certain deep-seated artesian wells the source is always the rain which falls in the immediate vicinity, as the physician knows when called to treat disease caused by seepage of the adjacent surroundings into the family well.
Part of the rainfall is quickly drained away by the surface channels, a part is evaporated, and a third, and for our consideration the most important part, is imbibed by the rocks and soil. The proportional disposition of the rainfall in the above manner varies with the climate and geologic conditions, but so far as underground waters are concerned it is necessary to consider only the water which sinks into the ground.
That portion of the earth visible to human inspection, known as the crust, is more or less saturated with water. In times of drought and in the arid region this is not always evident at the immediate surface, where evaporation is taking place, but a post hole, a plow furrow, a blast in a quarry, or a newly dug well reveals the dampness of the rock material. This moisture is sometimes invisible to the eye, but in general its quantity varies in proportion to the compactness or porosity of the rocks, the number of joints, fissures, or other crevices, and the topographic situation which controls the drainage.
If rainfall be long continued, the portion of the crust upon which it falls becomes completely saturated. Upon cessation of the rain, evaporation or drying begins at the surface, causing the line of saturation to sink deeper and deeper. Thus it is that in the Eastern States, where rainfall is excessive and evaporation slow, the line of saturation usually coincides with the surface, while in the arid regions it is often several hundred feet below. In this section, holes three hundred feet deep are often drilled through soil and rock as dry as powder without reaching the line of saturation, while on the East, as for example in New Orleans, water is so near the surface that dry graves can not be dug for the dead.
If the earth were of uniform porosity, temperature, and composition the water it contains would be uniformly distributed through it, as is the water in a well-soaked sponge. But this is not the case, for the outer portion of the globe consists of rocks of much less density than are those of the interior, while the downward percolation of water in some instances encounters the superheated mass of the earth's interior, and is forced back to the surface as steam, as in geysers and volcanoes, or enters into mineral combinations. Hence the available water is confined to that portion of the earth's crust between the lines of heated interior and surface evaporation. Even in this narrow belt the distribution of water is very irregular.
Inasmuch as there is a great diversity of geologic structure,