Page:Popular Science Monthly Volume 42.djvu/623

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the possibility of securing water at any given point must be determined by the local formations. All rocks imbibe moisture in proportions varying with their physical structure, a fact which can be demonstrated experimentally by saturating familar types of rocks. Glass is similar in water capacity to large areas of volcanic and other igneous rocks, and will absorb no perceptible amount of moisture; marble will drink in only a slight quantity; while chalk, sand, and brick will absorb nearly their own weight of water. The manner in which rocks absorb water is simple.

PSM V42 D623 Mesa structure of llano estacado.jpg
Fig. 3.—Mesa Structure of Llano Estacado.

In most rocks, however compact to the eye, there exist interstices, cavities, and other spaces in which water may enter and be stored. This is especially true of all sedimentary rocks, which comprise ninety-nine per cent of the earth's crust. A fine sandstone whose grains and intervening spaces are indistinguishable to the eye, when placed under the microscope resembles a mass of cobblestones in which the spaces occupy as much of the aggregate area as the solid particles. Into a gallon measure of dry pebbles varying in size may be poured half a gallon of water. The consolidated rocks which compose most of the mountain masses are more compact and less adapted for the storage and passage of water than the sedimentary rocks. Nearly all the minerals which compose them are impervious, as is readily seen in a large crystal of quartz, feldspar, or mica. The rocks of valleys and plains usually consist of detrital material less hardened by mountain-folding, and hence more pervious.

Rocks which have imbibed all the moisture they can contain are in a condition of saturation, and all water in excess of this

PSM V42 D623 Artesian functions of impervious and pervious strata.jpg
Fig. 4.—Artesian Functions of Impervious and Pervious Strata.

amount will pass off by gravity or evaporation. The excess above the water of saturation is available as the source of springs, but the supply of wells is from the water of saturation.

Each kind of rock has an individual capacity for the transmission of the water which it has imbibed, and this is entirely