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)