The irresistibility of drift sand is well seen where isolated hills lie in the path of the dunes. When such hills are narrow, and elongated in a north and south direction, they generally act as a wedge, divide the wind, and force any sand it may be carrying to pass on either side. When, however, a hill has northerly flanks of any considerable width, it will only form a temporary check to the southward passage of the sand. The progress of the latter may be arrested for a while, until hollows have been filled in and the general slopes reduced; but eventually the sand will pass up along the lines of least resistance to the summit. This is beautifully illustrated at Jebel Tarif, the great isolated, flat-topped, hill-massif lying between Um el Dabâdib and Kharga village, standing over 300 metres, or 1,000 feet, above the surrounding plain. Instead of being diverted by this obstacle, the sand, coming from the plateau to the north and north-west of Um el Dabâdib, surmounts the slopes, and continues in its normal direction across the flat-topped summit. The latter has been cut up into numberless parallel north- and south-disposed ridges, separated by deep troughs, which are now to a considerable extent filled with sand. Active erosion must, therefore, at the present day be confined to the upper portions of the slopes of the valleys and to the intervening ridges. Naturally in such a region the inhabitants have been quick to take advantage of any protection afforded by topographical features, and it is not surprising to find that the most important of the ancient monuments, as well as the greatest area of modernly cultivated land, lie in a comparatively sheltered position to the south of Jebel Têr. Even there, however, trouble is constantly experienced on the east side, owing to the tendency of the belt of sand-dunes, which comes from the vicinity of Qasr Lebekha, to spread laterally. Along the east side of the village the dunes are close up against the outlying houses, and the sand continually surmounts and pours over the walls which from time to time are erected by the natives to act as barricades. Sheltered places of the above description are rare, the majority of the smaller settlements being situated on the open plain, where the only precaution which can be taken is to select the sites for wells as far away as possible from the well-defined sand-belts. But in windy weather the sand-blast is not confined to the neighbourhood of the dunes; the wind rages across the open plains, carrying dense clouds of sand in suspension, and devastating the works of man wherever the latter offer the slightest obstacle to its progress. [Illustration: SAND EROSION ON SUMMIT OF JEBEL TARIF.]