It is impossible for the polyps to raise their stony structure above the sea. Some kinds can endure exposure to the air for a short time without dying, but only for a short time. A coral island, when first built, is merely level with the ocean-surface at low tide. But old Ocean carries on the work which the polyps have begun. Heavy rollers break incessantly against the reef, loosening blocks of coral, some of which are flung bodily upon it, while others are pounded into sand, which fills up holes and crevices. Gradually thus the whole becomes cemented into hard reef-rock. As this goes on the low island slowly rises, until it has climbed above high-tide level. Then the task can only be completed by waves of exceptional reach. Still the grinding of coral to fine sand goes on; and in time it gains a depth of some inches. And the waves carry seeds and cocoa-nuts to the spot; and shrubs and trees spring up, to sow again their own seeds, and by gradual decay to form mould. Thus it comes about that a little island is made ready for man to live on. One might expect these stages to be very slow indeed, but such events march at the double in tropical climates. In the Low Archipelago one atoll was found, in the space of only thirty-four years, to have been transformed from a mere rock-reef to a lagoon island, fourteen miles in length, with tall trees growing along almost the whole of one side. As a specimen of the sizes to which coral-buildings reach, the Maldive Archipelago may be named. It is a vast collection of islands and reefs, stretching to a length of four hundred and seventy miles, and in parts fifty miles broad. Barrier-reefs, reaching through many hundreds of miles, are even more remarkable. Travellers write stirring descriptions of the beauty of these erections. The vivid pen of Miss Gordon-Cumming, for instance, has painted many a picture of Pacific reefs, of thundering breakers, and dazzling white surf. We are told by her of “the patches of coral, sea-weed, and sometimes white sand, lying at irregular depths beneath a shallow covering of the most crystalline emerald-green water,” producing “every shade of aqua-marine, mauve, sienna, and orange, all marvellously blended.” And, again, of the wonderful masses of living coral which grow like garden-plants below the clear water, and of branching shrubs of all imaginable tints, such as pink, blue, mauve, and primrose. To pluck and carry off these ocean-blossoms would be a vain attempt, for the “gelatinous slime” to which the colours belong “drips away, as the living creatures melt and die, when exposed to the upper air.”