Although it has been stated by more than one writer that the great sand accumulations of the Libyan Desert originate in the denudation of the Nubian Sandstone which occupies large areas to the south of the oases, I have never myself met with any evidence in support of such a view. In my opinion the bulk of the sand must be regarded as originating in the arenaceous deposits of post-Middle Eocene age, which largely occupy and are exposed on the surface of the country between the Mediterranean Sea and latitude 29° N. We know from personal observation that some of the great sand-belts, which cross the intervening limestone plateau to the southern oases, commence in this region, and the supply of sand made available by the denudation of the beds in question is inexhaustible. During the passage of this sand across the great desert tableland the individual grains of silica doubtless become much rounded and reduced by attrition, though the total loss is probably more than counterbalanced by the very considerable additions received in the form of calcareous grains, derived from the limestones forming the surface of the high desert. An examination of the dunes of Kharga Oasis at once shows that the sand is not by any means composed solely of siliceous grains, but that there is a considerable proportion of white granules of limestone. A number of samples collected at random from the dunes in the central part of the depression were all found to contain a visible proportion of calcareous grains, and one of these, collected from the big dune to the south-east of Headquarters, was qualitatively examined for me by Mr. Garsed, and shown to contain 7·7 per cent. of calcium carbonate. It would be difficult to account for the presence of these calcareous grains on the assumption that the sand is derived from the Nubian Sandstone, though the denudation of the latter in and to the south of the oases must give rise to a very large amount of siliceous sand, which goes to swell the bulk of the dunes which have invaded this country from beyond the limestone plateau to the north. The dominant form of dune in the oasis is of crescentic or horse-shoe shape, a form specially typical of desert regions where there are prevailing winds in any one direction. These ‘barchans,’ as they are called, which are always disposed with the concave, steeply-inclined sides facing southwards, are found of every size, and exhibit many variations of the simple crescentic pattern. In many parts of the oasis the sand-belts are made up of a number of isolated and promiscuously disposed barchans; in others the dunes have joined together into a compact mass, in which the typical shape of the individual barchan is more or less obliterated.