268 Q. W. Gvabham— Wells of North- Eastern Sudan. 



Wells in" Areas of Crystalline Rock covered with Cotton' Soil. 



A well in such a situation requires very special conditions for its 

 existence. The Cotton Soil is impervious, and attempts to make wells 

 in it generally prove the driest spots during the rainy season when the 

 country around is flooded. The blanket of Cotton Soil must be broken 

 by an ' island' of rock, and then, if the rock is capable of decomposing 

 into permeable material, we have the requisite conditions for a water 

 supply. In all successful cases I have seen the soil is pierced by an 

 igneous rock. The water is held in the decomposed rock and debris 

 round the base of the hill, while the surface of the ground is largely 

 composed of sandy detritus from the hill-slopes, through which the 

 water can pass and saturate the porous material beneath. 



A few special cases may be mentioned: — 



Geili. The hill is formed of syenite, and around the main one there 

 are several ranges of low hills breaking through the plain. The 

 wells are nearly all situated on the plains that separate the different 

 hills. The water is found at a depth of about 60 feet in soft 

 decomposed rock, and is generally of good quality. 



Nasla. The rock here is a felsite, and the water is derived from 

 joints, as the rock does not decompose to a spongy mass. The natives 

 have wells at Um Rueishid, about 8 miles north, and these are in 

 similar rock yielding abundant water. The stone is far too hard for 

 the native to tackle with the implements at his disposal, and so, 

 instead of making a vertical well, he has taken the line of least 

 resistance where the joints allowed him to pick his way, and the 

 shafts are even horizontal in part of their course. 



El Sofeiya. These wells are in decomposed granite. There is no 

 actual hill, but the country slopes away in all directions from the 

 granite outcrop. The wells are situated at the top of the rise where 

 the soil is sandy, and there are outcrops of fresh rock around them. 

 The water is obtained at a depth of about 50 feet. 



There are many other cases, but those I have mentioned are fairly 

 tj'pical. The water is nearly always good, and only in one case have 

 I found it disagreeable. This was from some wells in decomposed 

 granite, and was found to contain magnesia and sulphates by 

 Dr. Beam, who tested a sample at the Wellcome Research Laboratories. 



Water has recently been obtained in a well sunk hj the Grovern- 

 ment at Khor Ogod el Bagar, about 20 miles from the river on the 

 road from El Sofeiya to the Atbara. It is situated in a shallow khor 

 traversing a Cotton Soil plain, and there are no hills within 10 or 15 

 miles. The rock is a foliated quartz-mica-diorite traversed by 

 pegmatite veins. At the time of my visit the work was in quite 

 fresh rock, and the chances of obtaining water in it seemed very 

 remote. Softer rock appears to have been found as the depth 

 increased, and from this the water is derived. The occurrence of 

 water in such a situation is an exception to the rule, and I think it is 

 more likely to be due to seepage from the river than to local rainfall 

 percolating through the Cotton Soil and fresher rock above. In other 

 cases, where the wells have been situated at a distance from the 

 river, harder and fresher rock was found as the depth increased. 



