THE distribution of desert sand dunes in the neighbourhood of the Nile Delta is remarkable. They form a fringe to the desert (where the latter supplies a suitable sand), the material being piled up in dune-tracts, or dune-massifs, where-ever its flow is locally checked by ground moisture. The water seeps up through the sandhill, keeping it moist and compact nearly to the surface. The ground plan of the dune-massif has little relation to the wind, which, however, playing upon the surface, throws it into waves. A typical example of the progressive development of their form appears to be as follows. First, a gently-rounded swell, then the lee side becoming rather steeper and the summit of the swell no longer central, but nearer the lee side: the eddy under the lee slope gathers strength and begins to undercut the bank of sand, causing it to slip, forming a straight cliff. This process continues until the eddy has cut back to the summit of the growing dune, which is then of nearly equal average steepness on windward and on lee side. The windward side is, however, a smooth curve of compact sand, whereas the lee side consists of two portions, the upper a straight cliff of loose sand, the lower a curved surface of tolerably compact sand. It is common to find the central portion of a dune showing the fully-developed, the ends the embryonic form. Where the eddy cuts down to a hard bed, the slipping cliff may constitute the whole of the lee slope.