Correlation with South America is exceedingly difficult and it is in dealing with this problem that the widest differences of opinion have arisen among geologists. Through nearly all the earlier half of the Tertiary period the two Americas were separated and, because of this separation, their land mammals were utterly different. Hence, the lack of elements common to both continents puts great obstacles in the way of establishing definite time-relations between their geological divisions. Only the marine mammals, whales and dolphins, were so far alike as to offer some satisfactory basis of comparison. When, in the later Tertiary, a land-connection was established between the two continents, migrations of mammals from each to the other began, and thenceforward there were always certain elements common to both, as there are to-day. In spite of the continuous land between them, the present faunas of North and South America are very strikingly different, South America being, with the exception of Australia, zoölogically the most peculiar region of the earth. In the following table of the South American Cenozoic, the assignment of the ages to their epochs is largely tentative, especially as regards the more ancient divisions, and represents the views generally held by the geologists of Europe and the United States; those of South America, on the contrary, give an earlier date to the ages and stages and refer the older ones to the Cretaceous instead of the Tertiary. CENOZOIC ERA (South America) Quaternary period { Recent epoch { Pleistocene epoch—Pampean Beds, Brazilian caverns { { Monte Hermoso age { Pliocene epoch { Catamarca age { { Paraná age { Tertiary period { Miocene epoch { Santa Cruz age { { Patagonian age { { Oligocene epoch { Deseado age (_Pyrotherium_ Beds) { { Astraponotus Beds { { Eocene epoch { Casa Mayor age (_Notostylops_ Beds) The Pleistocene and Pliocene deposits are most widely distributed over the Pampas of Argentina, but the former occur also in Ecuador, Brazil, Chili, and Bolivia. The other formations cover extensive areas in Patagonia, and some extend into Tierra del Fuego.