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Show 40 GEOLOGY OF THE HIGH PLATEAUS. streams sink their channels and every additional yard of deposit will be accumulated at a slower rate. It was the uplifting along great lines of dislocation which set this cone-building process going. The abrupt descents gave the creeks and brooks their power to transport this coarse debris, and those slopes are now long and steep. But as the work proceeds the mountains and tables are gradually rounded and smoothed down and the valley plains built up. As yet comparatively little has been accomplished in this direction, but the work is under full headway. In comparing what has been effected since the beginning of the displacements with work of the same character which has been accomplished in ages prior to the displacements, we shall be most forcibly impressed with the littleness of the one and the greatness of the other. It is a comparison of hundreds with thousands. More than that: the hundreds of feet of modern valley cones represent the utmost activity of a process which has worked without interruption and under conditions the most favorable, while the thousands of feet of ancient accumulations represent the same process in all degrees of activity, now intense, now fading and dying out, and then probably long intervals of cessation. Thus* whether we view the denudation of the High Plateaus or the accumulations in the valleys at their bases, we reach the same conclusions. The faults are very late occurrences in the history of the district. But when we come to ask what is the age, in terms of the geological chronology, to which they must be referred, we can give no further answer than this: they belong to a very late one. There is no record of Miocene or Pliocene in this disturbed region, and we have nothing to mark the lapse of time, except relatively, since the close of the Eocene. But in other parts of the world, where we have some knowledge of the strata, we infer that the Miocene was a longer age than the Pliocene and the Pliocene longer than the Quaternary, though these are impressions rather than conclusions, and to be held lightly. Judging, however, by the magnitude of results accomplished by erosion in the High Plateaus since the faults were started, and comparing these results with similar work accomplished in other localities, and taking into the account the conditions under which they were accomplished, it seems perfectly safe to say that if we carry back the faulting to the mid- |