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Show 158 GEOLOGY OF THE HIGH PLATEAUS. of land from the water, or whether it merely indicates a local exposure to denudation, it is not possible at present to say. TERTIARY LACUSTRINE FORMATIONS The history of the Plateau Country which is at present best known is the history of its Tertiary formations. This remains to be written; but materials for it have been widely, collated, and are in the possession of Professor Powell, who will, it is believed, discuss the subject at an early day. A more promising and instructive one probably is not to be found in the entire range of North American geology. Nothing more is needed here than a mere summary, which may serve as a guide and index to the meaning of the terms employed in this monograph. The Tertiary system of the Plateau Country is lacustrine throughout, with the exception of a few layers near the base of the series, which have yielded estuarine fossils. The widely varying strata were accumulated upon the bottom of a lake of vast dimensions, and were derived from the waste of mainlands and mountain platforms, some of which are still discernible. The region of maximum deposit was in the vicinity of the Wasatch and Uintas, where in the course of Eocene time more than 8,000 feet of beds were laid down. As we proceed southward, these heavy deposits attenuate, partly by a diminution in the thickness of the individual members and partly because the period of deposition ceased earlier the farther southward we go, until in the southern part of the province only the lower Eocene is found, or, indeed, was ever deposited. The High Plateaus occupy the belt through which this diminishing bulk and successive elimination of upper members is well seen. In the Wasatch Plateau, at the extreme northern part of the district, wetfind the two lower divisions of the Eocene present in great volume; and in the valley of the Sevier and San Pete we find what is undoubtedly a still higher division. At the southern portion of the district only the lower division can be clearly made out, though some of the upper beds may prove to belong to a later period. The present weight of evidence, however, seems to me to place them in one division, the "Bitter Creek" of Powell. In the southern plateaus, the Markagunt and Paunsagunt, we find |