cline 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

SEA

מ

impervious strata occur, the water seeps out as springs at their contact. The Llano Estacado, or great Staked Plain of Texas and New Mexico, is the largest of all the American mesas in area. Its geological structure is practically that shown in Fig. 3, consisting of a pervious surface formation, averaging three hundred feet, resting upon a foundation of impervious clays and other rock. The upper formation readily imbibes all the surface rainfall; hence the region is void of running streams.

Throughout this large area, once considered hopelessly void of water, good non-flowing wells are now everywhere obtained by boring to the lower depths of the saturated, sponge-like surface formation, while springs occasionally break out at the margin of the plains where the two formations are in contact.

While water-bearing strata should always be porous, and usually are but slightly if at all consolidated, the degree of consolidation has but little bearing upon the retaining function of impervious strata. Soft clay shale is practically as impervious as hard slate. In the West many people discredit the possibility of artesian water in many favorable localities, because of the absence of consolidated strata which they suppose are necessary to constitute the impervious stratum above the one containing the water. In fact, the less consolidated the rocks of a region are, the more favorable are the artesian conditions; and, inasmuch as the older formations of the earth are more consolidated, metamorphosed, and disturbed by greater tilting, faults, and folds, they are least favorable for the occurrence of artesian water. Upon the other hand, the later formations present the opposite and more favorable conditions, and with few exceptions the great artesian wells of the world are found in them. These later rocks play an important part in the geology of the arid region.

The adjoining section from the Gulf of Mexico to the Rocky Mountains, from Galveston to Las Vegas, New Mexico, illustrates some of the principles herein set forth. From the coast to the ninety-seventh meridian is a large series (a)

Negative.

9

of alternations of porous and impervious strata of the later geological ages, dipping at an almost imperceptible angle toward the sea and accompanied by slight scarp valleys along their western outcrop, which are the receiving areas for the artesian waters. The Atlantic coast plain from New Jersey to the Rio Grande nearly everywhere presents similar conditions, and abundant artesian wells have been obtained. This group of rocks rests upon another series of older rocks (b, c), which presents negative conditions for artesian water, owing to their inclination in a direction opposite to that of the topographic slant. No artesian wells of large flow have been, or are apt to be, obtained in this region. Above the west part of this series is the great mesa of the Llano Estacado (d), the non-flowing wells of which have been explained. A second negative area is shown in the portion of the diagram in northeastern New Mexico (d), where the inclination of the strata is again opposite to that of the topographic slant. Where the front of the Rocky Mountains appears (e), the principle that the mountain rocks are unfavorable for artesian conditions is shown by the faulting and excessive dip of the strata.

Let us now briefly examine the bearing of the foregoing principles on the question of underground water in the great arid

FIG. 9.-FILLED-IN VALLEY OF ARID REGION.

region proper, west of the Rocky Mountain front. Topographically this country, from the union of the Cordilleras in southern Mexico to the British boundary, consists in alternations of mountain and desert plain (Fig. 8). The mountains are isolated masses of hard, impervious rock, broken by faults, and dipping at angles which render the strata unpropitious for artesian exploitation. The wide areas of desert plain separating the mountain masses are of the older type of valleys described on a previous page, which are now filled to a depth of two thousand feet by the detrital deposits from the adjacent mountains (Fig. 9). The original valley floor, consisting of mountain rock, is entirely obscured by these deposits, and of no value to the artesian possibilities. The rainfall upon the mountains is rapidly shed by cañon-streams and arroyos to the level of the adjacent valleys, where it sinks into the ground, owing to the thirsty character of the valley formation, and gravitates downward toward the lower and usually central depths of the deposit, the underlying floor of mountain rock serving as a