60 GEOLOGY. 



The eastern slope of the Sierra Carmel shows the strata of cretaceous limestone inclining 

 eastward at an angle of about 20° ; its exposed face is variously marked up by irregular 

 trenched valleys and abrupt points and ledges, due to the natural denuding forces of water 

 drainage and atmospheric action. This slope terminates in an irregular valley below, having 

 its drainage to the north, and leading direct to the Rio Grande. Further south is conspicuous 

 the extensive igneous development of the mountain range, rising in jagged peaks to an Alpine 

 height, and presenting in the forest growth, which clothes its sides, agreesible features of 

 verdure, contrasting strangely with the river valley and its bare outline of desert hills. 



The most northern outlier of this igneous formation is the singular peak known as the 

 " Picotena." Lying at a distance of about 5 miles from the river, it rises abruptly from amid 

 the surrounding limestone ranges, shooting up a sharp conical peak of basaltic structure. This 

 peak, by its height and external features, presents a most striking landmark. 



The country stretching to the north and east in the course of the river is less interrupted by 

 high mountain ranges than has yet appeared on the line of our route, and presents features 

 precisely similar to those before noticed in connexion with the lower valley of the Pecos. 

 Igneous exposures disappear altogether, or are of very limited extent, and the limestone strata 

 are but little disturbed. The numerous deeply cut valleys leading to the river are bounded by 

 abrupt walls, rendering travelling, except in the direct line of their drainage, next to impossible. 

 In attempting to follow down the river with pack animals, the only practicable course was to 

 follow up to near its head one of these tributary ravines, thus reaching the general table sum- 

 mit, and then to pass over to and down another ravine leading to some uncertain point of 

 the river below. By this plan it not unfrequently happened that, in order to make a distance 

 of 5 or 6 miles by the line of the river, a detour of 30 miles or more was necessary. Each of 

 these detours, moreover, leads over a country destitute of water, except the uncertain rain 

 water retained in rocky wells, which generally occupy positions inaccessible to animals. 



This character of country continues hence uninterruptedly to the mouth of the Pecos river, 

 about 80 miles distant, presenting great uniformity in the general external and geological 

 features of country. 



Our course led along the eastern base of the Sierra Carmel, bringing to view, in connexion 

 with its larger development of igneous formation, a section of country extremely picturesque, 

 including well watered valleys, timbered mountains, and upland plains covered with a luxuriant 

 growth of nutricious grass. 



Indian traces abound in these vicinities, and the deep recesses of the adjoining mountains 

 afford secure retreats, where the animals plundered from the Mexican settlements are driven to 

 recruit, in preparation for their passage across the Rio Grande into Texas. 



To this character of country again succeed ranges of cretaceous mountains, showing a general 

 easterly dip of strata, and connected with upland basin plains mostly waterless. 



At a distance of about a hundred and fifty miles south from the Rio Grande we reach a 

 system of elevated basins, having frequently a drainage distinct from the valley of the Rio 

 Grande, forming extensive inland lakes fed by numerous rivers. The noted Bolson Mapimi is 

 the largest example of this lagoon formation. 



Several, however, of these lagoons on the northern edge of this elevated area give rise to 

 tributaries which empty into the Rio Grande. Of this latter class, the " Laguna Agua Verde" 



