After some oscillations of retreat and advance, the sea withdrew from Patagonia and the terrestrial accumulations of the _Santa Cruz_ stage were formed. These beds are partly composed of river-deposits, but chiefly of more or less consolidated volcanic ash or tuff, and have yielded a surprising number of beautifully preserved mammals. No other assemblage of South American Tertiary Mammalia is so well known and understood as the Santa Cruz fauna, and the very large number of all but complete skeletons which have been found strongly suggests that many of the animals were buried alive in the showers of volcanic ash. The Santa Cruz fauna is completely and radically different from any of the North American assemblages, and at that time no immigrant from the north had penetrated so far as Patagonia. In the upper Miocene the Andes stood at a much lower level than they do now; fossil plants, some of them collected at a great height in the mountains, are the remains of a luxuriant and purely tropical flora nearly identical with the vegetation of the modern forests of Bolivia and Brazil. Such a vegetation could not exist at the altitudes where the fossils occur and these demonstrate a great elevation of the mountains since those leaves were embedded. The same mild climatic conditions which prevailed in the northern hemisphere during the Miocene must also have characterized Patagonia, subtropical shells extending far to the south of their present range. Whatever may have been true of the land-bridge connecting South America with Africa during the early Tertiary epochs, it must have been submerged in the Miocene, otherwise there would not have been the open pathway for the Cetacea of Patagonia to reach the Atlantic coast of North America and _vice versa_. 5. _Pliocene Epoch_