| OCR Text |
Show 238 GEOLOGY OF THE HIGH PLATEAUS. group, overlaid by an enormous mass of volcanic conglomerate. Between the two are thin layers of those fine-grained marls and sandstones which have been derived from the decay of ancient lavas, and which were evidently deposited in water. Of the age of these intermediate beds it is possible to say but little. They are apparently conformable to the Bitter Creek below, but the conformity is no proof of continuity of deposition. They contain no fossils. The finer marly and arenaceous deposits are often of an exquisite apple-green color, and in some of the exposures the color is most charmingly delicate. The larger masses are from strong gray to white, when the grain is fine, and brown when it is coarse. Srnall decayed granules of volcanic sand, hornblendes, mica, and a green mineral, which may be epidote or " viridite," are intimately commingled. Veins of chalcedony and agate often cut the beds, and the fragments strew the soils and bad-land at the foot of the cliffs. The fault which uplifts the plateau has not been affected in any noticeable manner by its passage from the volcanic to the sedimentary region. It cut through a country which had apparently been long in repose; where time had been gradually smoothing down the inequalities, which had been produced by volcanic activity. When this new disturbance set in it seems to have laid out its line of operations regardless of existing inequalities, splitting whatever it found in its way. In the southern part of the Sevier Plateau it has sheared the old volcanic pile, and passing southward among the sedimentaries and conglomerates it treated them in the same fashion. The termination of the Sevier Plateau southward is effected by cliffs of conglomerate fringed with buttes. The conglomerate attenuates in that direction, and when its thickness has diminished to about 600 feet it is cut off by the undermining of the sedimentaries upon which it rests. At the end of the plateau the Sevier fault has diminished its throws to less than a thousand feet, and farther southward the throw reaches a minimum of about 600 feet, and thenceforward it increases again. This has produced a very slight sag, in which lies the Panquitch Hayfield, a broad valley-plain having an absolute altitude of a little less than 7,000 feet. |
