The marine Oligocene is better developed and better understood on the Gulf coast than elsewhere, and therefore forms the standard of comparison. "It was a period of profuse invertebrate life and steady sedimentation, especially of oceanic deposits in water of not always great depth. Some 2000 feet of strata, formed almost wholly of organic debris, were deposited in the peninsular region of Florida." (Dall.) In the Gulf region there is no decided stratigraphic break between the Eocene and Oligocene, but a change in the marine fauna. The Oligocene is but scantily shown on the Atlantic coast; some beds in the Carolinas are referred to it and traces of it occur in New Jersey, but it is generally concealed beneath the overlying Miocene. In the western Gulf region certain fluviatile beds are placed in this epoch, but upon insufficient reasons. Oligocene limestones are found in the Greater Antilles and very extensively in Central America, which seems to have been nearly or quite submerged. At the end of the lower division of the series there was some disturbance, raising northern Florida into an island, and shoaling the water where deposition continued.
The marine fauna of the Oligocene is an assemblage of warm-water animds, very much like those which now live on the coasts of the West Indies and Central America, some of the West Indian forms extending as far north as New Jersey.
On the Pacific coast the Oligocene is found in western Oregon and British Columbia and very extensively in Alaska, where the Kenai formation, 10,000 feet thick and containing beds of lignite, is exposed along the coast and at many places in the interior down to British Columbia. The overlying Miocene follows in apparent conformity.
In the interior, Oligocene formations are among the most important of all the continental Tertiaries. The lower division, the Uinta, is found in a relatively small area of northeastern Utah and northwestern Colorado, where it lies unconformably upon the Bridger, overlapping the latter upon the southern flanks of the Uinta Mountains. The Uinta, which is the last of the Tertiary horizons in the plateau region, is usually regarded as uppermost Eocene, but its fauna allies it more closely with the Oligocene. Its mammals show that the isolation from Europe, which had begun after the Wasatch, still continued.
The Middle Oligocene, or White River, covers a vast area, northeastern Colorado, western Nebraska, eastern Wyoming, and southwestern South Dakota, with outliers in the Black Hills and North Dakota, and a separate area in the Northwest Territory of Canada. The mode of formation of the White River beds has long been a subject of discussion; originally they were considered to be lacustrine, a view which is supported by their very regular stratification, but it is now very generally believed that they are chiefly fluviatile, and several of the old stream-channels, filled with cross-bedded sandstone and banks of conglomerate, have been observed (see Fig. 36, p. no). The fine clays which make up most of the beds are chiefly flood-plain deposits, but there are also beds of pure white volcanic ash, showing that the volcanic activity which was so marked in the Eocene still continued. A system of streams meandering over a nearly base-levelled plain, with very low divides between them, would in times of flood unite into a vast, but shallow and temporary lake, and such would appear to have been the conditions under which the White River beds were laid down.
The mammals of the White River prove that a Way of intermigra-tion for terrestrial animals had again been established with Europe. While many families did not join in this migration and each continent had several groups peculiar to itself, the number of identical and closely allied genera common to both, and the appearance in America of types which Europe had had in earlier times, is sufficient proof of the renewed connection. The contrast between the Uinta and White River in this respect is very marked.
The Upper part of the interior Oligocene is the John Day, which covers a large part of eastern Oregon, and a small area in central Montana. The Oregon beds are a very thick mass (3000-4000 feet) of stratified volcanic ash and tuff, with some fresh-water beds at the top. Evidently, gigantic eruptions were in progress and the vents were at no great distance, though too far away for the formation of a coarse agglomerate.