THE UBYAN PLATEAU— TUE MIBAOE. 819 The Libyan Plateau. The Libyan hills are lower than those skirting the right side of the river. Taken as a whole, the relief of Egypt presents the character of a plane inclined in the direction from east to west. From the crest formed by the coast range the highlands and plateaux diminish gradually in height down to the Nile Valley. From the western edge of this valley the ground also falls until at last it sinks below the level of the sea. On both sides of the strip of verdant and inhabited land fringing the Nile the zone of rocks is alike destitute of permanent dwellings. But the Libyan region being more uniform, void of lofty eminences and covered with sand, presents a more desolate appearance than the eastern zone. It already forms part of the great desert, which stretches thence westwards right across the continent to the Atlantic seaboard. Seen from the pyramid of Cheops, this Libyan plateau might seem to be nothing more than a boundless plain varied only by sand dunes. But this is merely the effect of an optical delusion, as we are assured by the few travellers who have ventured to penetrate into these dreary solitudes. Taken as a whole, the desert comprised between the Nile and the depression of the oases is a plateau of nummu- litic limestone rising to a height of 830 feet above the river level. The limits of this plateau are indicated by escarpments, while its surface is dis|)oscd in distinct sections by the erosive action of old marine waters. Hillocks of uniform elevation rising here and there above the plain serve to indicate the primitive level of the land. The base of all these promontories was undoubtedly washed in pre-quater- nary times by the Mediterranean, whose waves were broken into surf amid these rocky archipelagoes, where at present water appears only in the form of delusive mirages. The Mirage. Nowhere is this remarkable phenomenon of the mirage seen to greater advan- tage than in the Libyan and Arabian deserts. It often assumes the most weird and fantastic forms, the outlines of lovely landscapes, hills and valleys, verdant plains, everywhere interspersed with the treacherous appearance of broad lacustrine basins, glittering under the torrid rays of the tropical sun. And so vivid are these scenic effects, that the most experienced travellers, and the animals themselves, are at times deceived by the pleasant phantom and thus beguiled to their destruction. When crossing the Arabian Desert in the year 1883, Colonel Colborne tells us that on one occasion the mirage was intensely real. Before him stretched a large lake, its blue waters laughing in the sun, studded with gem-like islets clad in verdure, and bordered by castles, high pinnacles, turrets, and battlementa, and again by gleaming villages and smiling hamlets — the whole scene fairylike in its beauty, while presenting a most painful contrast to the arid sand and fierce heat and con- suming thirst from which the traveller was suffering. It is in vain that we rub our eyes and seek to disabuse ourselves of the illusion. The spectacle lies before us
