Page:Popular Science Monthly Volume 42.djvu/624

604 distinct from its capacity for imbibition. If the component particles of a rock—for instance, the quartz pebbles of a loose conglomerate or the grains of a sandstone—present an impervious surface, water will cohere to the individual surfaces until the entire specimen is enveloped in a coat of water. If the interstices are smaller than the average drop of water, the resistance of cohesion to the transmission of water will be greater; hence a chalk or a fine-grained brick will drink in much water, but will transmit it slowly, while water will pass rapidly through coarse gravel. The capacity for transmission in variously grained rocks and the accompanying cohesion is similar to that seen in passing water through sieves of different mesh. Thus, some sandstones of exactly the same capacity for imbibition as chalk transmit water six hundred times faster.

The rock materials of the earth with these different capacities for imbibition and transmission have been sorted into definite sheets or strata by the water which deposited them, and thus another important fact in the question of underground water is introduced—the stratification or arrangement of the rocks relative to one another. Earth water percolates downward through a



porous stratum until an impervious one is reached, while an impervious stratum at the surface will prevent the saturation of a pervious one below. Stratification performs the important function of controlling the distribution of earth water, of resistance, transmission, and storage. If the surface rock stratum is pervious and horizontal, it simply serves as a sponge to hold the water until disturbed by evaporation or seepage, unless the supply is constantly renewed by rainfall. (See Fig. 3.)

If an impervious sheet is above an inclined outcropping porous stratum (Fig. 4), it opposes the tendency of water to rise by hydrostatic pressure and retains it in the porous sheet. If an impervious stratum is beneath a porous one, it prevents the water of the latter from percolating to greater depths. If vertically arranged from folding, the including strata cut off the horizontal transmission of underground water. (Fig. 5.)

Water is transmitted by gravity in the direction of the inclination of the strata—i. e., with the dip; and if the topographic conditions are favorable, flowing wells can be obtained at lower points more or less distant from the outcrop. If the strata