1885.] 



Report on the Deposits of the Nile Delta. 



217 



disintegration of the solid rocks is effected by mechanical means ; 

 the most potent of these mechanical agents of disintegration are the 

 heat of the sun, causing the unequal expansion of the minerals which 

 build up the rocks, the force of the wind, producing constant 

 attrition of the disjoined particles, and torrential rains. 



This being the case, it will be readily understood that the sand- 

 grains will include felspar and other minerals in a nearly unaltered 

 condition, while in countries where the chemical agents of the atmo- 

 sphere come into play such particles would be more or less com- 

 pletely converted into kaolin. In the same way, the mud, instead 

 of consisting of scales of kaolin originating from chemical action, will 

 be formed of particles of the chemically unaltered minerals reduced to 

 the finest dust by purely mechanical agencies. 



The chemical analyses which have been made of these Nile-muds 

 tend to support these conclusions. Instead of containing large 

 quantities of combined water, as do all the ordinary clays, their 

 composition is that of a mixture of anhydrous minerals. As the 

 analyses hitherto made have none of them been undertaken with the 

 object of estimating the exact proportions of hygroscopic and com- 

 bined water in these muds, it would be well if some more careful 

 determination of these two constituents could be made. 



But there is fortunately a kind of evidence, derived from chemical 

 analysis, which is of the greatest value from its bearing on the 

 questions we are now discussing — that, namely, which is obtained from 

 a study of the composition of the Nile- waters. 



It must be remembered that the Nile is a river of a very peculiar 

 and exceptional character. The last tributary which it receives is the 

 Atbara, which falls into it in lat. 17° 38' N. ; from that point to its 

 mouth, in 31° 25' N. lat., the river does not receive a single affluent ; 

 for a distance of 1400 miles, indeed, it obtains no fresh supply of water 

 except what is brought to it by superficial torrents after heavy rains 

 in Lower Egypt. It has been clearly demonstrated that, after 

 receiving the Atbara, the Nile undergoes a continual diminution in 

 volume in its course through Egypt. This is no doubt in part due to 

 percolation of the water through the delta- deposits, and in part to the 

 water being drawn off in canals for purposes of irrigation ; but a large 

 part of this diminution in volume must certainly be ascribed to the 

 great evaporation which must be going on from the surface of the 

 river during the last 1400 miles of its course. 



The diminution which takes place in the volume of the Nile in its 

 downward course is illustrated by the following estimates : — 



The French engineers in 1799 showed that at Siut, 240 miles above 

 Cairo, the quantity of water passing down the river per second 

 amounted to 678 cubic metres at low water, and 10,247 cubic metres 

 during high water. 



