lands. Large sums of money have also been wasted in vainly bombarding the skies for rain, contrary to every known law of Nature. The Department of Agriculture has investigated the

Strata b.

a c Impervious Strata enclosing water-bearing

Conditions of Artesian Wells

a

FIG. 1.-PREVALENT CONCEPTION OF ORDINARY ARTESIAN CONDITIONS.

Drawn from a Geological Report.

underground waters of the Great Plains region east of the Rocky Mountains, but the underground supply of the true American Desert lying between the Rockies and the Sierras has been little studied.

This section includes one fifth the total area of the United States and most of the great central plateau of Mexico. It is marked by peculiar geographic, geologic, and climatic phenomena altogether different from those of the rest of the country, chief of which is the absence of surface water. Streams are rare even in the mountains, and, with the exception of the Colorado, the Snake, and the Rio Grande, not a drop of its surface water reaches the sea, so great is the evaporation and the capacity of the porous desert soils for absorption. Almost any Eastern State has a greater area of surface water than has all the arid region; and the smallest New England brook, could it be transported West, would be a great blessing. In this arid section there are many thousand square miles without a drop of water even for drinking purposes. Nearly every available stream has been appropriated for irrigation by the present population, and all improvement in the water supply must come from underground sources.

It is wrong to encourage anticipations of enormous supplies of underground water where rainfall is so slight; but when we remember that in this region water is of greater value than land, or rather that land is worthless without water, the procurement of even small supplies, sufficient for stock, for irrigating small areas, or for supplying the thirsty locomotive, will be of great value. In view of these facts it is well to understand the laws of the occurrence

and availability of underground waters, for not only have large sums been wasted in boring in unfavorable localities, but impracticable notions have been obtained from scientific treatises on this subject.

The laws of the distribution and utilization of underground water are as simple as those controlling the surface supply, but the popular fallacies concerning them are appalling. The most prevalent of these is that the waters originate at some remote point from their outlet, and flow in subterranean streams like the "blood in the human body," as a farmer once said, and that these streams must be tapped by the well borer or digger before water can be obtained. In nearly every community is some person supposed to possess the art of locating the exact spot above these currents by means of a switch called the divining rod. It is also a current fallacious belief that all underground water is due to rain which falls on the more or less distant mountains, and especially is this true in the region between the Rocky Mountains and the Mississippi, where every spring and well, even on the Texas

FIG. 2.-FAVORABLE STRUCTURE FOR ARTESIAN WATER, IN WHICH THE RECEIVING AREA IS A VALLEY.

coastal plain a thousand miles distant, is commonly explained upon the hypothesis that the water comes from this lofty range.

These prevalent impressions in the minds of those untrained in geology are more excusable than the widely prevalent idea conveyed by cuts in geological text-books that the usual and ordinary conditions for artesian wells are in great synclinal areas in which the strata can be seen markedly dipping from two including mountain borders against which their edges are upturned as shown in the following figure.

While there is no theoretical objection to this ideal conception, the conditions it represents seldom occur in Nature; on the contrary, as will be shown later, mountain rocks are not the source of great artesian wells; neither do they usually occur in synclinal valleys, but the most favorable conditions are gently sloping monoclinal plains in which the receiving areas, instead of being the upturned mountain rocks, are, in fact, the escarpment valleys of the plains. (See Fig. 2.)

To understand the distribution of earth water, it is necessary to be familiar with the true laws of its occurrence. The rainfall is the source of all underground water, and with the exception of

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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,

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.

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 mountainfolding, 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

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

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 finegrained 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

FIG. 5.-UNFAVORABLE CONDITIONS FOR ARTESIAN WATER.

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 in