1911 Encyclopædia Britannica/Tertiary

TERTIARY, in geology, the time-division which includes the Eocene, Oligocene, Miocene and Pliocene periods, in other words, it is the earlier portion of the Cainozoic era. By some authorities the term Tertiary is made to embrace in addition to the foregoing periods those of the Quaternary (Pleistocene and Holocene), i.e. "Tertiary" is made the equivalent of Cainozoic. On logical grounds there is much in favour of this interpretation; but having in view the state of geological literature, it is certainly better to restrict the use of the term in the manner indicated above. Tertiary rocks were among the latest to receive the careful attention of geologists, and the name was introduced by G. Cuvier and H. Brongniart in 1810 (Essai sur la géographie minéralogique des environs de Paris, 1810-11, 1st ed.).

Deshayes (1830) worked out the percentages of recent fossils found at several horizons in those strata, and upon this Sir C. Lyell (1852) founded the main periods, viz. the Eocene with 31/2 per cent. of recent forms, Miocene 17 per cent., Pliocene 35 to 50 per cent. Subsequent investigations naturally modified the numerical values upon which this nomenclature was based, but without altering the order of the periods. Later, E. Beyrich introduced the Oligocene period, and some geologists recognize a Palaeocene or early Eocene period. European geologists very generally use the grouping adopted by R. Hörnes:—

Younger Tertiary = Neogene (Miocene, Pliocene).

Older Tertiary = Palaeogene (Palaeocene, Eocene, Oligocene).

The great number and variety of mammalian remains has made it possible for the Tertiary rocks to be classified by their means: see A. Gaundry, Les enchainements du monde animal—mammifères Tertiaires (1878); W. B. Dawkins, Q. J. Geol. Soc. Lond. (1880); Forsyth Major, Geol. Mag. (London, 1899); and H. F. Osborn, J. L. Wortman, G. F. Matthew, for western North America, Bull. Am. Mus. Nat. Hist., xii. (1899).

During the Tertiary era the geographical configuration of the globe was steadily approaching that of the present day; but in the earlier part of the time there still existed the great equatorial ocean "Tethys," and there is evidence that East India and Africa, Australia and Asia, north Europe and North America were probably severally united by land connexions. As the period advanced, along the very line that had been occupied by the nummulitic sea (Tethys) the crust began to be folded up, giving rise to the Alps, Carpathians, Caucasus, Himalayas and other mountains, some of the early Tertiary marine formations being now found raised more than 16,000 ft. above the present level of the sea. Associated with these crustal movements were enormous outpourings of volcanic materials.

The faunal aspect of the Tertiary periods differs strikingly from that of preceding Secondary or Mesozoic; in place of the great saurian reptiles we find the rapid development and finally the maximum expansion of mammals. Snakes and true birds advanced rapidly towards their modern position. In the seas, bony fish and crab-like decapods increased in numbers and variety, while pelecypods and gastropods took the prominent place previously occupied by ammonites and belemnites, and, leaving behind such forms as Rudistes, Inoceramus, &c., they gradually developed in the direction of the modern regional groups. In the plant world, the dicotyledonous angiosperms gradually assumed the leading rôle which they occupy to-day.

The climate in northern latitudes seems to have passed from temperate to sub-tropical, with minor fluctuations, until at the close a rapid lowering of temperature ushered in the glacial period. (J. A. H.)