4 GEOLOGY. 



the temperature is as high as 10° Fahrenheit. Between these watering places occur what are 

 termed by travellers " dry stretches," being in some instances 50 miles in extent. 



In all our observations thus far, little disturbance is noticeable in the position of the strata. 

 To ordinary view they appear strictly horizontal ; the indications of the barometer and the 

 changes of the climate prove, however, a gradually increasing elevation. The height, as indi- 

 cated at the Leon spring, the most western point of the continuous table-land at which cretaceous 

 fossils were collected, is 2,807 feet. This shows a rise of 1,800 feet from the lowest series of this 

 formation, (the mouth of Devil's river,) and 2,200 feet above San Antonio, giving an average 

 rise of 1 feet to the mile. 



Quite constantly in the distance, to the south and west, rugged mountain ranges are visible, 

 evidently of igneous character, and connected with extensive disturbance of adjacent cretaceous 

 rocks. It is through these, as we shall hereafter see, that the Rio Grande forces its way, pre- 

 senting a series of chasms and deep cleft caiions of a most stupendous character. 



The first indication of a change in the general features of scenery, as sketched above, on the 

 line of the usually travelled road to El Paso, is encountered in the range of the " Sierra Diavolo," 

 or Limpia mountains. This range may be regarded as the southern continuation of the great 

 dividing ridge between the Pecos and the upper Rio Grande, including the Sacramento mount- 

 ains to the north, the Guadaloupe and Limpia mountains, with their continuation south, to 

 form the Sierra Rica of Mexico ; through the latter portion of this range the Rio Grande forces 

 its way a short distance below and east of Presidio del Norte. 



This range is characterized, at all the separate points observed, by the presence of igneous 

 rocks, varying considerably in structure and lithological character, as noted by Professor Hall 

 in rock specimens Nos. 12^ 13, 14, 15. 



The elevation attained by this range, on the line of the El Paso road, is from 5,000 to 7,000 

 feet above the sea. On entering this range from the east, we pass quite abruptly from the 

 horizontal limestone strata to the igneous exposures. 



The passage of this range is accomplished by a series of rather steep and rough ascents, fol- 

 lowing up the course of the Limpia valley. The main pass, known as the " Wild Hose Pass," 

 exhibits gigantic walls of rock, towering up on either hand to the height of 1,000 feet or more 

 above the valley below. 



The summit divide is composed of a coarsely- grained granitic rock, formed principally of feld- 

 spar, and varying in color, in the different exposures, from dark brown to a dull whitish. In 

 descending the more gradual western slope of this range, the rock exposures assume the char- 

 acter of a close porphyril.ic trap, of a reddish color. As we leave the main range, passing to 

 the west, we encounter extensive ridges of stratified limestone rock, associated more or less 

 closely with interrupted igneous exposures, and showing a general dip to the southwest, or away 

 from the Limpia mountains. The inclination, however, shows, in many places, a variable direc- 

 tion and intensity, depending on local causes connected with adjoining igneous exposures. 



A degree of metamorphism is also exhibited in rock exposures, having a gneissoid structure 

 and traversed by quartz veins. 



From the specimens collected, imperfectly characterized by fossils, Prof. Hall concludes that 

 these stratified rocks belong to the carboniferous period. 



Between these irregular mountain ranges and spurs, which in this section of country meet 



