| OCR Text |
Show 190 GEOLOGY OF THE HIGH PLATEAUS. flowed northward in great volume, but since their eruption the eastern Tushar fault, swinging westwardly, has uplifted full 3,000 feet the extension of the sheets in that direction. The lavas which flowed eastward are all trachytic, but represent two groups of trachytic rock, one being highly hornblendic, the other being almost pure feldspar and granitoid in appearance, with a very few small but well-defined crystals of biotite. The hornblendic variety is exhibited in much greater quantity than the other, is very coarse-grained in texture, and lies in masses of great thickness. In several places single floods are seen between 300 and 400 feet thick, as if erupted in a highly viscous state, and appearing to have moved with great slowness and much internal resistance. This appearance is not only common, but is highly characteristic of the most typical trachytes, and gives rise to the exceeding coarseness and roughness which the etymology of the name implies. Upon the western side of Dog Valley many masses of coarse dolerites and some basalts are found. Being among the latest outbreaks of the locality, they have suffered most from erosion, and their debris are widely distributed in the form of conglomerates over the surrounding regions. These conglomerates are wrell stratified, and when the exposures are viewed at a distance great enough to render the rocky fragments no longer distinguishable, they reveal a lamination quite as conspicuous as a succession of sedimentary strata. These conglomerates lie in the heaviest masses in the northwestern portion of the valley, and turn up against the southern end of the Tushar at an angle of 22°, showing a thickness exceeding 1,500 feet, without exposing its entire extent. No individual mass of conglomerate has been observed to extend over any large area, but they seem rather to have filled up depressions They increase and diminish rapidly in thickness, and obviously represent many local accumulations, which are not continuous among themselves. This arrangement is to be expected upon the theory that their origin is alluvial, a theory which (if it needs any special support) will appear to be abundantly sustained when we come to the examination of their formation at the present time in the larger valleys of the district. The elevation of this valley above that of the Sevier on the east is |
