50 GEOLOGY. 



The river here follows a general southeast course, making its way through strata of disturhed 

 carboniferous limestone, having usually a dip to the southioest. The river course thus cut- 

 ting the strata unequally, we should naturally expect not so much of a continuous canon as an 

 unequal development of rock on either side, presenting, it may be, bold and abrupt faces on 

 the one side, and comparatively low on its opposite, thus affording the means of following near 

 the river banks, by crossing from one side of the stream to the other. This, indeed, seems to 

 have been the course pursued by the surveying party, with their pack-trains, who were thus 

 enabled to keep up a connexion with the line of survey. 



We should also expect, as another consequence of this irregularity of feature in the rock 

 exposure, not such a marked contraction of the river bend and channel as we should be more 

 apt to find in the case of horizontal strata of equal development ; rapids would be less apt to 

 form, and lines of beach would be more frequent. Further on, in encountering the exposures 

 of igneous rocks, these features would vary, and here would be the points characterized by 

 greater obstruction to the regular course of the river, and also rendering a passage along its 

 banks more impracticable. 



Such are the general features, as well as they can be gathered from the maps of the survey 

 and the geological features of the country through which the river here passes. 



Approaching Presidio del Norte, the valley of the Rio Grande again opens ujdou a wide basin, 

 closely resembling in all its external features that seen above, near El Paso. The table-land, 

 however, attains a greater height above the river bottom, presenting steep bluffs, often 200 feet 

 high. The river bottom is also more contracted, rarely attaining a mile in width, and fre- 

 quently reduced by the adjoining table-land to a mere strip. The river spreads out, embracing 

 in its course numerous islands of dei^osit, and forming frequent sloughs along its main banks, 

 subject to regular overflows. It is to these several tracts, islands, and sloughs that cultivation 

 is chiefly conflned. 



On the Mexican side the Rio Grande receives the waters of the Rio Concho, flowing from the 

 southwest, and draining a large extent of country in the State of Chihuahua. This is the only 

 constant tributary to the Rio Grande yet met with in oiir course downward ; its waters at the 

 usual height are clear, flowing generally over a bed of limestone pebbles. 



The delta formed at the junction of this stream with the Rio Grande afi'ords a patch of soil 

 suitable for agricultural jDurposes, and is occupied by the Mexican settlement of Presidio del 

 Norte. The town itself occupies a conspicuous site, on high gravelly table-land, overlooking 

 both valleys. 



On the American side, about three miles below the junction of the river, the greatest amount 

 of bottom-land suitable for cultivation is met with ; it is connected with the site known as Fort 

 Leaton. 



The bottom-lands in this vicinity are variously occupied by scattering growths of cotton-wood, 

 willow, &c. The highest alluvial tracts are covered with a dense growth of mezquite ; the table- 

 land presents its usual desert vegetation. 



The natural boundaries of this basin consist of irregular mountain ranges, composed princi- 

 pally of carboniferous limestone, similar to that seen above. As a general thing, the strata 

 here appear less disturbed, but show not unfrequently a strong westerly dip. 



In an east and southeast direction lies the range of igneous mountains called the '^Sierra 



