OCR Text |
Show 64 in reaching its base, is composed of the sandstone of Cretaceous No. 1, which first exhibits southern dips, but on the flanks of the mountain eastern dips. At various poiuts below and during the ascent, trap- dikes rise above the surrounding level, sometimes to considerable elevations, having a southwest and northeast strike. Higher up igneous intrusions appear on a larger scale, and the ground is covered with fragments of a fine- grained siliceous mineral, apparently rhyolite. Several large mountains on the left and right of the road appear to be composed of this igneous product. On approaching the pass, the road traverses the red sandstone. The highest point includes two adjacent hills; the eastern measuring by the aneroid 9,460 feet, the western 9,425 feet in elevation. The former of these is composed of the red sandstone; the other, in its eastern half, of the same formatiou, which is here thin- bedded, but in its western half, of a light- colored intrusive dole rite, including crystals of a black mineral. CHAPTER II. THE VALLEY OF THE RIO GRANDE TO SAXTA FE. On the west side of the Sangre de Cristo Pass, the sandstone beds dip west 30° to 60°. Three miles west the sandstone is vertical, and a little beyond is a bed of Carboniferous limestone, also nearly vertical. From a locality in this region, General August Kautz, then in command at Fort Garland, procured a series of Carboniferous invertebrate fossils, which he kindly presented to the survey, and which have not been identified. From the same region Major Hartz procured from a sandstone rock the impression of a large Gouiatite which was added to the paleontological collection. Through his attention, along the upper waters of Sangre de Cristo Creek, near the locality of the Carboniferous sandstone, a bed of black shale is followed by a greenish sandstone, both dipping 15c east. Near this point the hill is covered with fragments of rhyolite, with CDncentric reddish ttains, like that which has been described from Eastern Nevada. A mile westward tbe dip of the sandstone is reversed, and for a few miles beyond it exhibits contrary dips. The road gradually descends, and eight or ten miles west of the pass we come upon the real axis of the range, the red feldspar- porphyry. I found settlers rooting out carefully a white- flowered Astragalus- like plant, with radical leaves of a light green, with a silky pubescence, on the ground of its being poisonous to cattle and horses. On my arrival at Fort Garland, I found these statements confirmed by Dr. Moffatt, post- surgeon. This gentleman informed me that the plant was fatal to stock, narcotizing them when eaten, the effects in some instances coming on slowly, and remaining sometimes as long as two years. As dangerous properties are rare in leguminous plants, I thought that these observations were worthy of record. * rhe felspathic rock is here easily decomposed, so that, instead of forming the crests of the mountains at the Sangre de Cristo Pass, it occupies the valleys of the western slope of the range. The elevated parts of the pass are composed of sedimentary rocks, while the mountain- peaks of the region are of rhyolite and other intrusive material. The granitic beds are stratified, dipping northeast. Tbey are either heavily- bedded feldspathic porphyry, or more finely- bedded hornblendic gneiss, with much hornblende in fine grains. The following figure represents some of these beds, which have an intermediate mineral character, and are located near tbe eastern border of the formation. The dibris of this formation forms hills, which furnish placer- gold diggings, which are now worked with some success. The western part of the granite belt is excavated into a basin, which forms a branch of the great baBin of the Rio Grande, or San Luis Valley, and which was once filled with a deposit of Tertiary age. To within eight miles of Fort Garland, I found north of theroai a series of benches composed of the sediments of this lake, viz: beds of olayey sandstone alternating with heavier beds of moderately fine to very coarse conglomerate. The conglomerate contains rounded pebbles of chlorite, quartz, red sandstone, basalt, & c, and its strata lie horizontally against the oblique beds of the granite toward the pass. Dr. Hayden has described this formation from more northern parts of the San Luis valley, and I discovered the corresponding members of it on my return trip on the western side of the valley, occupying tbe lateral valleys of the San Juan range. In the latter locality, it presents the same features as in the Sangre de Cristo Valley, being composed of alternating beds of clayey sandstone and coarse conglomerate, and reaching a thickness of 800 and 1,000 feet. On the upper waters of the San Antonio Creek it forms bad- land tracts, with the characteristic scenery. There can be, then, no reasonable doubt that, as proposed by Dr. Hayden, the npper as well as the lower part of the valley of the Rio Grande was once* entirely filled with the deposit of a Tertiary lake, which I have elsewhere identified with tbe horizon of the Loup Fork epoch. |