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The igneous protrusions which occur are composed of greenstone or basalt, and are traced from the San Saba mountain, by the head of the Leona, to Santa Rosa, in Mexico, where it unites with the main ridge, at an angle of about forty-five degrees. The point where they unite is rich in silver mines. At Santa Rosa the Spaniards had sunk extensive shafts and made a tunnel a mile and a half in length, which was not completed when the revolution of 1825 broke out; since then all extensive operations have been suspended, and the country, rich in minerals and in the production of the cereals and of tropical plants, has been a prey to the incursions of banditti and Indians, and at this time Wild Cat and his band of Florida Indians are settled near there, to add to the disorder and misrule of this beautiful region.
It has been observed that these metalliferous rocks generally occur at the junction of two systems, or where some unusual disturbance or change in the geological structure takes place. Hence we may expect to find these silver-bearing rocks along the boundary line, where the upheaving force, after subsiding near the bed of the Gila river, begins again to re-appear to the south.
The remaining mountain feature of North America, the Appalachian, is referred to here, only to illustrate by comparison the mountain system of the western part of the continent. That chain, grand as it is, sinks into insignificance when compared to those I have attempted to describe.' It is nearly at right-angles to the western chain of mountains, is less elevated, and sheds its waters, as is well known, clear on both sides ; on the one side into the Atlantic, and on the other side into the Mississippi and the Gulf of Mexico. On both sides, the slopes are comparatively gentle, and the soil fertile, and, refreshed by frequent showers, yields in abundance all that contributes to the wants of man; on the western side of this slope, between it and the desert border of the Rocky mountains, such an expanse of fertile country exists as can be found, in one body, nowhere else on the face of the globe, producing all the fruits of the earth, including those found in every zone, from the boreal regions to the tropics. Persons familiar with its character, as most who read this memoir are, will scarcely be able to comprehend, still less to believe, the character given to the more western and less favored regions described in this report.
In the fanciful and exaggerated description given by many of the character of the western half of the continent, some have no doubt been influenced by a desire to favor particular routes of travel for the emigrants to follow; others by a desire to commend themselves to the political favor of those interested in the settlement and sale of the lands; but much the greater portion by estimating the soil alone, which is generally good, without giving due weight to the infrequency of rains, or the absence of the necessary humidity in the atmosphere to produce a profitable vegetation. But be the motive what it may, the influence has been equally unfortunate by directing legislation and the military occupation of the country, as if it were susceptible of continuous settlement, from the peaks of the Alleghanies to the shores of the Pacific.
Between the two most distinctly-marked ranges of mountains, before described, (the Rocky mountains and the Sierra Nevada,) a succession of minor ranges occur, some of which are many hundred miles in extent, while others appear like isolated mountains, rising above the general level of the plateau. Most of them preserve a general system of parallelism; others present their lines of maximum elevation, forming very considerable angles with the general direction, and all, when traced upon a map, exhibit lines varying from right lines to every degree of curvature.
The whole system, plateau and mountain, seems to have been produced by a succession of forces analogous to each other in direction, but differing in intensity and occurring at long intervals. The prevalence of granite and other unstratified rocks throughout the Sierra Nevada suggests the probability of its being the oldest of the western range of mountains. The identity of its rocks, generally, with those of the Alleghany mountains, marks these two distinct and detached chains as probably contemporaneous. The rocks marking these mountains are of the description commonly traversed by gold and copper veins, as is the case in Oregon, California, Virginia, and North Carolina. . Travelling eastward from the Pacific along the bed of the Gila, we encountered similar rocks in a chain of mountains as far east as the Pimo village. This chain, characterized also by the presence of gneiss, mica, and talcose slate, has been traced as far south as the present boundary, where it crosses the Santa Cruz river, between longitude 110 and 111; and in that neighborhood we saw everywhere the remains of gold mines, from which the operators had been driven by the Apaches.
Pursuing our course eastward along the boundary from the meridian of 110°, we cross the San Pedro, the Guadalupe, and the San Luis range of mountains in the order in which they are named, the middle range being chiefly characterized by sienitic aggregates, granitic lava, and immense masses of conglomerate, or breccia. Precisely the same formation is found in the cañon of the Gila, some distance to the north, about the meridian of what is called, in my reconnoissance of 1846, Disappointment creek. And no doubt, when future surveys shall develop a more minute knowledge of the physical geography of the country, each of these ranges of mountains will find its equivalent to the north and to the south. With the present information, I shall not even attempt to connect them conjecturally.
Hypothetical geography has proceeded far enough in the United States. In no country has it been carried to such an extent, or been attended with more diastrous consequences. This pernicious system was commenced under the eminent auspices of Baron Humboldt, who, from a few excursions into Mexico, attempted to figure the whole North American continent. It has been followed by individuals to carry out objects of their own. In this way it has come to pass, that, with no other evidence than that furnished by a party of persons travelling on muleback, at the top of their speed, across the continent, the opinion of the country has been held in suspense upon the subject of the proper route for a railway, and even a preference created in the public mind in favor of a route which actual survey has demonstrated to be the most impracticable of all the routes between the 49th and 32d parallels of latitude: On the same kind of unsubstantial information maps of the whole continent have been produced and engraved in the highest style of art, and sent forth to receive the patronage of Congress, and the applause of geographical societies at home and abroad, while the substantial contributors to accurate geography have seen their works pilfered and distorted, and themselves overlooked and forgotten.
The San Luis mountains, a distant view of which is given from the Alamo Hueco springs looking west, rise abruptly from the plains about three leagues north of the parallel 31o.20, and, as they run south, assume by far the most formidable appearance of any range, on that parallel, west of the Rio Grande. They are called, in Sonora and part of Chihuahua, the Sierra Madre mountains, yet they do not fulfil entirely the conditions implied by that term, for I am credibly informed that the waters flowing from their base towards the Pacific coast often take their rise to the east of these mountains, and flow through chasms impassable for men, and fall down the western slope in rapid descent, producing sublime and picturesque cascades.