DEPOSITION OF SALTS. 469 
and with which the lands are either irrigated or occasionally 
overflowed. The river inundations in hot countries usually 
take place but once in a year, and, though the banks remain 
submerged for days or even weeks, the water at that period, 
being derived principally from rains and snows, must be less 
highly charged with mineral matter than at lower stages, and 
besides, it is always in motion. The water of irrigation, on 
the other hand, is applied for many months in succession, it is 
drawn from rivers and canals at the seasons when the propor- 
tion of salts is greatest, and it either sinks into the superficial 
soil, carrying with it the saline substances it holds in solution, 
or is evaporated from the surface, leaving them upon it. 
Hence irrigation must impart to the soil more salts than natural 
inundation. The sterilized grounds in Egypt and Nubia lying 
above the reach of the floods, as I have said, we may suppose 
them to have been first cultivated in that remote antiquity when 
the Nile valley received its earliest inhabitants, and when its 
lower grounds were in the condition of morasses. They must 
have been artificially irrigated from the beginning; they may 
have been under cultivation many centuries before the soil at 
a lower level was invaded by man, and hence it is natural that 
they should be more strongly impregnated with saline matter 
than fields which are exposed every year, for some weeks, to 
the action of running water so nearly pure that it would be 
more likely to dissolve salts than to deposit: them. 
Subterranean Waters. 
I have frequently alluded to a branch of physical geography, 
the importance of which is but recently adequately recognized 
—the subterranean waters of the earth considered as stationary 
reservoirs, as flowing currents, and as filtrating fluids. The 
earth drinks in moisture by direct absorption from the atmos- 
phere, by the deposition of dew, by rain and snow, by percola- 
tion from rivers and other superficial bodies of water, and 
sometimes by currents flowing into caves or smaller visible 
