Another case from the Cape Verde Islands, where reef corals do not exist at all, is shewn on Plate XXXIV. A reef flat, with raised definite edge and miniature boat channel complete, has been cut out of sandstone, the edge of which was protected by a growth of stony seaweed (lithothamnia), and vast numbers of the shelly tubes of that strange animal _Vermetus_[53]. These two organisms combine to form a continuous crust over the whole surface of the seaward edge of the sandstone, and so greatly delay its removal by the sea, but landwards, this protection being absent, the reef flat is hollowed out into a “boat channel.” This sandstone is a local deposit just to the south-west of the town of St Vincent, but the volcanic rocks of which the island is composed are cut down to a narrow flat in the same way, but less regularly. _Plate XXXIV_ [Illustration: Fig. 74. A sandstone reef near St Vincent, Cape Verde Islands] [Illustration: Fig. 75. An embryo fringing reef near Ramleh, Alexandria] A third case, from the Mediterranean near Alexandria, is so striking as to be worth illustrating, though only the embryo of a reef, as it were a ledge a few yards wide, has been formed as yet. The rock is a calcareous sandstone, a consolidated dune, and the protecting organisms are much the same as those found in the Cape Verde Islands, but here forming a less coherent coating to the rock. The regularity of the ledge laid bare by the retreat of a wave is very striking. Reefs may shew other features, no one arrangement can be taken as typical of all. Instead of the smooth slope and rounded ridge which compose the reef edge on this coast and that of Zanzibar it is usual, in many oceanic reefs, for the growing edge to be cut into by deep and narrow fissures, up which the great breakers send violent torrents of water.