CHAPTER I.

GENERAL GEOLOGICAL FEATURES OF THE COUNTRY.

Colonel W. H. EMORY, United States Commissioner:

WASHINGTON, D. C., April 1, 1854.

SIR: In completion of the duties assigned me as botanist and geologist to the United States boundary commission, I present the following:

I. GENERAL PHYSICAL AND GEOLOGICAL FEATURES OF THE COUNTRY.

The general features of the Mexican Gulf Coast, in connexion with the United States and Mexican boundary line, present a marked contrast with those observed on the opposite Pacific coast. Thus, instead of the high cliffs, abrupt headlands, and general bold and rugged outline exhibited on the Californian coast, the Texan shore-line, throughout its whole extent, presents a uniform, low dead level. Generally, indeed, the main coast is shut in from the open sea by ranges of sand islands formed by the waves of silted sea sand and comminuted shells. Inside of this line of islands shallow bays spread themselves into the indented coast, and here the numerous rivers flowing from the interior meet the tide-water. The tide range is moreover small, and thus the alternating differences of level do not favor the formation of navigable estuaries by which the main land may be approached. These features, collectively, give to this coast an inaccessible character, and serve to render its navigation both difficult and dangerous.

Its rivers are unapproachable, except by vessels of very light draught; while the inlets to its shallow bays, obstructed by variable sand-bars, present obstacles to navigation, sufficiently proved by the numerous wrecks that strew their beach. Proceeding inland from the line of sandy beach, a gentle slope spreads out in a uniform gradually rising plane, composed of dark rich loam, and covered with luxuriant pasturage. The scenery is rarely relieved of its blank outline by a clump of live oak trees surrounding a sunken morass. Farther on, at a variable distance of 10 to 20 miles, the surface of the ground shows gentle swells, still maintaining its fertile character, and displaying here and there groves of post oak and other timber. The river bottoms adjoining are occupied with a heavy timber growth, principally of elm, (ulmus crassifolia,) festooned with Spanish moss. The undergrowth comprises a complete maze of shrubbery, matted and tangled together by vines and creepers, and supporting a rank annual growth.

At a distance of 50 to 80 miles from the coast, the ground-swells become more abrupt and form distinct ridges, between which are collected the drainage of the country. Along the course of the numerous streams there is an exposure of the geological substratum, consisting first of loose gravelly strata, which contain erratic pebbles of siliceous or calcareous character; to this

succeed occasional exposures of a coarse-grained sandstone, No. 1. Still farther inland we meet with a form of soft calcareous earthy rock outcropping along the sides of hills, and constituting the first outlayer of that extensive cretaceous formation which characterizes so large a scope of country throughout middle and northwestern Texas.

From San Antonio, occupying the first step in the cretaceous series, at an elevation of 600 feet above the Gulf of Mexico, proceeding northerly on the line of the lower road to El Paso, we soon remark a rapid change in the general features of the country. The underlying limestone formation becomes more largely developed, and is less deeply covered with alluvial deposits. The rock stratum is frequently exposed in the beds of streams, which are everywhere thickly strewn with the water-worn pebbles of this formation. The streams here acquire an intermittent character, subject to sudden overflow and recession. Their course, when low, is marked by an irregular series of deep basin ponds connected together only by shallow brooks, cr even not at all above ground during dry seasons.

As we proceed, mural exposures of limestone rock become more frequent, and the same formation is met with on summits of the higher table-land. The alluvial tracts along the course of the larger valleys acquire a more arid character of soil, and support a stunted timber growth, in which mezquite makes its appearance. Fossil, fresh water, and land shells are quite abundantly scattered over the lower depressions of these alluvial bottoms.

At the crossing of the Rio Frio, near Fort Inge, occurs the first exposure of igneous rock. It is seen as an isolated knoll of dark-colored trap, showing at this place but slight disturbance of the adjacent cretaceous strata. This formation is thence observed to constitute a broken line, extending in a northwest course, and coming into view at several points along the road at variable distances of 5 to 10 miles.

