REPORT. 
CHAPTER I. 
General Remarks on the Physical Geography of the region explored. 
Extent and boundaries of the region.—Mountain ranges.—Trend and elevation.—Table-lands, their extent and elevation.— 
Llano Estacado.—Two slopes of the plain.—Altitude.—Bluff character of its northern borders.—Limit of the Llano on 
the east. 
The strip of country explored by Captain Pope, extending from Preston, on the Red river 
of Texas, in a direction south of west, to the Pecos river, and thence nearly west to the valley 
of the Rio Grande at El Paso and Dona Ana in New Mexico, embraces within its limits geologi¬ 
cal formations of great variety and interest. Its western end is crossed by the ranges of the 
great central mountain chain of the continent; its eastern reaches to the comparatively low 
plains of the Red river; while the central portion of the route is upon the extended and elevated 
desert plain of the Llano Estacado. 
As it is very desirable and interesting to note the' connexion of the geology with the promi¬ 
nent physical features of the region, I purpose to precede the more particular geological de¬ 
scriptions by some general remarks, based upon the observations of the survey, and which are 
given in the profile of the route. 
The physical features of the region, though strongly marked, are very simple. On the west, 
the mountain ranges break the monotony of the plains, and form, by their numbers and parallel¬ 
ism, a series of longitudinal valleys extending nearly north and south; but to the eastward of 
these, and over the greater part of the country Captain Pope traversed, an almost unbroken 
horizontal plain is found. 
These mountain ranges are three in number, and are there known by the following names: 
Organ mountains, Hueco mountains, and Guadalupe mountains. The prolongations of these 
ranges towards the north at Albuquerque and Santa Fe have other and local names—Sacramento 
mountains, Sierra Blanco, &c.; hut the whole series form a part of the main central chain, 
known in its northern portions as the Rocky mountains. The general direction or trend of the 
Organ mountains and the Hueco mountains is north and south—the former deflecting slightly 
towards the west. The Guadalupe range, however, does not conform to this direction, but 
diverges and trends towards the east; its mean direction, as it is given upon Captain Pope’s 
map, being N. 38° E. According to the report of Lieutenant Garrard 1 , this range extends 
southeasterly for seventy or eighty miles beyond the high peak, becoming more impassable as 
you proceed southward, and finally uniting with a chain of mountains having a northwest and 
southeast trend. The point which has generally been considered as the termination of the 
range, he found to be a spur running out into the Salt Plain. The northern terminus, accord¬ 
ing to Captain Pope’s observations, is about sixty miles north of the 32d parallel, where it 
1 Report of Captain Pope, page 63. 
