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Show FINAL EESCJLTS OF EEOSION AND UPLIFTING. 23 fauna. This formation once extended over the Grand Canon area probably as far south as the river itself, and possibly farther, but has all been swept away as far north as the southern end of the district of High Plateaus. From the region east of the High Plateaus also very large areas of it have been removed. The Upper Trias has also been greatly denuded, and the Lower Trias nearly as much so. The erosion of the Carboniferous has been small, being confined chiefly to the cutting of canonsâ€"most notably the Grand and Marble Canons, which are sunk wholly in that series, and in several places have been cut through the entire Palaeozoic series system. The average denudation of the Plateau Province since the closing periods of the local Eocene can be approximately estimated, and cannot fall much below 6,000 feet,* and may, nay, probably does, slightly exceed that amount. Of course this amount varies enormously, being in some localities practically nothing and in others nearly or quite 12,000 feet. It is a minimum in the High Plateaus Within that district the average denudation will fall much below 1,000 feet in the sedimentary beds. The enormous floods of volcanic emanations have protected them, and these have borne the brunt of erosion, and their degradation has given rise to local accumulations of sub-aerial conglomerates in all the valleys and plains surrounding the volcanic areas, thus increasing the protection. The general cause which has enabled these strata to survive within the limits of the High Plateaus while they have been so terribly wasted elsewhere may be stated briefly. Until near the close of the Pliocene the High Plateaus were not only the theatre of an extended vulcanism, but those portions which never were sheeted over by lavas were low-lying areas, where alluvial strata tended to accumulate. They remained, in fact, base levels of erosion during the greater part of Tertiary time. Turning now to the Great Basin, which lies even lower than the mean level of the Plateau Country, we find that the pre-eminence of the High Plateaus is due to a totally different cause. Here the difference of altitude is due altogether to differences in the amounts of uplifting. Since the * My own estimate exceeds by a few hundred feet that of Professor Powell and also that of Mr. Gilbert. The latter places it at about 5,500 feet. |