66 Darfur.

A great valley, the Wady Gendy, has its origin on the southern slopes of Jebel Marra, and probably runs into the Bahr el Arab ; whilst those already mentioned on the west are likely to be affluents of Lake Tchad.

To the south of Dara the watercourses are insignificant. Wells are usually found in the small depressions where rain water collects, but this becomes scarce in the dry season.

During the kherif the southern portion of Darfur becomes a huge lake and impassable.

The geological formation is very varied ; in the west, the mountains show a volcanic origin ; in the north and south, granite and sandstone are the prevailing rocks ; in the east, the soil is sandy and contains a quantity of iron which is worked to a small extent.

In the east and north-east, granite predominates, with the exception of a strip between Foga and El Fasher, where red and white sandstone crop up.

In the north, Wadi Millit and the hills in its neighbourhood are of gneiss. To the north-east of this granite again predominates at Saya, whilst still further north Jebel Tagabo is of sandstone.

Jebel Midubh contains both sandstone and granite ; this group has been much distorted by volcanic agency, and beds of lava are to be seen in all directions. To its south-west lies Bir em Malha, an extinct crater which, to outward appearance, is an insignificant hill, but has a depth of about 150 feet. Here is a small lake strongly impregnated with alkaline matter, while sweet water springs issue from the sandstone and granite declivities.

The greatest elevation of Jebel Midubh does not exceed 3,500 feet, and the plateau between it and Jebel Tagabo is about 1,200 feet.

The Jebel Marra group is also of volcanic origin ; lava and granite are to be found everywhere ; but there is no sandstone ; small peaks of pink granite crop up here and there between the mountains and El Fasher.

About Jebel Turah, one of the offshoots of the main group, the height of the plateau is about 4,400 feet, whilst the peak itself is about 5,500 feet.

Stretching from the main group in a westerly direction for a distance of 30 or 40 miles, is a huge djke of white quartz with a plateau of sandstone raised some 300 feet