gently toward the main bed of the valley. This plain, in receiving the drainage from the adjoining mountain ranges, is variously cut up by deeply-trenched arroyos, and terminates on the alluvial tracts below in gravelly bluffs of variable height.

This table-land is encountered wherever the course of the Rio Grande is not hemmed in by precipitous rocky cliffs, and is seen forming a belt of variable width on both sides of the river, extending to the base of the adjoining mountains. In all these situations it presents very uniform features.

An obvious analogy will be at once perceived between the latter formation and the widespreading upland alluvial plains, before noticed; in fact, a direct continuous connexion between them may be often traced. They evidently belong to the same general formation, representing basins filled up with alluvial and diluvial depositions, concealing, it may be, older tertiary strata below.

The pebbles contained in this formation can readily be traced to their original sources in the adjoining mountains, being of larger size and more angular near the base of the mountains, and smaller and more rounded at a greater distance. The earthy medium is generally a coarse sand or fine marl, argillaceous matter being less frequent. Occasionally the exposed bluffs show deposits of gypsum, which in some localities forms extensive beds. The most usual form of this material is in confused crystalline and fibrous masses, imbedded in loose marl. At other places a calcareous chalklike deposition is met with, occupying usually the upper stratum of the tableland.

A general saline character, pertaining to this formation, is also evidenced in the growth of saline plants or direct salt efflorescence in the lower depressions of valleys.

SECTION OF EARTHY TABLE-LAND FORMING THE BLUFFS OF THE RIO BRAVO ABOVE EL PASO, CORRESPONDING WITH THAT FORMING THE " 'JORNADA DEL MUERTO," TO THE NORTH.

As seen from any high mountain elevation, this table-land sweeps with all the exactness of a sheet of water, encircling as with a shore-line the bases of distant mountains, frequently completely insulating peaks and ridges, and everywhere masking the true connexion of the various formations.

The progress of subsequent drainage is also plainly seen in the various terraced elevations

which this table-land assumes. It may further be observed, briefly, that this is the formation that stamps the character of sterility on so large a scope of country forming those desert tracts known as "Jornadas," of which the "Jornada del Muerto" is a noted example. It is to this character of country, moreover, properly belongs the Spanish term "Llano Estacado," or Staked Plain, a term which has been less appropriately applied by travellers to the cretaceous tablelands of Texas, before noticed.

The proper alluvial tracts of the Rio Grande, as here met with on our route, exhibit a belt of variable width, from a mere narrow strip to several miles in breadth. Its lower portions are marked by frequent sloughs and old river beds. The body of the soil is sandy, but acquires a somewhat compact texture from the deposition of river slime, and is further enriched by the decaying vegetation that luxuriates on its moist bottoms.

The desert table-land is constantly encroaching on this alluvial belt, in the washing of its numerous stream beds, or the finest sand wafted by the winds. The roads occupying the river bottom are usually heavy, and whenever practicable are gladly exchanged by the traveller for the compact table-land.

The river itself presents few features of attraction. Its turbid waters sweep along during the flood season, in June and July, a swollen tide, spreading its enriching sediment through the various sloughs and lagoons that line its course, often cutting off all approach by land to the main channel. During low water, which includes the greater part of the year, the river contracts its dimensions, running in a very variable channel, over sandy shoals, interrupted by numerous islands and exposed sand-bars. Occasionally, in very dry seasons, it ceases to run altogether, and stands in stagnant pools.

The portion of the river bottom at present under cultivation in connexion with the El Paso settlements includes a large basin lying south of the El Paso mountains. In this is comprised the large alluvial tract known as "The Island," which is 30 miles in length by 2 to 5 in breadth. This island lies on the American side of the main channel, being separated from the adjoining land by an old river bed, which, except in very low water, still carries a variable stream. The bifurcation of these two arms of the river at the head of the island is taken advantage of to direct a stream of irrigating water through the centre of this tract of land, extending nearly its whole length, and furnishing from its main trunk side branches to supply the cultivated fields. Thus in usual seasons a sufficient supply of water is obtained to meet the wants of ordinary cultivation. At times, however, low water in the main channel is a certain precursor of drought; while at other times an unwonted abundance exposes to the danger of floods.

On the main banks of the river, including the Mexican town of El Paso, and Franklin, on the American side, these inconveniences are measurably obviated by drawing the irrigating supply from a higher source. This is accomplished by the construction of an artificial dam, located some two miles above these respective towns, thus allowing the construction of watergates and waste-weirs to regulate the supply of water according to need.

Hitherto we have observed the Rio Grande in its character of a variable stream, bordered by alluvial bottoms frequently of considerable width and extent; these again everywhere limited by gravelly table-land, sloping upward to the bases of distant mountains. A short distance above El Paso a new feature presents itself, and we have the mountains themselves encroaching directly on the bed of the river, which here passes in a contracted channel between rocky walls.

The rock exposures on the river bank exhibit disturbed strata of limestone, characterized by frequent fossils as belonging to the Cretaceous period. A greater or less metamorphism of this or a more ancient sedimentary rock is also exhibited, while on either side of the river tower up to a height of 500 to 1,000 feet rugged igneous rocks, having a granitic texture, and characterized by Professor Hall as "feldspathic or granitic lava." (No. 40.)

On the Mexican side, the various formations, stratified and igneous, are blended and intermixed in great confusion, the connexion between the various formations being obscured by the irregular exposure of igneous products, the greater or less degree of metamorphism of adjoining sedimentary rocks, and the presence of extensive diluvial deposits. The general surface is thus rendered extremely rugged and broken, the traversed roads being obliged to make a considerable detour from the course of the river.

