[Illustration: FIG. 205.—Former shore lines on the mountain wall surrounding the desert of the Great Basin. View from the temple in Salt Lake City (after Gilbert).] From such records it is learned that the Great Basin of the western United States was at one time occupied by two great desert lakes, the one in the eastern portion being known as Lake Bonneville (Fig. 206). With the desiccation which followed upon the series of pluvial periods, which in other latitudes resulted in great continental glaciers and has become known as the Glacial Period, this former desert lake dried up to the limits of Great Salt Lake and a few smaller isolated basins. Between 1850 and 1869 the waters of Great Salt Lake were rising, while from 1876 to 1890 their level was falling, though subject to periodic fluctuations, and in recent years the waters of the lake have risen so high as to pass all records since the occupation of the country. As a consequence the so-called Salt Lake “cut-off” of the Union Pacific Railway, constructed at great expense across a shallow portion of the lake, has been overflowed by its waters. The Sawa Lake in the Persian Desert, which disappeared some five hundred years ago, again came into existence in 1888 so as to cover the caravan route to Teheran. [Illustration: FIG. 206.—Map of the former Lake Bonneville (dotted shores), and the boundaries of the Great Salt Lake of 1869 (smaller area) and that of the present (after Berghaus).] The record in the rocks of the distant past reveals the fact that in some former deserts barriers were, in the course of time, broken down, with the result that an invading sea entered through the breached wall. The result was the sudden destruction of land life, the remains of which are preserved in “bone beds”, now covered by true marine deposits. A still later episode of the history was begun when the sea had disappeared and land animals again roamed above the earlier desert. Such an alternation of marine deposits with the remains of land plants and animals in the deposits of the Paris Basin, led the great Cuvier to his belief that geologic history was comprised of a succession of cataclysms in which life was alternately destroyed and re-created in new forms—a view which later, under the powerful influence of Lyell and Darwin, gave way to that of more gradual changes and the evolution of life forms.