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Show 158 APPENDIX HI. \ REPORT ON THE GEOLOGY OF A PORTION OF SOUTHERN CALIFORNIA, BY PROF. JULES MARCOU. Los ANGELES.- The hills which surround Los Angeles, forming a vast amphitheater open only on the west, are composed principally of friable sandstone of a yellowish-gray color, with some intercalated layers of sandy day and limestone of the same color. These rocks, which have all the appearance of real molasse, can be studied to the best advantage in the trenches which have been dng on the side of the old presidio, or fort, which overlooks the town on the north ; and also by following the trenches of the aqueduct and the cation of the Rio de los Angeles or Kio de Porciuncula, after leaving the town, as far as the valley of San Fernando. The strata are all much elevated, and dip in a south- southeasterly direction, at an angle varying from 30 to 35 degrees. The thickness of the sandstone- beds varies from 1 to 5 feet, but beds are nowhere found of sufficient hardness to furnish good building stone, on account of the friability of the sandstone, which is rapidly decomposed by the action of the atmosphere. Fossils are very rare, with the exception of the bones of Cetacea, which are in a bad state of preservation, and it is difficult to obtain good specimens. On the hill of the presiaio fragments of the oast of a small gasteropod are sometimes found in the sandy clay; of this not even the genus can be determined. On the road from Los Angeles to Cahunga Pass, descending the northern declivity of the hills, before reaching the plain of the Balonas or Bayona, we find several scanty petroleum- springs, and the limestone in the vicinity of the springs is strongly impregnated with asphaltum. The petroleum, or bitumen, when dried in the sun, becomes hard, and is then known by the name of " brea " to the Mexicans, who use it as a covering for the roofs of their houses and as a pavement for sidewalks. The hills of Los Angeles begin at the southeast of the lagoon, and of the Balona or Bayona ranch, rising at the east- northeast toward the canon of the Bio de los Angeles or Rio de Porciuncula, which cross them; they then rise in gentle acclivities against the granite of the Sierra Madre, not far from the ranches of the San Gabriel Mission; finally, they again descend toward the sontb, and next turn once more to the east, and form the small basin of El Monte. Their height is small, and varies from 50 to 300 feet above the bottom of the valley of Los Angeles. PLIOCENE ROCKS OF LOS ANGELES. The sandstone rocks of Los Angeles may be considered as a brackish deposit of an estuary, or of lagoons, subsequent to the formation of the valley of San Fernando, where they do not exist. The sierras of Santa Monica and San Fernando had already undergone a movement which had obliged the sea to recede more to the south and to the west. The formation of this sandstone is evidently quite recent, and, as it is placed between the Miocene rocks of San Fernando and the Quaternary alluvial rocks of the bottoms of the valleys, it must be regarded as being of the age of the Pliocene Tertiary. Fish- bearing limestone rocks of Los Encinos,- Below the molassic sandstone rocks of Los Angeles we find a group of limestone rocks whitish, chalky, and containing a small quantity of magnesia, stratified in rather thin layers, the entire thickness of which is from 100 to 150 feet. These white limestone rocks are found in the hills which face San Gabriel and along the road between San Gabriel and the San Fernando Valley. In these limestone rocks, especially in the upper portions, we meet here and there with remains of fishes, such as scales and vertebrae, generally in a bad state of preservation. Denudations have carried away these limestone rocks in which fish are found in the cafion of the Rio delos Angeles or Rio de Porciuncula, and likewise at the beginning of the San Fernando Plains; they reappear, however, in the San Fernando Valley, where they form a large portion of the bottom of the valley, and a part of the first " counterfort" of the mountains which surround it, beginning a mile and a half to the west of Cahunga Pass, as one goes toward Los Encinos. At the ranch of Los Encinos these limestone rocks attain their greatest thickness, which is nearly 200 feet; and they contain in their upper portions, close by the houses of Mr. Engine Gartner's ranch, a large quantity of fossil- fishes, large bones, vertebras, and ribs of Cetaoea, fossil-plants, Crustacea, and, it is said, even birds; also a quite common bivalve mollusk- the Feoten Peckanii Gabb. These fish- bearing limestone rocks of Los Encinos belong to the Miocene Tertiary rocks, of which they form the upper portion. As regards their age, they may be compared to those at Oeningen, on the banks of the Rhine, near Sohaffhausen; Switzerland, or to those of the quarries of Aix. in Provence, France. They are a deposit of brackish, almost fresh, water, notwitstanding the presence of a small Fecten and of Cetacea. It has not, as yet, been possible to collect the fishes in sufficiently large numbers, or to examine them in such a manner as to be enabled to obtain correct ideas with regard to the ichthyological fauna of Los Encinos. |