The natural explanatory inference from these facts is, that the internal force, here represented in the continued mountain range, was diffused over a large space, and not centralized on one particular line. Hence arises no great prominence of one central chain, but a number of independent ranges, serving to equalize the general elevation and give the character of an elevated plateau to the surface of the country.

Again, the same irregular action of the internal force, and especially the preponderance of recent eruptive products, favors a varied direction of the mountain ranges, by means of which areas are circumscribed and basins formed for the reception of aqueous depositions. Here, then, we see the origin of those extensive plains and stretches of table-land to which our attention has been so frequently directed in the preceding sketch.

These same characters probably apply more or less closely to many other localities connected with the general dividing range, whether north or south of the point we are examining.

We are now prepared to descend the western slope of this dividing ridge, and note the peculiarities of feature presented on our route westward.

Decending, then, by an equally steep slope as the eastern ascent, and about the same height, we come upon the alluvial plain below. The lowest depression of this plain is composed of a light alluvial soil, and thence sloping gently upward to the west, exhibits a gravelly deposit, till, at a distance of about eight miles from the base of the mountain just left, we come upon the abrupt descent of the Guadalupe Pass.

This noted pass, which has been so frequently traversed on the line of emigrant travel to California, is now so well known as hardly to need a detailed description.

This pass has been properly characterized as the first step of considerable descent from the Mexican plateau to the heads of valleys leading to the Californian gulf. It has now been clearly established that at a point farther to the north, near the parallel of 32° latitude, the descent westward may be accomplished by a more gradual slope, and without leaving the basin of drainage pertaining to the Gila river.

The geological structure exposed in this mountain pass is similar to that before noted as occurring in the upper slope of the Sierra Madre, including feldspathic lava, granitic in texture, associated with basalt, stratified porphyry, and closely cemented breccias.

These several forms, variously associated, serve to give a remarkable diversity and broken character to the rock exposure, presenting a confused outline of mingled crests, peaks, and ravines. Through these the road has to work its way by sharp turns and very steep descents. On attaining a lower level we pass down a ravine, gradually widening, which finally spreads into a small valley, watered by a fine running stream, and beautifully shaded by large sycamore and cotton-wood trees. This valley is closely hemmed in by steep rocky walls, marked by intricate ravines, and rendered picturesque by a varied assemblage of live oak, cedar, and other verdant shrubbery. In emerging from the higher points of the mountain range, the walls of this cañon exhibit various forms of stratified porphyry running into a breccia. The character of stratification has, at several points, a close resemblance to altered sedimentary deposits, showing a reddish color and a very uniform character of dip.

We finally leave this valley, mounting up a steep bank, composed of gravelly table-land, rising 200 feet above the bed of the stream, thence passing by a gradual and continuous slope toward the main valley of the San Bernardino. The table-land here has all the usual characters of this

formation in other parts, not differing essentially from that of the Rio Bravo or Gila valleys, and terminates by an abrupt bank, bounding the alluvial basin below.

This basin, forming, as it is said, the head of the Yaqui river, here shows a wide flat plain, extending from north to south, and having a breadth of three to five miles. On its western edge is situated the deserted settlement of San Bernardino. Adjoining this rancho are numerous springs, spreading out into rushy ponds, and giving issue to a small stream of running water. The valley is covered thickly with a growth of coarse grass, showing in places a saline character of soil. The timber growth is confined to a few lone cotton-wood trees scattered here and there.

Signs of previous cultivation are limited, this settlement having been engaged principally in stock raising. The numerous bodies of wild cattle now running at large over this section of country are the remains and offspring of domestic herds, now widely scattered and hunted by Indians.

The western side of the valley is precisely similar to its opposite, showing the same general character of gravelly table-land. This leads by a gentle ascent to a low point in the dividing ridge separating the valleys of San Bernardino and Aqua Prieto.

A remarkable tower-shaped peak rises in the centre of this ridge, a short distance south of the road, forming a conspicuous landmark. This ridge is seen to be composed of one or more of the variable forms of volcanic products so often noticed heretofore; the prevailing character is here a reddish brown granitic mass.

The descent on the opposite (western) side of the ridge to the alluvial bed of the Aqua Prieto is over a long, tedious slope, the gravelly table-land giving place to extensive tracts of clay or loam, supporting a patchy growth of coarse grass. The "Black Water" valley, at its lowest depression at this point, contains no constant running stream, its course being mainly occupied with low saline flats or rain-water pools. Extensive lagoons are said to occur in this valley a short distance south of where the road crosses.

The main tributary to this valley comes from the west, and is followed to its head on the line of wagon-road. Its bed consists of a wide ravine, coursing through pebbly strata, variously marked by the washings and drift deposits, caused by the occasional strong current derived from local rains. At other times its bed is entirely dry. The timber growth along its borders consists of hackberry and walnut.

