1885.] 



Report on the Deposits of the Nile Delta. 



219 



The examination of this table shows that the waters of the Nile, 

 judged either by the proportion of solids in solution, the percentage 

 of lime, or by the temporary and permanent hardness, exhibits some 

 very remarkable anomalies. That the variation in the quantities of 

 dissolved salts at different seasons of the year should be much greater 

 than in the case of any of the other rivers, is not surprising when we 

 bear in mind the great and rapid additions to the volume of the 

 river during seasons of flood. But it is startling to find that the 

 water of the Nile, instead of containing a much larger proportion of 

 saline matter than other rivers — as we might anticipate from the 

 enormous evaporation constantly going on from its surface — in reality 

 contains far less dissolved matter than any of the other rivers here 

 compared with it. This contrast is scarcely less striking in the case 

 of the comparison with the Severn and the Shannon, which flow over 

 Palaeozoic rocks, than in that of the Thames and the Lea, which 

 drain areas occupied by Mesozoic deposits. 



A little consideration will show, however, that this startling and 

 seemingly anomalous result is capable of a very simple explanation. 

 The substances dissolved in the water of rivers is of course derived 

 from the materials composing the rocks of the river-basin, through 

 the action of water holding carbonic acid or other acids in solution. 

 In this kind of action, rain which takes up the gases contained in the 

 atmosphere, and then percolates into the interstices of rocks, plays a 

 most important part. Rain is, in fact, the great agent of chemical 

 disintegration, and where rain falls the complex silicates composing the 

 hardest rocks are attacked, the silicates of potash, soda, lime, magnesia, 

 and iron being broken up and carried away in solution, while the 

 silicate of alumina takes its hydrated form of kaolin, and remains 

 behind or is removed in suspension. 



But in districts where there is little or no vegetation and the 

 rainfall is sudden and torrential, this chemical disintegration, as has 

 been already pointed out, is replaced by totally different kinds of 

 action. Under the influence of variations of temperature, having 

 an enormous range, the rocks made up of crystals having different 

 coefficients of expansion, in different minerals and indeed in different 

 directions in the same crystal, undergo mechanical disintegration, and 

 the fragments thus formed are driven backwards and forwards by 

 wind, being thereby subjected to constant attrition. Of this latter 

 kind of action, as we have seen, all the larger particles in the Nile 

 Delta deposits exhibit the most unmistakable evidence. 



Hence we are led by an examination of the composition of the 

 Nile- water to the same conclusion as was reached by the study of 

 microscopical characters of the mads and sands of the delta, that 

 while in the rainy districts of the temperate zones, the disintegration 

 of rocks is mainly effected by chemical agencies, in the desert areas 



