TERTIARY PERIOD, the space of time, geologically considered, immediately precediz the present, and occupying the earlier larger part of the Cenozoic era; also the rock system then formed. It is preceded by the Cretaceous. Tertiary strata were at first con• founded with the superficial alluviums of Europe and it was long'before their real char acters were realized. They occur most gen erally in patches,--- some of them of marine origin, others of fresh-water or of continental derivation. Rocks of this age were first de scribed by Cuvier and Bronginart in 1810 from the Paris Basin, where they are well developed and highly fossiliferous: The shells found in these deposits were recognized as different from those of the modern time, though related to them, while the bones of quadrupeds were found to be of extinct species. Similar strata from many other parts of Europe were subsequently found Those of Italy were found in low hills flanking the Apennines on both sides from the plains of the Po to Calabria and called by Basterot, who studied them, the Subapennines. The fossils of these beds were of a more modern type than those of Paris or London. In the neighborhood of Bordeaux, in the south of France, another series of Tertiary strata were discovered and described by M. de Basterot in 1825. The several hundred species of shells described from these beds were found to differ mostly from those of the Paris Basin and those of the Subapennines, and to possess an inter mediate character between the two. Subse quently it was found that strata contemporane ous with those of Bordeaux overlie the Parisian formation in the valley of the Loire, and under lie the Subapennine beds in Piedmont.
. In 1828 and 1829 Lye11 conceived the idea that the Tertiary beds might be subdivided ac cording to the percentage of living species in each. For this purpose, he and M. Deshayes, a well-known French conchologist, compared some 3,000 Tertiary with about 5,000 living species. The result arrived at was, that in the lower strata, or those of London and Paris, there were about 3%2 per cent of recent species, in the middle Tertiary of the Loire, Bordeaux, etc., about 17 per cent of recent species and in the upper Subapennine Tertiary from 35 to 50 per cent of living species. These results were published in 1833. In formations still more modern, which Lye11 studied in Sicily, where they attain a vast thickness, the per centage of living species was found to be 90 or 95. To these four series Lye11 applied the names Eocene, Miocene, Older Pliocene and Newer Pliocene. A still later formation (Post Tertiary) was 'called Pleistocene, in which the shells were all of recent types, but the mam mals partly of extinct species. The.most im portant recent modification of this nomenclature has been the introduction of the term Oligocene by Beyrish to include strata formerly classed partly as Upper Eocene and partly as Lower Miocene. The generally recognized divisions from the base up are now given as Eocene Oligo cene, Miocene and Pliocene. At present much
less stress is laid upon the numerical method of subdivision employed by Lyell and Deshayes. As the various deposits of the typical Tertiary bed.s of Paris, London, the Loire Basin and the Subaper.nine series became well known, a standard of comparison became established, by which similar deposits of other regions could be determined. This is the method employed to-day in deciding to which division a given deposit should belong.
The general characteristics of geography, vegetation and animal life of the Tertiary were similar to those of the present time, but land areas were of less extent and were more largely occupied by interior fresh-water basins. The climate of the early Tertiary was evidently warmer and more moist than that of the period following it, types of plants now strictly tropi cal then covering areas now under Arctic latitudes and influences. In the United States the Atlantic and Gulf costal plain under went repeated submergence and emergence. During the first hal f of the period, the site of the Pacific Coast Ranges was largely under water, but in mid-Tertiary the Coast Ranges were upheaved, and the coast line took nearly its present form. The great western interior had undergone uphealral at the close of the pre ceding period (Cretaceous) in the formation of the Rocky Mountains. During Tertiary. these underwent extensive erosion, building, on the great plains and in the intermontane basins, fairly thick beds of terrestrial gravel, sand and clay. A few lake deposits were also formed. Volcanoes were active during much of the period, from the Rocky Mountains westward, forming thick beds of ash and very extensive lava flows, as on the Columbia Plateau in Oregon and Washington. Marine Tertiary beds are found on the Atlantic and Pacific coa.sts. The most important of these are the strata of the Atlantic coastal plain with its expansion in the Gulf of Mexico. On the Atlantic Coast the Eocene beds are mostly clays and greensands which rest unconformably upon the Cretaceous strata and are unconform ably overlain by the Miocene beds. All of these beds are highly fossiliferous, shells predomi nating. In the Miocene is a great bed of diatomaceous earth (q.v.) from 200 to 300 feet thick. In South Carolina, Pliocene beds make their appearance. No Oligocene strata are known from the Atlantic Coast. On the Gulf Coast the Eocene is well represented and rests upon the Cretaceous. It consists mainly of marls, greensands, clays and sands. Both Oligocene and Miocene are represented on the Gulf Coast. The Oligocene is characterized by a warm-water or subtropical fauna. The Mio cene beds of the Gulf States represent the ad vent of the colder water fauna from the North. Pliocene beds of the age of those farmed in South Carolina are extensively developed in Florida.