10 GEOLOGY. 



we now take up the line of march westward, leaving the yalley of the Kio Grande at El Paso, 

 to follow out the most southern line of emigrant travel to the lower valley of the Gila river. 



On leaving the alluvial basin, in which El Paso is situated, we first ascend over a lower step 

 in the gravelly table-land, sloping gradually upwards, and presenting all the characters of 

 scenery before described. We pass mountain spurs on the right and left, composed of the lime- 

 stone rock, similar in appearance to the range noticed on the American side of the river, and 

 having the same general dip to the southwest, but at a smaller angle. 



Our route, following at first the regular Chihuahua road, passes nearly due south ; in about 15 

 miles from the river we reach a second terraced elevation of the table-land, rising as a steep bluff 

 80 to 100 feet above the lower step over which we have been passing. The character of this higher 

 deposit is here plainly exhibited in the face of the cliif, consisting of alternate layers of yellow 

 ferruginous marl and coarse brown sand, capped with a thin layer of highly calcareous marl. 



From the summit of this second elevation stretches a wide table plain, variously indented by 

 shallow valleys, and swelling toward the base of the mountain ranges. On approaching the 

 line of mountains lying to the southeast, we pass over a spur of limestone rock, connected with 

 this range, showing a dip of 15° to the northeast, a similar inclination being apparent in the 

 principal range. The rock formation appears to be identical with that before noticed near El 

 Paso, having a directly opposite dip, thus forming a synclinal axis, in the trough of which our 

 route seems to have been marked out. Leaving this latter range to some distance on our left, 

 we approach a long serrated ridge of mountains lying directly in our course; near the base of 

 the southeastern extremity of these mountains, occurs the first permanent water since leaving 

 the Kio Granie, about 32 miles distance. This locality is the "Samalayurca Spring." 



A short distance beyond this, commences the singular formation known as the "Medanos," or 

 Sand-hills. They here rise conspicuously to view from the plain below, presenting an exact 

 appearance of the sandy dunes along a stormy seacoast. It is difficult, at first sight, to discon- 

 nect this remarkable formation from such an obvious cause, and not to represent it as the sandy 

 ,beach of the extensive lake in which the deposits were made, forming the wide expanse of table- 

 land so often referred to. The present facts, however, do not warrant such an exclusive opinion ; 

 thus the separate grains of sand composing the sand-hills are seen under a lens to be angular, 

 and not rounded, as would be the case in regular beach deposits ; they are also extremely light 

 and penetrating, of which every traveller who has occasion to pass this locality in a dry, windy 

 day will have ocular demonstration. In fact, the peculiar features of this formation are suffi- 

 ciently explained in the topographical arrangement of the country, which presents an immense 

 plain, stretching out in the direction of the prevalent northwest winds. 



In overlooking the surrounding country from the projecting point of the adjacent gneiss range, 

 these sand-hills are seen to form a crescent, with its concavity toward the northwest, and 

 rising highest where the accumulated deposit is most sheltered by rocky barriers, from the 

 levelling influence of winds, other than those from the northwest. 



The spring which occurs in this locality near the base of these sand-hills occupies a natural 

 depression of the general plain. Its issvre spreads in a shallow pool surrounded by aquatic 

 plants and shrubbery. The central spring source forms a deep hole, about two feet in diameter, 

 bedded with quicksand, which is surges up intermittently at various points. The temperature 

 of the water is "70° Falirenheit. 



