Now when the rainfall decreased so that for the greater part of the year no freshwater stream entered the sea, and the materials of the edges of the delta became stationary, coral growth arose here and there, forming a fringing reef, the extension of which seawards must have been exceptionally rapid in this gradually shelving water. There was then an elevation of the sea-bottom, here comparatively slight, and the sea began to cut into the raised coral, carving it out into islets, surface reefs, tidal channels and lagoons. At the seaward edge of the reefs, the coral in the purer water could continue as it is doing now, a rapid and continuous growth, the uniformity of the conditions producing the remarkable unbroken reef already described. The disintegrating forces described in Chapter VIII break the inner side of the reef into sandy flats and pools, but the extension of these pools into wider spaces and continuous channels makes coral growth again possible in the shallows on their banks, so that all the inner sandbanks and rocks are capped with live corals which prevents their further demolition and gives the steep-sided canal-like form to the passages between them. _Living Reefs._ In detail the living reefs of the Red Sea are characterised by (1) their luxuriant growth of coral, (2) the absence of large stones or “negro-heads” along their edges, (3) their frequently crescentic or circular forms. There are remarkably few points on the coast where coral is not growing in abundance, and at the edges of reefs in the open sea its luxuriance is wonderful. Even at the inner ends of the canal-like harbours, where the water is stagnant and dirty, scattered colonies are found. The absence of rivers probably favours these growths, but even so the floods bring fresh water into the harbours occasionally, and colour their waters a deep red brown, like the Nile in flood, for several days together once or twice a year. The absence of strong currents carrying mud is another factor; where such occur, in Khor Dongonab, the vertical sided coral reefs give place to sloping bottoms of rock covered with stones formed by the growth of lithothamnia—stony seaweeds—but I only know this one place where these conditions prevail.