OCR Text |
Show SOUTHERN ERUPTIVE CENTER. 237 not, however, so well exposed, and much less can be said about it. A grand ravine has eaten its way into it from the western side and disclosed at the base propylite and hornblendic andesite in great masses, and exhibiting evidence of an early period of great erosion followed by the eruption of augitic andesites and many forms of trachyte, which buried the ancient piles beneath their floods. A few fragmentary exposures of old. conglomerate, consisting of the ruins of the most ancient lavas, are also revealed near the base. Some of these have been so thoroughly metamorphosed that they form almost a homogeneous mass, in which the cement has an aspect closely resembling the fragments it envelops, and is shot through with minute crystals of feldspar and secondary hornblende. When broken, the surface of fracture cuts the pebbles and cement indifferently. The propylites and hornblendic andesites are more profusely charged with hornblende than those of the northern vent, and the propylites are rather finer in texture. The great mass of rocks now visible in this part of the plateau are of the trachytic series and later in age. They are mostly of the 1 argilloid' varieties, but contain fewer porphyritic crystals of orthoclase than are usually found in such lavas, and are heavily charged with ferritic matter, giving them a dirty brown appearance. Those eruptions which flowed westward commingled with those which emanated from Dog Valley, about 12 to 15 miles westward. Of those which flowed eastward I know but little. I have no doubt that they are well exposed in many of the ravines which descend from the crest of the plateau towards the foot of the Aquarius. I have hastily crossed them once, but have no conception of them sufficiently clear to justify me in attempting to describe them. My field-notes indicate a broad expanse of trachytic and andesitic rocks inter-bedded with volcanic conglomerate sloping gently towards the east and appearing to emanate from the above-mentioned source. The eruptions from this source did not extend more than 6 or 7 miles southward. On the west side of the Sevier Plateau the last that was seen of them was in a deep carlon-like ravine, called Sanford Canon, opening into Panquitch Valley about 6 miles south of the head of Panquitch Canon. Here the strictly eruptive part of the plateau ends, and the continuation of it southward is composed of Tertiary beds of the Bitter Creek |