Sir Archibald Geikie has called attention to the swiftness with which the structure of the coral polyp's skeleton is effaced from the foundation and a compact mass of rock put in its place. The sea-water's chemical and dissolving action, and the vast amount of mud and sand produced by the breakers are chiefly responsible for this. As the rock is being formed it is always being cemented. On the portion of a reef laid dry at low water, the coral rock looks in many places as solid and old as some of the ancient white limestones and marbles of the land. In pools where a current of water keeps the grains of coral sand in motion, each grain may be seen to be rounded. This is because on each particle of coral the dissolved carbonate of lime in the water is always being deposited (like the sediment in the bottom of a kettle). A mass of these rounded or egg-like grains all gathered together in a lump is called _oolite_, from the Greek word "oon" (Latin "ovum"), an egg. In many limestones, now forming parts of agricultural land, this _oolitic_ structure is strikingly shown, and there can be no doubt that in such cases it was produced just as now coral reefs are being formed before our eyes. In the coral tracts of the Pacific Ocean there are nearly three hundred coral islands, besides extensive reefs round volcanic islands. Others occur in the Indian Ocean. Coral reefs abound in the West Indian seas, where in many of the islands they have been upraised into dry land--in Cuba to a height of 1100 feet above the sea-level. The Great Barrier Reef that fronts the north-eastern coast of Australia is 1250 miles long and from ten to ninety miles broad. It will thus be seen that, apart from any other consideration, the animals of past ages leave permanent records of their existence merely by the accumulation of their dead bodies. Nevertheless, alike on land and on sea, the proportions of organic remains thus sealed and preserved is only a small part of the total population of plants and animals living at any given time. CHAPTER VI COLD AND ICE ON THE EARTH