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Show % 48 GEOLOGY OF THE HIGH PLATEAUS. late A. R Marvine, shows a series of broad platforms, uplifted with a single monoclinal flexure or a fault on either side. The width of these platforms varies from 20 to 45 miles, and from these masses the individual mountain-piles have been carved by erosion. The restored profiles obtained by replacing the material removed by erosion are not indeed horizontal nor straight lines, but ordinarily convex upwards, with slight curvature, becoming abrupt or even passing into a great fault at the margin of the uplift. Inasmuch as almost any configuration of the strata which is convex upwards, be it never so little, is called an anticlinal, these platforms would probably be so characterized by most geologists. But what a contrast to the short, sharp waves of the Apalachians! If we analyze the form carefully, it will become apparent that we have to do with a structure which has nothing in common with a true anticlinal except this slight convexity, and which possesses characters which the true anticlinal does not. It has already been indicated that faults and monoclinal flexures are homologous terms. They represent varying degrees of abruptness in the passage from the thrown to the lifted side of a displacement. In the case of the fault the shearing is confined to a single plane ; in the case of a monoclinal flexure the shearing is distributed through a narrow zone between two planes. Both mean essentially the same thing. In the Park Mountains we have uplifts with a fault or equivalent monoclinal on one side or on both. Most frequently it is on both sides, but the shearing is almost invariably more strongly emphasized on one side than on the other. It rarely happens that the fault is clean and trenchant, but is accompanied with much fracturing and shattering of the thrown edges of the strata, and there are cases when the dragging of the fault has been accompanied by the overturning of a great slice of strata torn from the thrown edges. Instances are abundant where the rocks in the flanks of these ranges in the vicinity of the faults have been subjected to the most " heroic" treatment; but at short distances from the faults in both directions the disorganization quickly diminishes. Upon the summits of the platforms the traces of violence and distortion attending the upward movement are much less. Where erosion has laid bare the most ancient rocks they are ordinarily found to be more |