It is in its external form that a coral reef shews features which give it an individuality above that of a mere heap of stones. Generally it rises with a steep slope from the sea bottom which ends in a low precipice, above which is another and more gentle slope to the highest point of the reef, a foot or two above lowest water, which is near its outer edge. Passing landwards the reef level is lower again, and we may have a boat channel or series of lagoons, where the native canoes can travel on calm water however the sea may rage outside. This is succeeded by a flat of bare rock, which rises slowly up to the base of the undercut coral cliffs as in imaginary section in Diagram 1 and the Photograph on Plate XXXII. We have here three striking features, viz. precipitous reef edge, raised border and reef flat with boat channel, strongly differentiating the shore of a coral sea from the more or less even slope we are accustomed to at home, resulting in a nearly waveless shore and breakers out at sea, an endless line of purest white dividing the green of the shallows from the blue-black of the deep water. _Plate XXXII_ [Illustration: Fig. 71. Undercut cliffs of Rawaya. The whole foreground is reef flat formed by the planing down of the land] Imagine land newly raised from the sea upon which coral growth is only beginning. In section its coastline would be a more or less gradual slope (to take the simplest case) as the line A, B in Diagram 2, sea level being represented by the line C, D. Suppose the scale to be such that the depth C to A is about 50 fathoms. Now it is found that under the best of conditions reef corals do not grow at this depth; if the conditions are less favourable so the maximum depth at which [Illustration: Diagram 2. The Commencement of a Reef] these corals grow is decreased. (That this fact is the crux of the problems to be discussed later may as well be noted at once.) Coral growth will be most luxuriant in the shallow water, and the first stage of our reef will be a mound of coral of the shape shewn in section by the dotted area. Between E and D this comes to the surface, and the corals, projecting above the water at lowest spring tides, are killed at the top, so that E to D becomes an almost flat surface of dead corals, some of which may, however, be still living where their bases are immersed in clear sea-water. A continuation of this process gives us a reef flat of considerable area, indicated by the line F, D, the slope A, F becoming correspondingly steepened. At the point F the waves have thrown up a long low mound of coral fragments and shells, which, in the way described below, may be consolidated into a ridge of solid rock. It is easy to see how an extension of coral growth would make A, F a regular precipice, as F approximates to C. What happens after F, the reef edge, grows out to water 50 fathoms deep, where no living foundation can be laid for