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
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 rook, is entirely obscured by these deposits, and of no value to the artesian possibilities. The rainfall upon the mountains is rapidly shed by canon-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