ARTESIAN WATERS IN THE ARID REGION. 609 



of alternations of porous and impervious strata of the later geo- 

 logical 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 arte- 

 sian wells have been obtained. This group of rocks rests upon 

 another series of older rocks (b, c), which presents negative con- 

 ditions 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 prin- 

 ciples on the question of underground water in the great arid 



Fig. 9. — Filleh-in Valley of Arid Eegion. 



region proper, west of the Rocky Mountain front. Topograph- 

 ically this country, from the union of the Cordilleras in southern 

 Mexico to the British boundary, consists in alternations of moun- 

 tain 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 gravi- 

 tates downward toward the lower and usually central depths of 

 the deposit, the underlying floor of mountain rock serving as a 



