On the other hand, the foundations of these strangely isolated reefs may be like a certain island which, rising high above sea level, shews its structure, a centre of olivine rock fringed with coral. This island is variously known as Zeberjed, St Johns, and Emerald Island, the latter name due to its possession of mines for peridots, which are worked by the Khedive of Egypt. Its position is 23° 30′ N., distant about 60 miles from the African coast, a formation quite independent of the sides of the Rift Valley. It is an example of the “Block Mountains” described by Professor Gregory, portions of the original earth surface which have remained standing when the surrounding country dropped down to form the trough of the Rift Valley, not a mass of land thrust upwards and subsequently carved into peaks and valleys by running water, which is the way ordinary mountain ranges are formed. One gets a good idea of the structure of the Red Sea coasts on leaving the Gulf of Suez for the voyage south, before the ship’s course passes far from land. On the western horizon is a range of wild mountains, a grey plain ending in a yellow shore-line separating them from the sea, and the off-lying islands are of the same colour. The plain is formed of gravel from the high hills, its yellow border seawards being coral limestone, and the islands also. In the sea are numerous reefs, here of very intricate plan, lines of white breakers separating the deep blue black water from large areas of green and brown shoals in waveless lagoons. There are deep channels between these reefs and the shore, which is itself fringed by a shallow reef with its edge at low water level but bearing perhaps one or two fathoms of water on its surface within. This being the simple structure of both sides of the whole Red Sea trough I may proceed to describe in detail one section of the coast, that bounding the territory of the Anglo-Egyptian Sudan, between 18° and 22° N. This section includes two (Ras Rawaya and Ras Salak) of the three promontories which break the straight line of the west coast north of Masawa, the third being Ras Benas, further north. The map opposite shews clearly the fringing reef which lies along the whole coastline, the numerous harbours, of which Port Sudan, Suakin, and Trinkitat are of commercial importance[64], the deep channel separating the fringing from the barrier reefs, and the atoll of Sanganeb on which the lighthouse is built. [Illustration: Fig. 78. Coast of the Anglo-Egyptian Sudan Sandstone hills shaded, _small_ islands black. Coastline double, the outer line being the edge of the fringing reef. The thin lines enclosing roughly oval or elongated areas at sea are the barrier reefs. Figures on sea represent depths in fathoms.]