At its source there is a fine spring, issuing from ledges of stratified porphyritic rock, identical in character with that noticed at the foot of the Guadalupe Pass. The stratification is inclined to the northeast, and along the line of its tilted ledges the spring issue forms frequent pools of limpid water.

From this point we pass in a circuitous course to the southwest, winding among rocky spurs, and thence passing up an upland valley, agreeably diversified with groves of live oak and covered with luxuriant and nutritious mountain grasses. On this route we pass gradually to a divide which leads, on its western aspect, to an eastern branch of the Upper San Pedro valley. The country here begins to assume most attractive features. To the north and west rise high mountain ridges clothed with pine and oak groves; the intervening country is everywhere carpeted with fine grama grass, the nutritious quality of which is exhibited in the well-conditioned character of the numerous wild horses and cattle that luxuriate over this favored region. Water

is frequent in the valleys, and everything indicates a capacity for cultivation, the grazing capabilities being unequalled by any tract heretofore passed over.

Beyond this the San Pedro valley spreads out in diverging branches to the east and west, thus drawing tributary a very extended mountain drainage.

It is this latter character which sufficiently accounts for the fact that the San Pedro is the only branch of the Gila River, coming from the south, which furnishes an uninterrupted stream of running water along its whole course.

At the point where the main valley of the San Pedro is reached we find an alluvial belt, variable in width, and occasionally marshy. These bottoms are flanked by terraced table-land of unequal heights, composed of a hard gravelly soil, and supporting a close sward of grama grass, giving a peculiarly smooth shorn look to the general face of the country.

Occasional exposures of igneous rock, or the projecting spur of some mountain ridge, serve to diversify the scene; and quite constantly in the higher branch valleys is exposed a form of igneous conglomerate. This latter formation is exposed in irregular bluffs along the edges of these valleys, presenting washed faces and precipitous walls crowned with terraces. These higher points are frequently set off with the remains of deserted dwellings, plainly located with a view to defence. Other eminences, commanding extensive views, are occupied by rocky breast works, serving the double purpose of watch-towers and strongholds of retreat. Associated with these are also extensive rocky enclosures, in which the cattle were secured. All these points are suggestive of the condition of constant warfare to which this commencing civilization was subject, and under which it was at last obliged to succumb.

These upland valleys are only sparsely wooded by occasional cotton-wood or walnut trees. As we approach the mountains, however, the timber growth becomes more abundant, and the lower ridges are occupied by extensive groves of oak, which, on the higher points, are associated with pine and cedar. ·

From the head of the "Nutria" (southwest) branch of the San Pedro, up which our road passes, we commence the steep ascent of the mountain ridge lying between the Santa Cruz and San Pedro valleys. The character of this range is exactly similar to what we have before described as pertaining to all the higher mountains passed over on our route, west of the Sierra Madre.

The height of the pass leading to Santa Cruz is not less than 1,000 feet above the respective valleys on either side, being equally steep and rugged on either slope. The same ridge, extending toward the south and southwest, forms a continuous line of high mountains, lying between the San Pedro and Santa Cruz valleys; the preferable route for crossing is probably that taken by Col. Cooke in 1846.

The upper route, being the one more commonly followed, strikes the Santa Cruz valley near its head source.

The direction of this valley is at first nearly due south, giving the idea that its drainage is on the line of the rivers flowing south to the California Gulf. It is indeed so laid down on most of the maps of this region, but this is manifestly incorrect. About three miles south of the town of Santa Cruz the valley makes a sharp elbow; thence doubling on its former course, it continues north and northwest, being the same valley in which, lower down, are

located the towns of Tubac and Tucson; thence leading toward (though probably hardly ever reaching) the Gila River, near the Pimo settlements.

The situation of the town of Santa Cruz is highly picturesque, lying embosomed amid lofty wooded mountains. Its soil is fertile, abundantly watered, and susceptible of easy irrigation; its elevation gives it a cool temperature, suited to the production of northern fruits and cereal grains.

A cut-off, over the mountain range intervening between the two courses of the river, leads, by a distance of 18 miles, to a lower part of the valley, maintaining in the main the same general features, but showing a marked change in the climate. This latter fact becomes still more apparent in our progress downward, as shown by the comparative forwardness of vegetation. Thus a short journey of three days (or 80 miles) from Santa Cruz, between February 27th and March 1st, 1852, showed a difference in the advance of vegetation equal to a full month in time; so that while at Santa Cruz the cotton-wood trees were barely budding, the first day's journey displayed their loose catkins, the second the opening leaf, and the third the full leaf.

