96 PROCEEDINGS OF WASHINGTON MEETING. 



These beds, like all the post-Tertiary deposits, arc chiefly marked by their noncon- 

 solidation, the sands, clays and gravel being almost as loose as when first deposited. 

 White chalky lime strata, or " tierra blanca," resembling the Cretaceous beds, are 

 numerous; but upon examination they are always found to he conglomeratic and 

 composed of debris of the "jeso," or decom'posed gypsum of the Red hods, and the 

 chalky strata of the Cretaceous, mixed with the mountain debris. 



These beds were clearly laid down in the mountain troughs or valleys by lake 

 sedimentation, and are of later age than the Llano Estacado formation. They 

 never enter into the disturbed mountain structure, but are deposited unconformably 

 like a matrix around the mountain bases. Their depth or thickness would be dif- 

 ficult to estimate, hut it varies from nothing at the mountain edge to at least 1,000 

 feet in thickness in the southern center of the basin. 



The northern end of this valley or basin presents several peculiar phenomena, 

 the principal among which are the celebrated white sand-. These are composed of 

 rounded grains of gypsum instead of silica, and throughout their extent water is 

 easily secured by digging a few feet. The northern end has also been covered by a 

 great flow of lava or "malpais," mentioned later on, which it is alleged flowed down 

 the valley some thirty miles from the alleged craters in township lit, range 10, first 

 standard parallel. Although this flat or valley has not upon its surface a single 

 running stream or even a drainage channel, so that its surface is the most complete 

 picture of aridity imaginable, yet beneath it lies an illustration of one of the most 

 important artesian basins in the west. The rainfall in this region is mostly upon 

 the mountains that surround the basin, standing some 3,000 feet above its plain, 

 and the water flows rapidly down their sides until it readies the plain. Many of 

 these streams, like the Rio Tularosa and the Tres Rios, are perennial, and others 

 all along the mountain range carry great volumes of water during the winter and 

 autumnal seasons. Immediately upon leaving the impervious mountain rock and 

 upon reaching the plains these streams disappear completely, a phenomenon which 

 cannot but impress the observer with wonder and astonishment. They do not 

 evaporate, as has been alleged, nor do they sink into caverns, as most people think, 

 but they are imbibed, literally drank up. by the soft, sponge-like formation of the 

 plain, and are stored below the line of saturation. The shedding of its rain-waters 

 by the impervious mountain rock and its imbibition by the spongy plains rock is 

 the key to the whole question of underground waters in the arid region, for not a 

 single flowing well has ever been obtained west of the tooth meridian and south of 

 the Dakotas in the consolidated mountain structure. 



The Hueco-< >rgan basin is accompanied by many terrace benches around its hol- 

 der. These are of two kinds: (1) remnants of ancient shore lines; and (2) delta 

 deposit- of dohri- brought down by present fl 1- upon the mountains. The ter- 

 races are especially well shown in the pass of the Rio Grande at El Paso, where on 

 the northern side seven or eight tiers of them above the river level can he traced. 



The Mesilla Basin. — -West of the < Irgan-Frankhn range there is another extensive 

 basin which is occupied by the valley of the Rio < rrande and extends from near old 

 Fort Selden to Frontera. four miles west of El Paso. Tins basin is hounded on 

 the west by small mountain blocks running north toward the Fort Selden eruptives. 

 [n this basin are situated the towns of Mesilla ami Las Cruces, two of the most 

 flourishing place- in New Mexico, and extensive agriculture is carried on by irriga- 

 tion from the Rio < rrande. 



The formation of this hasin is the same as that of the Hueco-Organ basin, and 



