34 THE GEOLOGICAL STRUCTURE, ETC. 



floor of impervious Cretaceous shales insures the return of this 

 moisture to the surface, in the form of powerful springs. The 

 underlying Cretaceous beds are similarly arranged. 



The rainfall of the drainage area of the Mississippi amounts 

 to 620 cubic miles per annum. The elaborate experiments of 

 Messrs. Humphreys and Abbot have shown that of this amount 

 only 107 cubic miles reaches the Gulf of Mexico by surface 

 water channels, leaving 513 cubic miles to be accounted for. 



Experiments as to evaporation have been made at different 

 times in various parts of the world. These show that the 

 amount of rainfall lost by evaporation varies within very wide 

 limits, doubtless depending upon different climatic conditions. 



In the case of the Mississippi, assuming that 50 per cent, of 

 the rainfall is evaporated, this would leave a balance of 310 

 •cubic miles, which must precolate below the surface. To pro- 

 vide against all possible contingencies, taking even 75 per cent, as 

 evaporated, this would still leave the large quantity of 155 cubic 

 miles per annum as available for underground use. In other 

 words, 3,611,310,743,484 gallons per diem must disappear 

 beneath the surface of the drainage area of the Mississippi 

 watershed. 



In the Great Plains of Nebraska, Kansas, Oklahoma, and 

 Eastern Colorado, there is no lack of means whereby the surface 

 waters reach the permeable strata, in addition to that which 

 falls directly upon the catchment area. 



" There are a number of so-called ' creeks,' some of them 

 *' of considerable length, having broad sandy beds and numerous 

 ■" tributaries, draining large areas of country, and through which 

 " great floods of water pour at times, yet which have no outlet 

 " into any other stream or visible body of water. Among the 

 " most notable of these are White Woman C^'eek and Bear Creek. 

 *• The former rises in Eastern Colorado, in Kiowa County, 

 •' traverses Greely and Wichita Counties, in Kansas, and ends 

 " in a broad basin in the centre of Scott County, through the 

 " bottom of which the water, which sometimes comes down in 

 " great quantities, quickly escapes to the sheet-water below. 



" This creek drains an area of some 1,500 square miles ; 



" i.e., it is the outlet for such of the surface waters as escape 



above the 'surface of the ground. Bear Creek rises in Baca 



