EEPOBT OF CHEMICAL LAHOHATORY 



The actual amount of clay in the above soils was approximately : — 



1st foot 

 2nd „ 

 3rd ,, 

 4th „ 



58 per cent. 

 56 ,, ,, 

 61 ,, ,, 

 64 „ „ 



It is not held that the above is proof that cementation of the clay particles by calcium 

 and magnesium carbonates does not occur. It doubtless does occur to a certain extent, but 

 it is felt that some more potent cause is active in the majority of cases. In the regions of 

 no rainfall, e.g. Dongola Province, the soils were at first thought to contain practically no 

 clay whatever. Agitation with water, or with water and ammonia, resulted in the bringing of 

 a quantity of material into suspension, but practically the whole of this settled out at the 

 end of a few hours. It was at first assumed that this was due to soluble salts, but the 

 absence of these in the majority of cases was soon proved. It was then concluded that 

 the material present was fine silt, its preservation from decomposition being the result of 

 the total absence of rainfall. Later, these soils were treated with water containing a small 

 amount of sodium carbonate, and were also puddled by the aid of a camel-hair brush 

 as described on page 37, with the result that a large amount of clay was separated. The 

 results of the mechanical analyses carried out in this way at once explained the behaviour of 

 the soil in the field.' It should be stated that the proportion of calcium carbonate in these 

 soils was usually low — in one case only 0'36 per cent. — so that the explanation of the 

 behaviour of the soil to water by its cementing action does not appear to be tenable. 



In the writer's opinion, the intense baking which these heavy soils receive during the 

 hot season plays a very important part, in that it coagulates the clay and renders the soil 

 more permeable and productive. This is in addition to the well-known fact that the 



' These soils puddle very rapidly when not sufficiently drained 



