Although no natural formation of the Yemen can assist one in correctly determining its inland frontiers, the same cannot be said of the two great divisions into which the country is split up. These are so apparent that, from the earliest geographers to the present day, they have remained unchanged and fully recognised. But in order to appreciate this, a few words must be said as to the formation of the country. While the interior consists of vast mountain-ranges and elevated plateaux, some of which lie at an altitude of over eight thousand feet above the sea-level, the seaboard consists, both on the west and south, of low-lying sandy deserts and plains, varying in breadth from thirty to nearly a hundred miles. The only exception where a spur of the mountains approaches the sea is at the headland of Sidi Sheikh, the south-west corner of the Red Sea—a spur of land a few miles in width exactly opposite the island of Perim, from which it is divided by a narrow channel. It may be remembered that only a few years back there was a false report that France had purchased this advantageous spot from the Turkish Government. The formation of these maritime plains is such that it may be safely surmised that a very considerable portion, at least of what is now desert, was at one time covered by the sea. So fast, indeed, has been the silting action, that more than one former port now lies well inland. As an example of this, Sir R. L. Playfair, in his excellent ‘History of the Yemen,’ mentions the town of Muza, once a flourishing sea-port, now over twenty miles inland. In many places, too, shells and chips of coral are to be found at great distances from the coast. The same retrograde action of the sea can be traced, too, at Aden, which was, no doubt, at one time an island, and is now joined to the mainland by a low isthmus, formed by the silting of sea-sand upon a submarine basis of rock.