CHAPTER III. 



GEOLOGICAL FEATURES OF THE EIO GRANDE VALLEY FROM EL PASO TO 



THE MOUTH OF THE PECOS RIVER. 



Bt c. c. parey. 



Having completed our general sketch of the external features of the country, as represented on 

 the line of route in nearest connexion with the United States and Mexican Boundary Survey, we 

 now retrace our steps, to detail more particularly the course of the Rio Grande, especially in its 

 connexion with the extensive canons by which its course is marked, above and below Presidio 

 del Norte. In these we gain insight into the geological structure of a large and interesting 

 scope of country, also connected with scenery unsurpassed for singularity and grandeur. 



About "rO miles below El Paso the mountains on either side of the valley converge, and 

 present a lofty barrier in the direct course of the Rio Grande. 



Through these the river makes its way by deeply-cut chasms, exposing the geological forma- 

 tion and structure in the sectional faces presented by its precipitous walls. 



We also see in this connexion the lower limits of that extensive aqueous deposit, forming 

 what may be termed the Great El Paso Basin, which, by subsequent drainage in the progressive 

 deepening of the bed of the Rio Grande, has brought to view the various terraced elevations 

 marked along the course of the present valley in table-land bluffs and extensive gravelly 

 plateaus. 



In fact, in our progress down the river we shall have constant occasion to notice the connexion 

 between these canons, as marking the limits of upper basins of deposit. Thus the general 

 course of the river represents a continuous series, in descending steps, of basins, more or less 

 extensive, then a canon, forming, as we may say, the spout of the basin, which again opens on 

 a basin of lower level. 



This simple statement embodies the great principle of formation that characterizes all this 

 district, and gives to its topography a significance at once clear and instructive. 



It is in these barriers, then, these mountain dams, that the character of the valley, as a whole, 

 can be best studied , and the chasms by which the river pierces them furnish the true key to 

 their geological development. 



That portion of the Rio Grande thus marked by canons and basins, extending from the first 

 obstruction 70 miles below El Paso to Presidio del Norte, did not come under my own personal 

 examination. 



7M 



