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the line of complete saturation of the bed. This level is somewhat lower than that of the lowest surface outcrop. If there are several incisions, the question is not entirely one of static equilibrium. For instance, if water is naturally received by the stratum in one part of its outcrop and discharged from another lower part, the conditions in the intervening area are those of dynamic rather than static equilibrium, and the water will rise in a well to a level intermediate between the receiving and discharging levels.

The artesian water bearing strata of the State east of the Pecos River are composed mostly of extensive sheets of sands, clays, and limestones, succeeding one another in orderly arrangement, except along the Balcones zone of faulting, and in general having a gentle inclination toward the sea, so that intraveling northwestward, although constantly ascending in altitude, one encounters the outcropping edges of rock sheets of lower and lower stratigraphic position. This produces the simple arrangement of a tilted plain built up of a series of alternately impervious and pervious layers. The rain falling upon the outcropping edges of the latter, sinks into the embed, and by gravity is conducted seaward down the plane of its inclination to lower levels beneath the surface. Each different stratum, including any particular water-bearing stratum, becomes embedded deeper and deeper to the southeastward of the point where it outcrops at the surface. This structure is very simple and its detail can be traced out, measured, and mapped as accurately as that of the successive layers of stone in a building. To do this is the work of geologists, and it is not a matter of speculation and hypothesis, but simply the application of specially acquired knowledge, similar to that which one must possess to be proficient in any profession.

The foregoing principles are all applicable in explaining the numerous artesian wells and springs of the Rio Grande Plain and the headwater springs of the Edwards Plateau, but the explanation requires also an understanding of the order and arrangement of the strata, and we therefore proceed to consider the geology of the region.

GEOLOGY OF THE REGION.

The rocks of this region consist of various beds of marl, clay, limestone, sandstone, etc., a few igneous rocks (proportionally a very small part of the whole), and some alluvial or surface deposits derived by erosion from the older formations. With the exception of the igneous and alluvial rocks, the strata are all composed of material which was laid down beneath the surface of the ocean, and embedded in them are found many marine fossils, the remains of animals which inhabited the waters of the old ocean. These fossils are of the greatest value in determining the geologic position of the beds containing them, and therefore we give from each of the principal beds a few illustrations of the most important and most abundant forms, by the aid of which the layman can readily recognize the geologic horizons. (Pls. LI-LXIV.)