| OCR Text |
Show TERTIARY FORMATIONS. 205 or three species of fresh-water mollusca seem to sustain his view, but the fresh-water forms of the Plateau Province so frequently have a very great vertical range, that they are apt to mislead in just such cases, and require collateral evidence to justify such a conclusion. On the other hand, there is no indication in the appearance of the rocks of such a break of the continuity, and the whole of the Tertiary here exposed seems to belong to one series without unconformity and without any break in the conditions necessary to continuous deposition. It has, therefore, seemed to me unadvis-able to intercalate a vacant horizon in a series which to all appearances is continuous. The white limestone at the summit of the formation is a very conspicuous member and forms the surface of the plateau for a considerable distance south of Pan quitch Lake, where it is laid open by ravines and exposed in buttes capped by basalt. It reaches a thickness of rather more than 300 feet in some places, but is usually much less. It is very impure; sometimes very siliceous, holding agate or chalcedony, and is also sometimes marly. The total thickness of the Eocene beds is from 1,100 to 1,200 feet. The epoch of final emergence from the lacustrine condition seems to have been earlier here in the southwestern part of the Plateau Province than in the middle or northern portions. This is indicated by the earlier age of the most recent lacustrine beds; for as we proceed northward later and later members gradually make their appearance. In the south, not more than the lower third of the Eocene is present; in the middle district, barely more than one-half; while around the southern slopes of the Uintas nearly or perhaps quite the whole of it is revealed. It may be conjectured that the Lower Green River beds once existed here and were eroded and wholly removed before the volcanic eruptions began. This cannot be wholly disproven, but the view is extremely improbable; for in the epoch immediately following the final emergence the conditions were not favorable to a rapid erosion; the region was not at that time an elevated one; it could scarcely have exceeded a few hundred feet in altitude above sea level, and there were no important displacements nor dislocations. The Bitter Creek beds cover many hundred square miles of continuous |
