In regard to lakes and salt-marshes, the permanence of those occupying the depressions of Areg, Bahrein, Sittra, and Qattara can only, I think, be adequately explained by regarding them as fed, at least in part, by underground supplies coming into them from the south. The total area of the lakes of Bahrein, Nuemisa, Sittra, and Moghara is nearly 20 square kilometres, and that of the salt-marshes (_sabakha_) is not less than 5000 square kilometres. The depressions are situated in a region which is nearly rainless; in Siwa the mean annual rainfall is only about a quarter of an inch, and that in the depressions farther south, such as Bahrein and Sittra, is doubtless even smaller. The mean daily evaporation from the lake-surfaces cannot well be less than some 4 mm., which would mean a lowering of the lake-levels by evaporation of 1½ metres each year unless there was some inflow to make up for the loss. And though the rate of evaporation from the salt-marshes, area for area, is doubtless very much smaller than that of the lakes, the 250-fold greater extent of the marshes makes it certain that the total quantity of water annually evaporated from them must far exceed that from the lakes. It appears unlikely that the loss by evaporation in the lakes and marshes can be entirely made up from local rainfall and by seepages from the northern slopes. The rocks forming the surface of the great Miocene plateau, 200 metres high, which separates the depressions from the sea, are chiefly limestones and clays; the beds are nearly horizontal, but such slight dips as exist are believed to be towards the sea. The average annual rainfall on the coastal portion of the plateau is about 6 inches; but it falls off rapidly inland, till it is only about a quarter of an inch near Siwa. The heaviest rainfall on the plateau thus occurs along a strip parallel to the coast, where it is largely drained off towards the sea by the gullies which indent the plateau-edge. Of that which falls on the plateau-surface farther inland comparatively little is absorbed, owing to the generally non-porous nature of the uppermost rocks; after a heavy shower, water lies on the surface in shallow pools for a few days and is soon evaporated. So impervious to water is the limestone in this region, that the Romans excavated chambers in it to form reservoirs, of which many hundreds still exist. Nor can we think that much surface drainage-water from the country to the south ever finds its way into the depressions; for there is an almost complete absence of drainage-lines entering them. At the feet of the northern scarps of the Qattara depression, and along the north-eastern shores of the lakes in the Wadi Natrun, there are, indeed, small springs which show that some of the rain falling on the plateau does actually penetrate the rocks and escape by seepage into the depressions. But the amount of this seepage appears to be insignificant compared with the