On the American side is conspicuous a high mountain range, nearly parallel to the river, at a variable distance of from 3 to 10 miles, and observing a regular north and south course. This range is seen to be composed of stratified limestone, dipping very uniformly at an angle of 45° W.S.W., or toward the river; in the face of this dip rest the various igneous outbursts, associated with the disturbed cretaceous beds. This limestone is determined by Professor Hall to belong to the carboniferous series, being a northern continuation of that before noticed at Eagle Springs.

The remarkable character of the stratification is conspicuous at a great distance, its deep gullies presenting fine sectional views, and the different exposures of its sloping surface exhibiting variously curved lines, as the strata are thus brought to view by the action of the denuding forces.

The highest summit of this range presents a sharp, jagged crest, such as might readily be

mistaken at a distant side view for an igneous formation. Connected with this upper crest, we also notice outweathering masses of siliceous rock, (No. 38;) these frequently assume grotesque forms and positions, representing various tower-shaped prominences.

At another point this mountain range is interrupted in its usual stratified character by the presence of a porphyritic exposure, (specimen rock No. 41,) forming a dyke, passing through the entire ridge from east to west, and constituting the highest point in this range.

This igneous mass is variously associated with the adjoining limestone strata, lying either above or below, without showing any local variation of the ordinary dip, or exhibiting metamorphic changes at the point of junction.

On its eastern aspect, this range exhibits a precipitous slope, revealing the thickness of the formation in the regular succession of the uplifted strata, as thus exposed, from summit to base; there is developed at several points a thickness of not less than 1,500 feet. No very marked change is observable in the character of the rock from above downwards, or any local evidence of a change of formation.

We have, however, evidence from erratic fossils of the presence of a lower order of rocks, belonging to the Silurian period, in this vicinity. Such a formation has been assigned to a corresponding location west of El Paso, by Wislizenus, and there is little doubt but a careful examination along the lower line of these uplifted strata would bring to light this lower class of rocks.

This range continues to the north, forming the Organ mountains, at which point these stratified rocks give place to various forms of igneous products, as indicated in rock specimens Nos. 42 to 46, inclusive.

About seven miles north of El Paso, the mountains adjoining the river give place to the more usual character of gravelly table-land and alluvial bottoms, as noticed below. The table-land is here seen swelling to its broadest dimensions, encircling the distant mountains in every direction, and stretches northward to form the dreaded “Jornada del Muerto;" thence sweeping round the northern point of the Organ mountains, it constitutes the extensive desert tract between the Rio Grande and the Sacramento mountains. To the west the same formation is seen, variously interrupted by mountain ranges and isolated points of igneous rock, extending to the base of the Sierra Madre.

In further continuation of our general sketch of external and geological features of country, 2 M

we now take up the line of march westward, leaving the valley of the Rio Grande at El Paso, to follow out the most southern line of emigrant travel to the lower valley of the Gila river.

On leaving the alluvial basin, in which El Paso is situated, we first ascend over a lower step in the gravelly table-land, sloping gradually upwards, and presenting all the characters of scenery before described. We pass mountain spurs on the right and left, composed of the limestone rock, similar in appearance to the range noticed on the American side of the river, and having the same general dip to the southwest, but at a smaller angle.

Our route, following at first the regular Chihuahua road, passes nearly due south; in about 15 miles from the river we reach a second terraced elevation of the table-land, rising as a steep bluff 80 to 100 feet above the lower step over which we have been passing. The character of this higher deposit is here plainly exhibited in the face of the cliff, consisting of alternate layers of yellow ferruginous marl and coarse brown sand, capped with a thin layer of highly calcareous marl. From the summit of this second elevation stretches a wide table plain, variously indented by shallow valleys, and swelling toward the base of the mountain ranges. On approaching the line of mountains lying to the southeast, we pass over a spur of limestone rock, connected with this range, showing a dip of 15° to the northeast, a similar inclination being apparent in the principal range. The rock formation appears to be identical with that before noticed near El Paso, having a directly opposite dip, thus forming a synclinal axis, in the trough of which our route seems to have been marked out. Leaving this latter range to some distance on our left, we approach a long serrated ridge of mountains lying directly in our course; near the base of the southeastern extremity of these mountains, occurs the first permanent water since leaving the Rio Grande, about 32 miles distance. This locality is the "Samalayurca Spring."

A short distance beyond this, commences the singular formation known as the "Medanos," or Sand-hills. They here rise conspicuously to view from the plain below, presenting an exact appearance of the sandy dunes along a stormy seacoast. It is difficult, at first sight, to disconnect this remarkable formation from such an obvious cause, and not to represent it as the sandy beach of the extensive lake in which the deposits were made, forming the wide expanse of tableland so often referred to. The present facts, however, do not warrant such an exclusive opinion; thus the separate grains of sand composing the sand-hills are seen under a lens to be angular, and not rounded, as would be the case in regular beach deposits; they are also extremely light and penetrating, of which every traveller who has occasion to pass this locality in a dry, windy day will have ocular demonstration. In fact, the peculiar features of this formation are sufficiently explained in the topographical arrangement of the country, which presents an immense plain, stretching out in the direction of the prevalent northwest winds.

In overlooking the surrounding country from the projecting point of the adjacent gneiss range, these sand-hills are seen to form a crescent, with its concavity toward the northwest, and rising highest where the accumulated deposit is most sheltered by rocky barriers, from the levelling influence of winds, other than those from the northwest.

The spring which occurs in this locality near the base of these sand-hills occupies a natural depression of the general plain. Its issue spreads in a shallow pool surrounded by aquatic plants and shrubbery. The central spring source forms a deep hole, about two feet in diameter, bedded with quicksand, which is surges up intermittently at various points. The temperature of the water is 70° Fahrenheit.