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A general description of the topographical features of the country along the boundary between the United States and Mexico, (traversing the whole breadth of the continent,) cannot be made comprehensive, without presenting in the same view the great outline of the continent itself.

It is now well known that the most extensive feature in the continent is the plateau, or tableland, which traverses this country from the unexplored region of the north to its southernmost extremity, varying in width from five miles to one thousand, attaining its greatest elevation in the Andes of South America, its least elevation and breadth on the Isthmus of Panama and in Central America, and its greatest breadth about the parallel of 38° north latitude. On the northern portion of the continent, this plateau attains its greatest height in Mexico, where it is ten thousand feet above the level of the sea. Its lowest depression is along the line of boundary, about the parallel of 32° north latitude, where it is about four thousand feet above the sea. Thence it ascends again, and preserves an elevation varying from seven to eight thousand feet, to near the 49th parallel, where it is again depressed. This plateau, both in

North and South America, occupies the western side of the continent and is traversed by ranges · of mountains, the highest peak of which, in North America, is Mount Elias, 17,000 feet above

the sea, and in South America is Mount Aconcagua, 21,500 feet above the sea. The climatic features in this plateau, within the United States, are excessive dryness and great changes of temperature between night and day, often as much as 65o.

The principal ranges of these mountains in North America, naming them in the order of their proximity to the coast of the Pacific ocean, are, first, the Cordilleras of California and Oregon, or the Coast Range of mountains ; second, the Sierra Nevada, (which, as its name denotes, is a ridge of mountains and craggy rocks, covered with snow ;) third, the Sierra Madre, another range of mountains, which was supposed to separate the waters flowing into the two oceans; and, fourth, the Rocky mountains.

The idea conveyed by the name Sierra Madre is very generally adopted by the Mexicans, yet I doubt very much if any continuous ridge or chain of mountains can be found which separates the waters flowing into the Pacific from those flowing into the Atlantic. I am also quite well satisfied that the mountains known as Sierra Madre, in New Mexico, are not the same range as those known by that name in Chihuahua and Sonora, and that both are distinct from the range west and south of Monterey of the same name ; but the Coast Range, the Sierra Nevada, and the Rocky mountains, preserve a very considerable continuity throughout the limits of the United States. The Coast Range follows the generally northwest direction of the beach of the Pacific coast, and, for a very considerable distance, rises abruptly from the sea. Along the whole coast it is in view of the navigator, presenting an imposing and ever-changing panorama. It may be said to terminate at Cape San Lucas, the southern extremity of Lower California.

It is the slope towards the sea of this range of mountains which forms the western border of the arid region, and is, in my judgment, the only continuous agricultural country west of the 100th meridian. There are many detached valleys and basins affording facilities for irrigation, where the cereals, the vine, and all the plants which conduce to the comfort of man, are produced luxuriantly; but they form the exception rather than the general rule, and are separated by arid plains or mountains.

The Sierra Nevada, the Cascade Range, and the Rocky mountains, preserve a general parallelism to each other and to that of the Coast Range. Commencing at the north, they can be traced continuously until we reach to within a few degrees of latitude of the region

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VIEW TOWARDS THE EASTERN SLOPE OF THE CALIFORNIAN CORDILLERAS TAKEN FROM NAAR CARRIZO CREEK.

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UPHEAVINGS BORDERING THE COLORADO DESERT ON THE FOOT OF THE CALIFORNIAN CORDILLERAS

TAKEN FROM CARRIZO CREEK-LOOKING SOUTH EAST

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of the boundary, where occurs, in all the ranges except the Coast Range, the remarkable depression in the continent, or rather absence in the continuity of the ranges of mountains, hereafter to be described.

