DETAILED ACCOUNT OF SOME RECENT EXPERIMENTS. 25 



dry, and it is only when there has been a shower that they give 

 passage to a stream of water which bears sand and boulders if 

 the shower has been abundant enough. The more important chan- 

 nels in the fan, those that are, for the time being, so situated as to 

 collect a great amount of water, carry their burden farthest and 

 send tongues of gravel reaching the plain. Like all rivers they 

 are apt to shift their position, and the coarse gravels in the aban- 

 doned channels become gradually buried under the deposits of fine 

 silt known as " loess/' which are derived from the disintegration of 

 neighbouring rocks or consist of wind-borne dust floated from 

 afar, and which gradually get spread over the land by the slow and 

 gentle action of rain-wash. The same process is many times 

 repeated till the plain is occupied by a succession of irregular 

 tongues or patches of loose permeable gravel alternating with layers 

 of silt which are highly impermeable owing to their fine texture 

 and argillaceous composition. The tongues of gravel decrease in 

 coarseness as they slope towards the plain; they are entirely en- 

 closed by the impermeable clays except at the apex of the alluvial 

 fan where, by increasingly coarse material they all come into com- 

 munication with the chaotic accumulation of boulders at the de- 

 bouchure of the river from the hills. Owing to the torrential nature 

 of the rivers and the peculiar structure of the alluvial fans, the 

 gradients are high, so that there is a considerable difference of level 

 between the coarse gravels at the head of the fan, and the plain over 

 which the deposits are distributed. Thus arises a condition which 

 is eminently suited to artesian action. The coarse nature of the 

 deposits at the head of the slope of the "daman" favour 

 percolation ; the water gradually finds its way into the buried 

 tongues of gravel, and the impermeable clays which enclose these 

 gravels prevent all natural escape, thus producing an" artesian 

 reservoir which is eminently of the " perfect" type. Where these 

 reservoirs can be tapped they are extremely useful, since by means 

 of them it is possible to distribute to the arid soil of the plain the 



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