On approaching the line of the great table-land formation of Northwest Texas, we find near its base the sources of most of the minor streams of this region. These sources frequently exhibit magnificent basin springs, of which that at San Felipe is a noted example.

We have here reached the main development of this extensive cretaceous formation, partially concealed from view towards the coast, as above noticed, by alluvial deposits, but here standing out in bold relief, variously exposed in extensive ridges, bounding, more or less closely, valleys

Character of valley denudations in the cretaceous table-land formation. Upper portion of San Pedro river, Texas.

of denudation, or else stretching in vast upland plateaus, thinly covered with soil, and supporting a close even growth of upland grasses or scanty shrubs.

The true character of this formation may be satisfactorily studied in the course of its principal streams, the Pecos and Devil's river. As exposed along the course of these valleys, the view is bounded by steep mural cliffs, composed of limestone, disposed in nearly horizontal strata. This rock exhibits quite a variable texture, its weathered face showing an uniform gray or bluish tint, while its recent fracture has a much lighter color. Owing to its irregular texture, it frequently exhibits a cavernous structure, displaying in its various exposures all the grotesque features of ruined castles, forts, and dilapidated masonry; examples of which may be seen by reference to numerous sketches.

The river valleys either expand into more or less extensive alluvial basins, or are completely hemmed in by steep mural faces, forming chasms along their course, to which the Spanish term of cañon is generally applied. Thus, in following out the course of valleys in this district, we have a series of basins connected by cañons; the relative extent of these distinct topographical features being dependent on the local character of the formation, or the varied influence of previous denuding forces.

The alluvial tracts partake to a great extent in the sterility of the plateaus with which they are connected, seldom showing evidence of fertility, and in a great measure destitute of timber growth.

In the case of the Pecos river, which may be regarded as the main type of streams belonging to this table-land formation, we observe a contracted but constant body of water coursing through alluvial tracts, or clearing its way through rocky cañons.

In the former case, its tortuous course is marked out between deep banks of earth, so that its turbid waters are for the most part invisible till you come directly on its brink. The average width of the stream, during most of the year, is about 50 feet, and 8 feet in depth. Only limited portions of the adjoining valley are subject to that degree of overflow, such as constitutes what is commonly understood as bottom-land. Owing to the steep and crumbling nature of the banks, travellers often experience no small difficulty in watering their animals; the water itself, though highly charged with reddish sediment, is not unpalatable.

In its passage through cañons, this stream, like the Rio Grande, cleaves its way between. steep walls of rock; its course during low water being occasionally set off by lines of sandy or pebbly beach, and forming frequent rapids.

All the small intermittent streams of this region are copiously bedded with rounded pebbles, derived from the adjoining limestone formation.

The view from the summit elevations presents not an unbroken table-land, but rather a series of terraces, exhibiting occasionally truncated peaks, and showing a general increasing elevation westward. The mean level is, moreover, marked by depressed valleys, containing dry pebbly beds of streams, and frequently expanding into wide basins. The descent to these valleys is generally abrupt, and is the chief obstacle in the construction of roads, which, with this exception, are marked out with ease, and are unexcelled for purposes of wagon transportation.

The supply of water over these arid tracts, except in a season of rain, is confined to a few isolated springs, occupying the lower level of some of these depressed valleys, or occasionally bursting out from the base of high rocky ledges. These springs, though generally affording a copious and constant flow of water, are not sufficient to give origin to river tributaries, their issue being quickly absorbed in the lower course of their arid beds. In several of these springs

the temperature is as high as 70° Fahrenheit. Between these watering places occur what are termed by travellers "dry stretches," being in some instances 50 miles in extent.

In all our observations thus far, little disturbance is noticeable in the position of the strata. To ordinary view they appear strictly horizontal; the indications of the barometer and the changes of the climate prove, however, a gradually increasing elevation. The height, as indicated at the Leon spring, the most western point of the continuous table-land at which cretaceous fossils were collected, is 2,807 feet. This shows a rise of 1,800 feet from the lowest series of this formation, (the mouth of Devil's river,) and 2,200 feet above San Antonio, giving an average rise of 7 feet to the mile.

