G. W. Grahham— Wells of North-Eastern Sudan. 267 



rock beneath, should one exist. The amount may vary from the 

 thinnest covering to over a 100 feet in depth. It covers the 

 country like a blanket, forming level plains broken only here and 

 there by hills of hard rock that rise up like islands in a sea. 



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From the brief description of the country just given it will be 

 seen that wells can be classified according to the way the water is 

 derived. They fall at once into two groups, namely, those in crystalline 

 areas and those in sedimentary rook. The first group can be divided 

 again according as the wells occur in Cotton Soil districts or in the 

 more arid regions to the north. The wells in the Nubian Sandstone 

 Series are not susceptible to this distinction. We shall now pass on 

 to consider these different classes of wells separately. 



A general map of the area (Fig. 1) shows the positions of most of 

 the places mentioned, and a dotted line on it roughly indicates the 

 northern limit of the Cotton Soil. 



