Page:Popular Science Monthly Volume 42.djvu/626

 606 in a direction opposite to the general slope of the country, no matter how favorable the conditions, they will furnish no flowing artesian supply, for water can not rise above the height of the receiving area. (Fig. 5.)

If strata are excessively inclined, as in most mountain regions, artesian wells are improbable if not impossible over any wide area, for the strata soon dip below all available borings; hence the generally accepted idea that artesian wells are peculiar to regions of great stratigraphic dip is fallacious. A dip of one per cent is scarcely visible to the eye, but it will carry a stratum downward 52·8 feet per mile; a dip of ten per cent is hardly noticeable, but will carry a stratum 528 feet in a mile; a dip of forty-five degrees will carry a stratum deeper in a mile than any drill has yet penetrated.

If the earth's surface were level, and a homogeneous mass, earth water would be at a uniform depth throughout, as in an undrained field. But the surface is broken into mountains and plains, and scored by valleys, and the line of saturation sinks toward the level of these, where springs are often found escaping at the level of the streams. There are in Nature two kinds of valleys: (1) Unfinished, or active valleys, which are in the process of being cut out at the present time by the streams seeking base level; and (2) finished, or ancient valleys, which originated in past geologic time, and have been partially refilled with the débris of the adjacent region. All the valleys in the mountains proper, and of the eastern United States, belong to the first class, which may be called stream valleys, and their function is to furnish a channel for the passage of the surface waters to the sea. The valleys of the second class, or basin valleys are characteristic of the great arid region, and, with one or two exceptions, they are void of running surface water.

In mountains the surface and underground water is constantly seeking the level of the surrounding valleys, owing to the action of gravity. In general, mountains owe their existence to the superior hardness and imperviousness of their strata, and are of little importance to the problems of underground water.

Basin plains surrounded by the great areas of mountain surface are more favorably situated for the occurrence of underground water in quantity than those with a smaller surrounding area of mountain slopes, for impervious mountains serve to concentrate the rain-water which runs down their slopes upon the pervious valleys, thereby increasing the available water supply beneath the latter. (Fig. 6.)

The water of saturation in buttes and mesas, which usually consist of horizontal strata, is reduced by gravity toward the level of the surrounding plain, or, when alternations of pervious and