266 G. W. Grabham— Weils of North- Endern Sudan. 



the south, and one passes through all intermediate grades between 

 these extremes. In the area between latitudes 12° and 22" north, 

 water is the most important factor in determining the positions of 

 routes, and it is with part of this area that it is proposed to deal. 

 South of this, water is more abundant and other factors have to be 

 considered. 



The country is formed of a series of old sedimentary rocks, among 

 which many different kinds of igneous materials have been intruded. 

 These were subjected to extensive earth movements before the 

 volcanic and intrusive rocks immediately underlying the Nubian 

 Sandstone were erupted. In dealing with our subject we may speak 

 of all these collectively as the Older Crystalline Kocks. They are all 

 impermeable except where they are sufficiently decomposed to allow 

 the water to percolate along joints and pores. The Nubian Sandstone 

 Series rests on these Older Crystalline Eocks and fills up the hollows 

 of the old land surface. This series consists mainly of sandstones, but 

 it also includes beds of clay. As the latter never appear to have any 

 great lateral extent, the whole series may be regarded as permeable. 



Artesian conditions have not been met with, and water supplies are 

 dependent on rainfall, seepage from the river, and local geological 

 conditions. 



The surface deposits vary with the climate. The rain-bearing 

 monsun only reaches about as far north as Berber, and beyond this the 

 rainfall is limited to showers often with intervals of years between 

 them. At Khartoum we have about 4 inches per annum, and the 

 amount increases as one goes southwards and eastwards. Kassala, 

 due east of Khartoum, has about 12 inches of rainfall, and the 

 following list' serves to give an idea of the variation in quantity 

 along a more or less east and west belt, about 120 miles south of 

 these places : — 



El Obeid, 12 inches. "Wad Medani, 17^ inches. 



Dueim, 8 inches. Gedaref, 24 inches. 



Gallabat, a town south-east of Gedaref, has a rainfall of SS^- inches. 



On the Maritime Plain the rain does not accompany the monsun, but 

 occurs in the winter. Suakin has about 12 inches of rainfall per 

 annum. 



In regions of the Sudan Plains, where the rainfall amounts to 

 about 8 inches or more, the surface deposit is typically Cotton 

 Soil, and where there is less the conditions approximate to those 

 described by Mr. H. T. Ferrar.* The Cotton Soil is very similar to 

 the Indian 'Eegur'^ in physical characters. It is fine-grained, 

 absorbs a very large quantity of water during the rains, and, as this 

 evaporates in the dry season, the soil becomes fissured with wide 

 cracks often 6 feet deep. As far as water supplies are concerned, it 

 is quite impervious, for though it will absorb a large rainfall, it is so 

 fine-grained that the water cannot pass through to reach a porous 



^ Eainfall statistics derived from Physiography of the Nile Basin, H. G. Lyons, 

 Cairo, 1906. 



2 Geol. Mag., 1907, p. 459. 



2 Manual of the Geology of India, 2nd ed., revised by R. D. Oldham, p. 410. 



