OCR Text |
Show 65 Eight miles east of Fort Garland several masses of basalt form rough hills, with a southwest and northeast trend, and the road rises to a higher level, while the creek cuts its way through a small cafton. The characteristic features of the Rio Grande Valley now come into view in flat- topped mesas, with steep sides, capped with abed of basalt. Their sides are covered with masses broken from the face of the stratum of basalt, allowing of no vegetation or a few yuccas and sage- brush. The floor of the valley from this point to the Rio Grande, a distance of twenty miles, and for a greater distance to the west side of it is level and barren, being covered over with sage ( Artemisia) and brushlike Composite. Near Fort Garland its soil is everywhere mingled with gravel of the decomposed pink porphyry. The flat- topped black basaltic mesas are distributed on both sides of the Rio Grande, and form prominent objects for thirty miles below Fort Garland. The Rio Trinchera, passing the fort, enters the Rio Grande on a level plain, but not many miles below its month the river enters a cafion cut into the bed of basalt, which constitutes the floor of the plain, and only emerges at intervals during a course of one hundred miles to 1 he south. Two round basaltio masses are distinguished among the mesas, the San Antoino and the Ute Mountains. Yio. 1.- Strata of feldthspathic porphyry and gneiss on Sangre do Cristo Creek, near the pass. The prevalence of the basaltic rock gives the valley of the Rio Grande a forbidding character both to the agriculturist and the geologist. The concealment in a deep cafion of the great river, which, under other circumstances, would have been to it what the Nile is to Egypt, has relegated a great part of its surface to comparative sterility. This is relieved by the many creeks of pure water which issue from the mountains and carry fertility in their courses across the east side of the valley. Such are the Costilla, the Colorado, the San Cristoval, and the Honda. Near the Colorado Creek, at the foot of the mountain, I observed an interesting example of the decomposition of basalt. This rock is usually in this region porphyrittc, including small masses of a light- colored feldspar, which is often weathered out on exposure, leaving a vesicular structure of the surface. At the cafion, wherjB the Colorado Creek issues from the mountain, it traverses a vertical mass of protruded basalt of 800 feet in elevation. In the least altered portions, near the summit, the base of the mineral is a bluish iead- color, contrasting strongly with the small masses of white feldspar. At a lower elevation, the base is rusty- brown or yellow, the white bodies far less distinct. In the lowest part of the bluffs, say for 200 feet,* the rock has a homogeneous appearance, and is pare white, like kaolin. From these white rocks, near the base, issues a chalybeate epring, sour with excess of sulphuric acid. At its point of exit is a deposit of alum. At the month of the Rito Honda, the Rito Grande flows through a cafion of 800 feet in depth. No Tertiary beds are visible until we reach the valley of Taos. This fine tract of land, drained by the Taos Creek and its tributaries, occupies an amphitheater in the Sangre de Cristo Mountains, which sends a strong spur off to the Rio Grande on the south side of the valley. Numerous villages constitute the general settlement of Taos, not the least interesting of which is the Indian town, or pueblo, of agglomerated houses of that name. The mesas bounding this valley on the north are composed of a coarse gravel of worn pebbles and cobble- stones derived from the mountains. They are cemented together slightly by a calcareous substance to a depth of 2 to 4 feet, and the deposit has more the appearance of being a late drift than a part of the Pliocene lake-deposit previously described. The transported material shows plainly that the western flanks of the mountains are chiefly composed ot gneiss, qnartzose granite, aud quartz- |