224 H. G. McKINNEY. 



immense swamps south of Fashoda and Lake Dembea in Abyssinia, 

 are natural storage reservoirs which moderate the height of floods 

 and maintain the regularity of the supply in the river ; while the 

 volcanic soils from Abyssinia, the organic matter from the swamps 

 in what is known as the Sadd Region, and the lime brought down 

 by one of the southern tributaries, combine to form a sediment 

 which could scarcely be surpassed in fertilising properties. The 

 rivers of Upper India are in flood throughout the summer months 

 owing to the great areas of snow-covered mountains near their 

 sources, but their waters have no such fertilising properties as 

 those of the Nile. As the natural result of this, the alluvial 

 deposits of Egypt are incomparably more fertile than those of 

 Upper India. 



Extensive natural irrigation. — Before leaving the general con- 

 ditions under which irrigation works on an extensive scale are 

 practicable, it is necessary to refer to a variation of these con- 

 ditions under which irrigation takes place, either naturally or 

 with comparatively little assistance to the operation of nature. 

 It may be stated as a general rule, that the formation of deltas 

 and alluvial plains is due to successive inundations extending 

 over long periods of time. The conditions indicated by such 

 plains are best understood if we look back in imagination to the 

 time when the river to which these plains owe their existence 

 flowed in a well-defined valley, and with tributary streams flowing 

 in through ravines and minor valleys along its course. The silt 

 brought down from the upper parts of the catchment area must 

 have been deposited in layers which gradually obliterated the 

 outlines of the lower part of the valley, covering dale and hill 

 alike under a continuous sheet of alluvium through which the 

 waters of the river had to make their way in ever changing 

 channels and under ever-increasing difficulties. Where the course 

 of a river extends for a long distance under such conditions and 

 with a low rate of fall, the river channel contracts till it becomes 

 unequal to the task of carrying on a high supply. Hence there 

 is an increasing tendency to the spread of the flood waters and to 



