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Show 280 GEOLOGY OF THE HIGH PLATEAUS. quite flat, giving a tabular summit to the mass about 5 miles long and nearly 2 miles wide. An almost impassable talus surrounds the scarped edges of this cap, and renders the ascent difficult except at a few points upon the eastern side. The lavas are hornblendic trachytes and augitic andesites, heavily interbedded and made up of numerous flows. These rest upon a layer of Lower Tertiary, of which the thickness is not precisely known, but which cannot well exceed 700 feet. Whether the diminished volume of the Tertiary here is due to an originally small amount of deposition or to an erosion of the upper members prior to the volcanic overflow is not yet determined, but I incline to the former explanation. And the general indications seem to be that over the area occupied by the eastern part of the Aquarius and Thousand Lake Mountain the Tertiary deposition was locally much thinner than elsewhere. Immediately beneath is the Jurassic white sandstone. The Cretaceous is absent from its place in the stratigraphical series. Yet a few miles to the northeastward the whole vast Cretaceous system is rolled up and cut off on the slopes of a grand monoclinal. This monoclinal is the Water Pocket Fold, which is probably the grandest feature of the kind in the Plateau Country, so far as known, and perhaps the most typical. Its first appearance is beneath Thousand Lake Mountain (see Atlas Sheet No. 4), where the trend is east-southeast and it gradually swings around towards the south-southeast, reaching to the Colorado River, in the heart of the Glen Canon. It crosses the river into unknown regions. Upon the northwestern side of the mountain it is covered up by Tertiary beds and lava sheets and is wholly concealed, so that neither its northern nor its southern terminations are at present known. The great fault upon the west side of the mountain cuts across this monoclinal nearly at right angles, and has dropped the platform to the west several thousand feet. The age of the great flexure is evidently older than Tertiary time, for the Lower Eocene beds lie nearly horizontally across the upturned edges of the whole Cretaceous system and upon the deeply-eroded surface of the Jurassic sandstone. Inasmuch as the entire body of Cretaceous strata, including the Laramie beds, appear in succession as we cross the strike of the flexure and as they are all upturned upon its flanks, |