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Show RELATION OF FAULTS TO ANCIENT SHORE LINES. 45 time and possibly are Pre-Tertiary. Mr. Gilbert has studied the Water-Pocket flexure, and believes that its epoch belongs to the interval which separates Tertiary from Cretaceous time. The Echo Cliff flexure is probably much younger. The San Rafael flexure remains to be studied. None of them appear as yet to have any sympathy with the Pliocene-Quaternary faults of the High Plateaus. It yet remains to speak of another interesting relation of the later system of faults. They have throughout preserved a remarkable^and persistent parallelism to the old shore line of the Eocene lake, following the broader features of its trend in a striking manner. The cause of this relation is to me quite inexplicable, so much so, that I am utterly at a loss to think of any subsidiary facts which may be mentioned in connection with it and which can throw light upon it. It seems best, therefore, to allow the main fact to stand by itself, and not to confuse it with any others with which it has no certain relation. The faulting and flexing has been associated with a general increase in the altitude not only of the district of the High Plateaus, but of the country south and east of them. The uplifting has by no means been confined to the few tabular masses. Wherever we look in the western part of the Plateau Province the signs of this elevation are unmistakable. In some localities it was much greater than in others, but the signs of it are common to all. It is betrayed in the drainage channels. At a comparatively recent epoch there has been a sudden renewal of activity on the part of the streams, by which they have taken to canon-cutting with renewed energy as if their slopes had been increased, and this is especially observable in the Colorado itself, where the effect has been a maximum. The tributaries have responded and have acted in like manner. Just prior to the advent of this regional uplifting, the aspect of the region appears to have been that which would naturally have resulted from a long period of stability at the same altitude. The canons and intervales were wide and long stretches of the rivers were at or near their base-levels, havino-eroded as deeply as possible, then slowly widened their valleys and made flood-plains. All at once a new era of canon-cutting set in, and profound narrow chasms were sawed in the strata and are to-day sinking deeper. |