Now, although opinions may differ as to the part played in the past by wind and sand in the shaping and formation of the oases-depressions of the Libyan Desert, there can be no question of the effects they are producing at the present day. Wind-borne sand is indeed the curse of the oasis, and although its evil effects may be mitigated or altogether staved off for a time, the longer this force of Nature is opposed by man the greater and more overwhelming is its final victory. If a full account of the human occupation of the oases could be written, it would be very largely the history of an incessant combat between man and Nature; and although man may for a time gain the upper hand, and even make the sand-laden winds to some extent serve his purposes, he is in the end generally forced to abandon those places in which he has been at such pains to establish himself. In the Libyan Desert, both on the plateaux and in the depressions, blown sand has a marked tendency to collect into dunes occupying definite north and south belts of country, lying parallel to the direction of the prevailing winds. This peculiar and fortunate disposition depends on a number of circumstances, such as the mean direction of the wind, the points of greatest supply of the material, and on other causes which are not perhaps as yet thoroughly understood. In Kharga Oasis the best marked and most important belt lies somewhat to the west of the central axis of the depression. At the north end, in the neighbourhood of Um el Dabâdib, it consists of a number of isolated but parallel lines of dunes, which southwards pass over and round the hill-massif of Jebel Tarif, eventually uniting and forming a more compact belt of sand, with an average width of 8 or 10 kilometres, which continues right through the depression into the desert to the south. This line of sand sets a limit to the extension of cultivation to the west. A second belt comes from the plateau to the north of Qasr Lebekha, passes immediately to the east of Jebel Têr and the village of Kharga, and continues southwards until it spreads out and is arrested, more or less by artificial means, in the vicinity of Gennâh. Other smaller and less defined belts exist to the south of Ain el Tawîl, to the east of Headquarters, and near the foot of, and parallel to, the eastern wall of the depression. [Illustration: A BELT OF DUNES NEAR QASR LEBEKHA.]