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Show 294 GEOLOGY OF THE HIGH PLATEAUS. rocks present a great deal of variation in their aspect. A body of lavas so enormous as that which caps the Aquarius cannot be discussed with profit until it has been studied long and patiently, and inasmuch as my own observation has been extremely superficial, I do not feel justified in attempting to give any further account of them. The structure of the plateau is best studied upon the southern slopes. Here the most striking feature is a large monoclinal, already alluded to as a companion to the Water Pocket fold. It comes up from the southeast, crossing the lower end of Potato Valley, and trends along the slopes northwestwardly, disappearing beneath the lava-cap. The throw of the monocline is to the westward. Upon its flanks the Cretaceous system is turned up and dips westward beneath the southwestward extension of the general plateau mass. The edges of its strata are truncated by erosion, and over them lies unconformably the Tertiary. (See Atlas Sheet No. 7, Section No. 7.) The upthrow of the monocline heaves up the Jurassic white sandstone, which is seen rolling up in a huge wave 1,200 to 1,800 feet high across the lower end of Potato Valley. The position of this flexure relatively to the plateau mass is peculiar and very striking; indeed, at first sight it appears altogether anomalous. We are accustomed in the western regions to see the strata rolled up on the flanks of a mountain range like a great wave urged onward towards a coast and breaking against its rocky barriers. But the Escalante flexure is like a wave sweeping along parallel to the coast, the crest-line of the wave being perpendicular to the trend of the shore. Its line of strike runs up the slope and disappears beneath the Tertiary near the summit of the plateau. A fine steam of water (Winslow Creek) runs upon this monocline parallel to its strike, precisely as Water Pocket Creek runs upon and parallel to the course of that flexure. The age of the Escalante monocline is evidently Pre-Tertiary. It has been exhumed by the general erosion after having been buried beneath Eocene strata, and after these strata had been overflowed in great part at least by many hundreds of feet of lavas. The stream had its course laid out prior to this erosion, and held its position after it had cut through lavas and Eocene beds into the underlying Jurassic sandstones. The area included between the Escalante fold on the west and the |