Greater aridity also characterizes the lower portion of the valley, and the live oak, so common above, gives place to heavy growths of mezquite. The adjoining mountains on either hand become in great measure bare of trees, and present steep ledges of igneous rock exposed along their broken range. The immediate edges of the valley are flanked by a conglomerate formation, similar to that noticed on the Upper San Pedro. Accompanying these changes the stream contracts, and finally, in certain points along its course, ceases to run, and the usual desert features of all waterless tracts in this region are exhibited.

We thus pass the settlements of Tomocacori, Tubac, San Xavier, and Tucson, together with numerous deserted ranchos occupying various points along the valley. After leaving Tubac, which is situated about midway between Santa Cruz and Tucson, the valley expands into a wide open basin, the mountains receding on either hand, and the dry valley, now almost exclusively occupied by mesquite, is bordered by a wide stretch of gravelly table-land. On this table-land we meet, for the first time on our route, that most remarkable vegetable production, the Cereus giganteus. Further on it becomes abundant, its stiff trunks and branched arms rising up here and there like sentinels, and giving a most peculiar character to the landscape scenery.

Approaching the town of San Xavier, noted for its superb church, contrasting strangely with the mud hovels surrounding it, we again come upon running water, with its constantly associated fertility and verdure. In this vicinity occur rocky knolls, composed of a dark-colored trap-rock, which formation becomes still more largely developed in the vicinity of Tucson, forming extensive ridges having a tabulated form and very irregular outline.

The settlement of Tucson occupies the lowest line of constant running water, and consequently the last fertile basin lying in the course of this valley. Below this, on the north, succeeds the extensive desert tract lying between Tucson and the Gila River.

In pursuing our course down the valley, the adjoining table-land gradually merges into the desert plain over which our road passes. Hardly, however, did it seem to deserve the name of a desert at the time of our crossing it. Owing to the refreshing influence of recent rains, a rapid growth of evanescent flowers gave its otherwise barren surface the aspect of a flower garden, regaling both the sense of sight and of smell with a profuse and varied assemblage of tints and scents. Water sufficient for our animals was found in ravines by the side of the road, and

a journey of eighty miles, otherwise dreaded, was, by an agreeable disappointment, rendered highly pleasant. Our journey was made in the first week in March; doubtless another month might have changed its features materially.

Our course lies quite regularly to the northwest, a broken line of mountains lying on our left, while to our right lies the extensive high mountain range northeast of Tucson. Directly in our course is a singular pinnacled peak, being the "half-way point" between Tucson and the Gila; approaching this, we pass by a gradual ascent over a gentle ridge, forming a depressed point in a continuous mountain range extending from the pinnacled peak, on our left, northeast toward the Gila valley. Near the summit of this ridge we pass small alluvial tracts, then occupied by a luxuriant growth of young grass, and cut up by deep gullies containing abundant supplies of rain water. The rock exposure here has a more ancient appearance than any before passed, indicating an approach to the granite ranges of the Californian Cordilleras. We descend the northern slope of this ridge, passing over extensive clay flats washed by recent rains into frequent gullies, these finally centering in one form the irregular bed of a rain stream leading direct to the Gila river.

The portion of the Gila valley thus reached is where the river, emerging from the high mountains occupying the mouth of the San Pedro, spreads out into the extensive alluvial bottoms, occupied in part by the settlements of the Pimo and Maricopa Indians.

The gravelly table-land here forms a gentle slope, leading from the distant mountains, and indenting the alluvial belt below. This latter consists of an upper level, supporting a shrubby growth of mezquite, and a lower bottom subject to river overflows. On these upper portions the Indians usually construct their dwellings, thus overlooking the lower cultivated fields. The amount of land here capable of cultivation is quite extensive, forming a belt on each side of the river often several miles in width, and extending east and west for 20 miles or

more.

The stream of water, then at its average height, (in early March,) measured about 40 yards in width with an average depth of 2 feet, the volume, however, being considerably diminished by the extensive irrigating ditches drawn from above.

The line of the river bank is at this season set off with lagoons and marshes, and everywhere bordered with a dense willow growth, rendering it difficult of approach.

The dams, which serve the purpose of drawing off the irrigating water, are constructed of old willow trunks and snags; these, in the course of time, entangling the loose soil and sediment borne down by the river, furnish a bed for the willow growth, thus becoming more permanent with age.

From a rock knoll of true granite, abutting on the river on the American side, a fine view is obtained of the general character and external features of this interesting locality.

The character of the Gila valley, from this point down to its mouth, did not come under my personal inspection. All accounts represent a great uniformity of general features already sufficiently detailed.

Thus we have a succession of basins, limited by mountain barriers, through which the river forces its way, forming cañons of greater or less extent.

These basins are again occupied by more or less extensive stretches of gravelly table-land, representing the desert features of this region; through these are marked the alluvial tracts,