide, and come back again, more or less, with the falling tide. Tide, wind, and wave forever act on the surface of the zones, but their action is the greatest on those which are landward. There are other wreckers of the coast; for the heat of the sun, the winter’s frost, the rain, and the chemical action of the air, one and all crumble and break off pieces of rock or earth. These fall on to the tidal shore, and are rolled here and there, and up and down, to be turned into mud, sand, and pebbles. The cliffs and bold headlands are worn year by year, and during centuries they lose much, and retire landward. Needles and “no man’s lands” stand out on the shore, or out at sea, testifying to the former extension of the land; and shore exists where there was once high solid rock. The shore consists of the worn surface of the old land, rock, or earth, and this is usually hidden by stone or stuff which has fallen from the cliffs, and by sand, or mud, or pebble and stone, which the tide has swept along. But often the jagged or rounded remains of the former rock project out of the sand, mud, and stone on the shore, and they may be bare, or covered with sea-weed. In other spots, the hard rock is hollowed out into places which let the water stand in them like so many puddles, pools, and ponds, when the tide has gone down. These are often crowded with marine plants and animals of the shore. The rolling stones, the wash of the tide, and the rush and drawback of the waves, are ever wearing off the surface of the shore and grooving it, or planing it flat, and in some places where the stones do not collect, this is very evident; but where they form great masses of pebbles or shingle, it can not be readily seen. There are many shores around Great Britain, where the rock is hard, which are rarely covered with pebbles, bowlders, and sand; and the sea-weed grows on them and protects them against the sea. But usually the rock is only exposed here and there, and the stones which collect and cover much of it come from a distance, and are on the move at every tide. In some places, where the coast is composed of clay or soft sandstone, the shore is muddy, soft, and may be uncovered or covered by stones.