454 SUBTEERAlSrEAN WATEES. 



many of the most fertile districts of I^orthern and "Western India, 

 and changing them into sterile deserts. It consists principally of 

 sulphate of soda (Glauber's salts), with varying proportions of 

 common salt. These salts (which in small quantities are favorable 

 to fertility of soil) are said to be the gradual result of concentrar 

 tion by evaporation of river and canal waters, which contain them 

 in very minute quantities, and with which the lands are either ir- 

 rigated 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 proportion 

 of salts is greatest, and it either sinks into the superficial soil, car- 

 rying with it the saline substances it holds in solution, or is evap- 

 orated 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 I^ubia lying above the reach of 

 the floods, as I have said, we may suppose to have been first cul- 

 tivated in that remote antiquity when the Nile valley received its 

 earliest inhabitants, and when its lower grounds were in the con- 

 dition 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. 



SubteTranea/n Waters. 



I have frequently alluded to a branch of physical geography 

 the importance of which is but recently adequately recognized — 

 th(; subterranean waters of the earth considered as stationary 

 reservoirs, as flowing currents,* and as filtrating fluids. The 



* Among the anomalies of the geography of Australia, mention is often 

 imade of the fact that the rivers run the wrong way. In truth, a large portior 



