OCR Text |
Show 62 the strike of the formations of the country to the south, on the west side of the Galli-nas Mountains, reaching the Rio Puercoat a point fifty miles southwest from Los Ojos. Considerable time was occupied in this expedition, side- trips being coutinually made east and west of the line, the latter as far as the Alto del Uta, forty miles west of the Gallinas Mountains. The same route was traversed in returning as far as the Rio Chama and the town of Tierra Amarilla. From this point, the expedition took the direct course across the San Juan Monntaios to Conejos, and across the Rio Grande Valley to Fort Garland. The route from this point to Pueblo was the same as that followed on the outward trip. As is now well known, the ranges composing the Rocky Mountains form a scries of Echelons, which have a generally north and south course, and descend to the plain at their southern extremities. The result is that when this arrangement prevails the trend of the entire mass of ranges is not identical with that of the constituent ranges, but is southwest and northeast. Thus the Front range, which bounds the plains continuously for two hundred and fifty miles, disappears in the Shyenne Mountains, near to Pike's Peak. The second range, or Wet Mountains, disappears at the en trance to the Sangre de Cristo Pass, after having culminated in the Greenhorn Mountain. The third, or Sangre de Cristo range, extends one hundred miles south of the pass, and sinks into the plains not far southeast of Santa Fe\ The fourth range bounds the valley of the Rio Grande on the west, and has received various names in its different extensions. It is the San Juan, Navajo, Gallinas, and Nacimiento Mountains of the present survey. These are sometimes known under the collective term Sierra. Madre. and they exhibit a reduction in elevation as compared with their northern coutinuation in Colorado; a reduction which continues to the southward until, in Central New Mexico, they no longer constitute a continuous range. It is seen therefore that the expedition passed round the southern extremity of the Wet Mountain Echelon, and crossed the two axes of the Sangre de Cristo and Sierra Madre. Observation therefore extended to the structure of the western border of the Mississippi drainage, to the entire width of the drainage- area of the Rio Grande, and to the eastern portions of the area mostly drained by the Great Colorado. The subject will therefore be considered under three heads, viz: the eastern slope of the Rocky ranges; the Rio Grande Valley j and the western slope of the Sierra Madre. CHAPTER L THE EASTERN SLOPE OF THE ROCKY MOUNTAINS. The Rocky Mountains axes are well known to be composed of a feldspar- porphyry, where not exceptionally igneous and intrusive. These axes were forced through superincumbent sedimentary strata, the remnants of which now rest upon their flanks. Those of the sedimentary strata which extended across the region now occupied by the mountain- ranges were necessarily lifted to an almost or quite vertical position, in which they now remain. Other beds, deposited after the commencement of the process of the elevation and before its conclusion, were necessarily raised so as to he more obliquely against the sides of the axes or of the older sedimentary beds ; while strata deposited after the close of the process of elevation extend to and rest upon the slopes of the ranges nearly as they were originally deposited, in a horizontal position. At Manitou, near Colorado Springs, the porphyritic granite is immediately covered by sandstones and limestones of Silurian age, of 70 feet in thickness, mostly of a reddish color. This formation is succeeded in ascending order by 279 feet of gray, purplish, and yellow limestone supposed to be of Carboniferous age. Above this is a series of red or variegated sandstones, often containing conglomerate beds, and often very massive, of 1,200 to 1,500 feet in thickness. No fossils have yet been found in this horizon, so that its age has been conjecturally called Triassic. This is followed by about 31 feet of calcareous shales, with thin beds of sandstones, in which fossils of Jurassic age occur in other localities. Above these Jurassic beds is a stratum of white gypsum of 57 feet in depth. It is below the Cretaceous No. 1, and is sometimes included in the Jnrassic. ( See Dr. Peale, Report of the U. S. Geol. Surv. of the Territories, 1873, p. 198.) From this point upward the members of the Cretaceous may be traced. After an interval of 60 feet of shales and soft sandstones, the gypsum is followed by abed of white or pale massive sandstone of 200 or a few more feet in thickness. This is Cretaceous No. 1, or the Dakota group, a very important base- line in estimating the position of other strata in New Mexico. Its hardness and consequent resistance to erosive forces have left to it a prominent position as the axis of the first range of foothills along the mountains for very great distances. It is followed in the ascending Beries by soft and dark- colored shales, usually rich in invertebrate fossils, whioh are known as the Benton group, or No. 2. The Niobrara group ( No. 3) consists usually cf impure limestone either of a siliceous or argillaceous character. It forms the crest of the second and lower line of foot- hills, and is often highly fossiliferous; common tpecies being Ostrea congesta and Inooeramusproblemaiicus. The following beds are again |