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Show 103 Proceeding from the head of Cation de San Diego to the westward, we cross the trachytic formation, aud reach Rito Pefias Negras, a tributary of the Guadalupe, where the Carboniferous limestone is again seen. This rock contains, in this locality, oxide of iron, and all the shells found in it are more or less covered with a red layer of this substance. Going farther west, across ridges and valleys and through magnificent ine forests, we gradually lose sight of the volcanic rocks; but before leaving them we will take a glance at their lithological character. Near the headwaters of the Bear Creek, in the vicinity of Abiquiu Peak, the gray feldspatbic matrix of the trachytes is densely crowded with large needles of hornblende and crystals of sanidine, while, approaching the valley of the Rio Polvadera, labradorite, with a beautiful blue iridescence, becomes a conspicuous constituent, sanidine almost disappearing, and hornblende diminishing gradually until it entirely vanishes in the rocks of the Canon de Santa Clara, south of Abiquiu Peak, the trachyte in this vicinity being full of sphernlite. To the south of this canon is a very siliceous rbyolite, devoid of any crystalline constituents, but colored red in spots by oxide of iron. Still farther south, near the valley of the Obsidian Creek, the trachyte becomes exceedingly rich in sanidine crystals of small size, hornblende being almost entirely absent. In more than one locality are unmistakable evidences of repeated trachytio outbursts, whereby the once- formed tufas have been brought to fusion. Not far south of Abiquiu I encountered a rbyolite with a reddish matrix, in which fragments of a gray rbyolite are embedded. This rock I consider a tufaceous deposit, which was subsequently subjected to an incomplete fusion by the heat of the neighboring trachytic protrusions. All along the upper portions of Guadalupe River the granite and gneiss become uncovered by the Carboniferous strata. The granite is of a dense structure, and the feldspar of a reddish color. The mica is the black variety, or biotite, while The quartz particles are of a subtranslucent character, as seen in certain jaspers and milky opals. On one side of the Guadalupe Valley we noticed a singular laud- slide, which probably occurred quite recently : a tract of land of about 10,000 cubic feet had moved to the bottom of the valley, a distance of 50 or GO yards. Leaving the Guadalupe we have to cross another range, the Nacimiento, before reaching the Mexican village of Nacimiento. This range forms the northeastern boundary- line of a wide sandy terrace and mesa country, which I have termed the 11 Nacimiento Desert." TUB NACIMIENTO DESERT. This barren waste comprises all the land between the Rio Puerco in the east to Fort Defiance or Canon Bonito in the west, ami from Mount Taylor in the south to Nacimiento and Ojo San Jose in the north, covering more than five thousand square miles, and having an average elevation of about 5,900 feet above sea- level. The vegetation is exceedingly poor, with the exception of a few localities that possess a higher elevation than the average. One of those forest islands amid this sand- waste is formed by a very high mesa north of Bacon Springs and northeast of Fort Wingate. The topo- . graphic features of this desert consist in a continuous system of mesas and valleys hardly equaled anywhere on the globe. The trapezoidic forms of mesas, ranging in height from 50 to 300 feet, stretch many miles in monotonous uniformity. In the eastern section are the Mesas de los Portales, de la Ventana, de los Torreones, and de la Piedra Lumbre; in the northern the Mesas de los Lobos and del Raton ; in the southern, Mesas de Joro, Pintada, and Rotouda; and in the western section Mesas Trastchi- Tchibito, and Tistsitloe, ( Navajo names.) Ouce upon the mesas it is difficult to fiud a place to descend, and only by chance is a narrow trail found that leads to the valley; in any event the descent is a dangerous undertaking. Some forty miles to the west of Nacimiento one of the larger mesas forms the divide between the Atlantic and Pacific, here hardly marked by an elevation, while usually high mountain ranges represent the dividing- line between these oceans. The Rio Puerco of the east, and its tributary, the Rito Torreones, flow through the mesa system to the eastward into the Rio Grande, while the Chaco Creek, the head of which is forty- two miles west of Nacimiento, forms a tributary of the Rio San Juan. These once powerful streams are now scarcely more than dry arroyos. The questiou arises, how was this extensive mesa system produced ? Of course by erosion; but how was the erosion brought about? Erosion can be active in the form of rain, rivers, submarine cnrreDts; but none of these agents were at work here. The most probable theory i « in the wearing action of the tidal motions of the slowly- receding waves which began when the retreating Cretaceous ocean had as its shores the same deposits that were formed before at the bottom of its depths. This view is confirmed by the existence of many very deep and at the same time very short canons in these mesas representing former bays of the receding Cretaceous ocean- canons whose character forbids the idea that they are the result of erosion by supposed rivers. In a similar manner, according to my belief, were the great mesas of the Moqni country in Eastern Arizona formed. Tbe chief formation throughout the Nacimiento Desert is the Cretaceous; the older formations, like the Triassic and Carboniferous, being exposed in only a few localities |
