This world was once a haze of fluid light, as the poets and the men of science agree in informing us. As soon as it began to cool down a little, the heavier materials naturally sank towards the centre, while the lighter, now represented by the ocean and the atmosphere, floated in a gaseous condition on the outside. But the great envelope of vapour thus produced did not consist merely of the constituents of air and water; many other gases and vapours mingled with them, as they still do to a far less extent in our existing atmosphere. By-and-by, as the cooling and condensing process continued, the water settled down from the condition of steam into one of a liquid at a dull red heat. As it condensed, it carried down with it a great many other substances, held in solution, whose component elements had previously existed in the primitive gaseous atmosphere. Thus the early ocean which covered the whole earth was in all probability not only very salt, but also quite thick with other mineral matters close up to the point of saturation. It was full of lime, and raw flint, and sulphates, and many other miscellaneous bodies. Moreover, it was not only just as salt as at the present day, but even a great deal salter. For from that time to this evaporation has constantly been going on in certain shallow isolated areas, laying down great beds of gypsum and then of salt, which still remain in the solid condition, while the water has, of course, been correspondingly purified. The same thing has likewise happened in a slightly different way with the lime and flint, which have been separated from the water chiefly by living animals, and afterwards deposited on the bottom of the ocean in immense layers as limestone, chalk, sandstone, and clay.