5322 NORTH-EAST AFRICA. penetrated mucli farther southwards than at present. But in that case it is difficult to understand how these fossil woods could have been stranded in such a good state of preservation, and, moreover, without being associated with any of those vegetable or animal marine organisms which are always found adhering to driftwood. Nor is any theory advanced to explain how this flotsam and jetsam could have been borne over lofty mountains to the upland plateaux of Abyssinia. On the other hand it is impossible to suppose that these petrifications can have been brought down by fluvial currents such as that of the Nile, because they are nowhere associated with any alluvial deposits. Hence these sterculiaceae of the Nilotic basin must be regarded as still in situ, or at least in the immediate vicinity of the places where they originally flourished. The opinion which finds greatest
- favour with geologists is that the vegetable fibres were gradually petrified under
the action of thermal waters, such as still occur in various parts of Egypt, and especially in the region of the oases. Becoming saturated with these waters, the fallen trunks would be gradually changed to stone, just as they become converted into peat or turf in the swampy districts of more northern latitudes. Doubtless the petrifications of herbs and other vegetation at present going on round about the geysers of Iceland and of Montana in North America, differ from those of the Egyptian deserts in their general appearance and process of formation, for in these districts the plants are changed not into particles of quartz but into amorphous flints. But allowance should, perhaps, be made for climatic dif- ferences and for the long action of time. Close to the " petrified forest " of Cairo is observed a dome-shaped sandstone hill, the Jebel-el-Ahmar, or ** Red Mountain," the interior of which is easily quarried, thanks to the softer character of the deeper strata. May not this sandstone hill, isolated amid the surrounding nummulitic limestones, have been gradually accumulated by the action of some ancient geyser ? And to the similar action of thermal springs may we not attribute the preservation of the trees on the neighbouring plain, which at that time was doubtless thickly wooded ? The Western Oases. To the west of Egypt as well as to the west of Nubia a chain of oases is developed which describes a curve almost parallel to the course of the Nile. The first of these oases is that of Kurkur, which although scarcely more than 60 miles from Assuan, has never been inhabited. At about the same distance in a north-westerly direction stretches the so-called *' Great Oasis" of the ancients, now known as that of Khargeh, from the name of its chief town. Including the palm- groves of Beris, it occupies a depression stretching north and south for a distance of 90 miles. It does not, however, form one continuous oasis, but rather an archi- pelago of small oases, a cluster of cultivated islands separated from each other by intervening tracts destitute of vegetation. West of Khargeh lies the oasis of Dakhel, or Dakleh, that is to say, the "Interior," also known as the Wah-el-Gharbieh, or "Western Oasis." Dakhel is
