metres lower than the general ground-surface[34]; and I have edged with red the only areas in the west and south in which the two surfaces are within 100 metres of each other—that is to say, the areas within which comparatively shallow depressions with underground springs _must_ lie, if our contours of the two surfaces are drawn with even approximate correctness. It will be seen from the map that this restricts the search very considerably. In fact, if Zerzura is within the frontiers of Egypt, it lies in all probability either to the west of longitude 26° 20′ and north of latitude 26°, or to the east of longitude 27° and south of latitude 23° 30′. We may dismiss from our field of search all the broad tract of rising ground which extends from near the Dakhla escarpment south-westwards towards Owenat; for unless Zerzura is in a depression of great depth, it cannot possibly lie within this tract. This consideration shows that most of the previous rough estimations of the possible position of Zerzura must have been erroneous; and indeed it is remarkable how few of the various explorers’ tracks traverse the only two areas within which Zerzura, if it exists, almost certainly lies. The northern area has only been crossed by the Rohlfs expedition of 1874, and that near its eastern edge; while the western part of the southern area has only been crossed by Prince Kemal el Din’s expedition of 1925. Of all the Arab traditions, only those which would place Zerzura far to the west of Farafra, or far to the south-south-west of Dakhla, can now have any high degree of credibility. It can hardly, I think, be doubted that the various traditions refer to more than one place; and in view of the almost totally unexplored state of the only two areas in which our new contour-maps indicate the possibility of unknown oases existing, it is quite conceivable that at least one oasis may lie within each of them. In the northern area, Siwa would probably make the best starting- point from which to conduct a search, as it is easily reached by motor- car from Alexandria or Cairo, and exploratory journeys southwards from Siwa would mostly lie along the direction of the dune axes. In the southern area, Terfawi or Sheb would form the best base for exploration; at each of these places there is a good water-supply. I am inclined to think that the southern area is more likely to yield results than the northern one. Water was found to exist at a point some 15 kilometres west of Terfawi, and though no other source was observed on the way to Owenat, it was impossible to see very far on either side of the track, so that such a source might have been passed within a few miles without notice. Terfawi itself is very difficult to find, being inconspicuous even when one is fairly close to it; and as it is in a sandy area, tracks leading to it are soon obliterated. Another factor which favours this southern area is that, the country being all Nubian sandstone, the