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Show EFFECTS OF GEE AT DENUDATIONS. 21 the earth, behaving as a quasi-plastic body, have reasserted its equilibrium of figure by making good a great part of the loss by drawing upon its whole mass beneath f Few geologists question that great masses of sedimentary deposits displace the earth beneath them and subside. Surely the inverse aspect of the problem is a priori equally palpable. That some such process as this has operated in the Plateau Country looks at least plausible, and if there could be found independent reasons for believing in its adequacy the facts certainly bear it out. Yet its application is not without some difficulties, and the explanation is not quite complete. Granting the principle, it will still be difficult to explain how these local uplifts were inaugurated, and we can only refer them to the operations of that mysterious plutonic force which seems to have been always at work, and the operations of which constitute the darkest and most momentous problem of dynamical geology. On the whole, it seems to me that we are almost driven to appeal to this mysterious agency to at least inaugurate and in part to perpetuate the upward movement, but that we must also recognize the co-operation of that tendency which indubitably exists within the earth to maintain the statical equilibrium of its levels The only question is whether that tendency is merely potential or becomes in part kinetic, and this again turns upon the rigidity of the earth. But it is easy to believe that where the masses involved are so vast as those which have been stripped from the Kaibabs and from the San Rafael Swell, the rigidity of the earth may become a vanishing quantity. The great erosion of the Plateau Province was most probably accomplished mainly in Miocene time, but continued with diminishing rapidity throughout the Pliocene. But it is necessary to say that the terms Miocene and Pliocene have here no definition. They cannot be correlated except in a very general manner with events occurring outside the province. We have only a vast stretch of time, with an initial epoch near the close of the local Eocene. The greater part of the denudation is assigned to the Miocene, because the conditions appear to have been more favorable to a rapid rate of destruction in that age than subsequently. The climate appears to have been humid, while the elevation was at the same time gradually increasing, both conditions being favorable to a rapid disintegration and removal |