Quite constantly in the distance, to the south and west, rugged mountain ranges are visible, evidently of igneous character, and connected with extensive disturbance of adjacent cretaceous rocks. It is through these, as we shall hereafter see, that the Rio Grande forces its way, presenting a series of chasms and deep cleft cañons of a most stupendous character.

The first indication of a change in the general features of scenery, as sketched above, on the line of the usually travelled road to El Paso, is encountered in the range of the "Sierra Diavolo," or Limpia mountains. This range may be regarded as the southern continuation of the great dividing ridge between the Pecos and the upper Rio Grande, including the Sacramento mountains to the north, the Guadaloupe and Limpia mountains, with their continuation south, to form the Sierra Rica of Mexico; through the latter portion of this range the Rio Grande forces its way a short distance below and east of Presidio del Norte.

This range is characterized, at all the separate points observed, by the presence of igneous rocks, varying considerably in structure and lithological character, as noted by Professor Hall in rock specimens Nos. 12, 13, 14, 15.

The elevation attained by this range, on the line of the El Paso road, is from 5,000 to 7,000 feet above the sea. On entering this range from the east, we pass quite abruptly from the horizontal limestone strata to the igneous exposures.

The passage of this range is accomplished by a series of rather steep and rough ascents, following up the course of the Limpia valley. The main pass, known as the "Wild Rose Pass,' exhibits gigantic walls of rock, towering up on either hand to the height of 1,000 feet or more above the valley below.

The summit divide is composed of a coarsely-grained granitic rock, formed principally of feldspar, and varying in color, in the different exposures, from dark brown to a dull whitish. In descending the more gradual western slope of this range, the rock exposures assume the character of a close porphyritic trap, of a reddish color. As we leave the main range, passing to the west, we encounter extensive ridges of stratified limestone rock, associated more or less closely with interrupted igneous exposures, and showing a general dip to the southwest, or away from the Limpia mountains. The inclination, however, shows, in many places, a variable direction and intensity, depending on local causes connected with adjoining igneous exposures.

A degree of metamorphism is also exhibited in rock exposures, having a gneissoid structure and traversed by quartz veins.

From the specimens collected, imperfectly characterized by fossils, Prof. Hall concludes that these stratified rocks belong to the carboniferous period.

Between these irregular mountain ranges and spurs, which in this section of country meet

the eye in every direction, the intervening surface spreads out into wide basin plains of an alluvial character. These basins receive and absorb the scanty streams of the adjoining mountains. Rarely indeed, except in the highest mountain recesses, is running water visible, the occasionally copious rains furnishing only a temporary current along the course of the numerous streambeds. The water thus accumulated in rocky basins or marshy lagoons, affords the only supply for travellers, over these arid wastes. During the dry season these plains spread out their dreary tracts, unrelieved by a single feature of fertility, occupied by innutricious grasses or a scattered growth of dry shrubbery, among which the repulsive form of the "Spanish bayonet" (Yuca) is a conspicuous feature. Owing to their exposed and elevated position, these plains are subject to great extremes of temperature. They are mostly shut off from the Rio Grande by a variable mountain range, composed of the carboniferous limestone, variously associated with igneous rocks. The passage to the valley is accomplished by following down the natural cleft made by some rain stream. These passes exhibit fine sectional views of the tilted limestone strata, exposed in various conditions of disturbance, in some places inclined at an angle of 80° to the west, and at other points exhibiting evidences of igneous action in metamorphic changes.

We have thus reached, on the line of the ordinary wagon-road, the upper valley of the Rio Grande, the external features of which, as more directly connected with the line of boundary, will claim a more detailed notice. At first, however, a more rapid sketch must suffice, while continuing to notice the general features of scenery and geological structure presented on the route westward to the lower valley of the Rio Gila.

As we pass from the rocky cañon, by which we enter on the Rio Grande valley, we first come upon a gravelly plain, generally presenting a smooth and more or less uniform surface, sloping