The Sierra Nevada, in latitude 33° N., branches; one great division unites with the Coast, Range, forming the elevated promontory of Lower California, and presenting, when figured on the map, the appearance of the letter Y, (Tulare valley resting in the fork of the letter ;) other branches or spurs are thrown off in a southeast direction, crossing the Gila at the mouth, and many miles above, and traversing the newly-acquired territory in the meridian of Santa Cruz and Tucson. - That range, as well as the Sierra Madre and the Rocky mountains, about the parallel of 32°, lose their continuous character, and assume the forms that are graphically described in the western country as lost mountains--that is to say, mountains which have no apparent connexion with each other. They preserve, however, their general direction N. W. and S. E., showing that the upheaving power which produced them was the same, but in diminished and irregular force. They rise abruptly from the plateau, and disappear as suddenly, and, by winding around the bases of these mountains, it is possible to pass through the mountain system, in this region, near the parallel of 32°, almost on the level of the plateau ; so that if the sea were to rise 4,000 feet above its present level, the navigator could cross the continent near the 32d parallel of latitude. He would be on soundings of uniform depth, from the Gulf of California to the Pecos river. He would see to the north and to the south prominent peaks and sierras, and at times his passage would be narrow and intricate. At El Paso he would be within gun-shot of both shores.

I noticed this remarkable depression in the continent, in an exploration made by me in 1846, and called to it the attention of Mr. Buchanan, then Secretary of State ; and it was upon this

not to take a line north of the 32d parallel of latitude, in the boundary between the United States and Mexico.

Passing to the south of this parallel, in about that of 31°, we find the plateau rising rapidly to the table-lands of Mexico, the ranges above described are no longer traceable, and the plateau gives evidence of having been disturbed by tremendous plutonic forces, and the mountains assume a loftier and more rugged and diversified appearance. As I have said before, the Sierra Madre range of mountains cannot be traced distinctly with our present information.

The Rocky mountains, near the head-waters of the Rio Bravo, throw off spurs, which add to the confusion and make it difficult to separate the range from that called in New Mexico the Sierra Madre.

It may be a question whether the Rocky mountain range is not divided by the Rio Bravo; and if so, that which I have designated as the Sierra Madre of New Mexico will, in that case, become a spur of the Rocky mountains. The geological formations to which I shall presently refer, seem to favor this hypothesis. If this hypothesis be true, the Sierra Madre of New Mexico and the Rocky mountain system are the same, and are only divided by the Rio Bravo. But this is a question which does not affect the general topographical description of the country, and may be disregarded here. What I have described refers more particularly to the country west of the Rio Bravo.

The Rocky mountain system, commencing in the north, beyond the source of this river, and beyond the limits of the 49th degree of north latitude, is the distinguishing feature of the

Vol. I -6

country east of that river until we reach the great plains lying between the base of those mountains and the valley of the Mississippi. The axis of maximum elevation preserves a general parallelism to the Sierra Nevada range. Its principal chain, after passing the 36th parallel of latitude, becomes less elevated, and finally terminates in the Organ mountains near El Paso, re-appearing again to the south and east, and becoming at last merged in the great mountain masses in Mexico.

Another branch of these mountains diverges about the head of the Pecos, and running south with unequal elevation, crosses the Rio Bravo between the 102d and 106th meridian-of longitude, forming the great bend in that river, and producing one of the most remarkable features on the face of the globe-that of a river traversing at an oblique angle a chain of lofty mountains, and making through these. on a gigantic scale, what is called in Spanish America a cañon--that is, a river hemmed in by vertical walls.

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These mountains to the south of the river expand in width and height, attaining a great elevation in the neighborhood of Monterey, Saltillo, and Buena Vista, and forming one side of the Bolson Mapimi, and it is my impression that these mountains are identical with what is called in Nuevo Leon the Sierra Madre. : .

A third, but subordinate range, branches from the main chain about the same parallel as that last described, and terminates in the Llano Estacado or the staked Plains, from which issue the Red river and other rivers of Texas. From the foot of the Llano Estacado the country falls, sometimes by steps, but most generally by gentle slopes, to the shores of the Gulf of Mexico, the crust only broken by the washing of water, and in a few places by the protrusion of igneous rocks. The view of the bed of Devil's river will give a very good idea of the manner in which the general level of the surface forming this great cretaceous plane is broken by the action of water.

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