The land, or reef islands, may be either portions of the reef elevated above sea level, containing fossil corals in the positions in which they grow, or it may be partly formed of a mass of corals thrown up by storms backed generally by an accumulation of sand. The coral rock thus elevated may be, as in the Red Sea, but little different from the original material of which it was formed, but more generally it is much altered. The continual wetting by spray or rain and drying under the tropical sun has a very marked effect in hardening and consolidating elevated coral, or coral sand. The upper parts are dissolved, and as the water sinks into the porous corals and becomes supersaturated with lime, the latter is crystallised out, thus filling up all cavities with _crystalline_ limestone. Thus in the end the highly porous heterogeneous limestone becomes a rock of exceeding hardness, crystalline and homogeneous. All the more delicate organisms are dissolved, only the largest remaining recognisable. At the same time as sea-water contains magnesium carbonate as well as limestone, and the former is less soluble than the latter, it tends to be deposited more quickly, so that it comes to replace the original limestone to some extent[54]. The alteration in the external appearance of the rock is very marked. Instead of the yellow, rather shapeless, cliffs of the Red Sea coast, in most other parts of the world, where tides supply spray and there is a considerable rainfall, we have coal-black rock with a very peculiar surface, all covered with sharp points and knife edges separating depressions left by the solution of the stone by water, hence the name “coral rag” applied to such rock. Where it forms the shore of a sheltered bay its homogeneity causes the undermining by the sea to go on to an astonishing extent before the unsupported piece falls away from the cliff to which it is attached. Such projections of the rocks which may be much longer than those shewn on Plate XXXV, also illustrate the hardness of this recrystallised material, for on striking one with a hammer a loud clear bell-like note is produced. Given the right conditions and we have the same peculiar result in the Red Sea and even in the Mediterranean. For instance, a considerable swell breaks at times on the narrow reef fringing the east side of the Tella Tella Kebir Islands, thus keeping the cliff behind it drenched with spray. In consequence the rock has become like that of Zanzibar and British East Africa. And generally, wherever the coral rock is exposed to spray it takes on these characters partially or completely, as is the case at the bases of all the cliffs along a narrow band just about sea level, where the rock is “’twixt wind and water.” Here the outer part is converted into a black, hard, and pitted crust, higher up it is harder than normal but above gradually passes into the slightly altered rock of the normal cliffs. Such a crust also covers the reef flats of the Red Sea,