TERTIARY SYSTEM. A term applied in geology to the group of rocks included between the Cretaceous and Quaternary systems. It is one of the two periods constituting the Ceno zoic era, the Quaternary being the other period. The term Tertiary was first suggested when it was considered that all strata were divisible into three groups, primary, secondary. and ter tiary. The first two have been replaced in most localities by other names, hut the third is still used. although not with its original significance. The Tertiary is one of the most interesting periods of geological history, presenting most complete sections and a great abundance of or ganic remains. and yet there has been much dif ficulty in classifying its different members cor rectly. The rocks are mostly unconsolidated and have rarely been laid down over great areas, so that in the United States. Alone, there are no less than three o• four separate regions of Ter tiary rooks. in each of which a different series of subdivisions has to be adopted. The follow ing classification of the American Tertiary is that compiled by NV. II. Dall: These main epochs and stages are recognized in all the Tertiary areas of the United States, of which there are four, viz. the Pacific Coast. Western interior. Atlantic Coast, and Gulf States. The section in the last-mentioned area is taken as the type.
At the end of the Cretaceous period a great topographic revolution took place in many parts of the world, and at that time the American continent had practically received its present form. The marine Tertiary deposits are found, therefore, cliiell\• along the borders of the con tinent, while tile interior :n•eas are of fresh water formation, or perhaps in part :eolian de posits. .Nlany of the highest moutitain ranges of the world, such as the Alps. Atlas, Caucasus, and Himalayas, were uplifted in the Tertiary period, their height being due to the fact that they are young geologically and consequently have not suffered greatly from erosive agencies.
The life of the Tertiary period presents many similarities to that of the present, although modern types had already begun to appear to some extent in the Cretaceous. In early Ter tiary time the climate was very mild over the entire globe, and there was all abmidant plant growth far to the north, plants being found in the rocks in many parts of Greenland. A grad ual cooling of the climate followed, until at the end of the Tertiary there began the formation of the great continental glaciers that subse quently spread over so much of the temperate zone. The faunal changes that occurred during this period were in many respects remarkahle. Of the smaller animals—the ammonites, belem nites, and other molluscan types that swarmed in the Cretaceous—few lived in the Tertiary. The great reptiles had also disappeared, but their place was taken by still more gigantic mammals. The fishes, amphibians, and birds closely resembled modern types.
The Tertiary deposits inclose a variety of economic minerals. In the Cordilleran region many of the metalliferous veins are probably of Tertiary age, as are also some of the bituminous coals and lignites, in this :name area, notably in Washington and Oregon. In the Tertiary beds of the Atlantic and Gulf States are many de posits of brick. pottery. and fire-clays, while much of the green sand obtained in the Atlantic coastal plain region is of Tertiary age. In Texas deposits of limonite are known in this formation, while in Florida and South Carolina there are great supplies of rock phosphate. Petroleum and mineral tar are obtained from the Tertiary deposits of southern California.
HinuoGn.tenY. Dail, A Table of North Ameri can Tertiary llori:ons, Eighteenth Annual lie port 'United States Geological Survey, part ii., page 323 : Dana, Manual of Grology; Geikie, Text Book of Geology. Other references will be found under the titles referred to below. Sce GEOL OGY: PALEONTOLOGY; EOCENE EPOCH; MIOCENE EPOCH; PLIOCENE EPOCH.