PERIDOTITE, a plutonic holo-crystalline rock composed in large part of olivine, and almost or entirely free from felspar. The rocks are the most basic, or least siliceous plutonic rocks, and contain much iron oxide and magnesia. Hence they have dark colours and a high specific gravity (3.o and over). In some peri dotites, such as the dunites, olivine greatly preponderates over all other minerals. It is always in small, rather rounded crystals without good crystalline form, and pale green in colour. Most of the rocks of this group, however, contain other silicates such as augite, hornblende, biotite or rhombic pyroxene, and often two or three of these are present. By the various mineral combina tions different species are produced, e.g., mica-peridotite, horn blende-peridotite, enstatite-peridotite. Of the accessory minerals the commonest are iron oxides and chromite or picotite. In some peridotites these form segregations or irregular masses which are of importance as sources of the ores of chromium. Platinum and the nickel-iron compound awaruite are found in rocks of this class in New Zealand. Red garnet (pyrope) characterizes the peri dotites of Bohemia. The diamond mines of South Africa are situ ated in pipes or volcanic necks occupied by a peridotite breccia which has been called kimberlite. In this rock in addition to dia mond the following minerals are found ; hypersthene, garnet, bio tite, pyroxene (chrome-diopside), ilmenite, zircon, etc.
Some peridotites have a granular structure, e.g., the dunites, all the crystal grains being rounded and of nearly equal size ; a few are porphyritic with large individuals of diallage, augite or hypersthene. Some are banded with parallel bands of dissimilar composition, the result probably of fluxion in a magma which was not quite homogeneous. The great majority of the rocks of this group are poikilitic, that is to say, they contain olivine in small rounded crystals embedded in large irregular masses of pyroxene or hornblende. The structure is not unlike that known as ophitic in the dolerites, and arises from the olivine having first separated out of the liquid magma while the pyroxene or amphibole suc ceeded it and caught up its crystals. In hand specimens of the rocks the smooth and shining cleavage surfaces of hornblende and augite are dotted over with dull blackish green spots of olivine; to this appearance the name "lustre-mottling" has been given. Although many peridotites are known in which the constituent minerals are excellently preserved, the majority have undergone much mineralogical alteration. The olivine is specially unstable and is altered to serpentine while the pyroxene and amphibole are in large measure fresh. In some cases the whole rock is changed to an aggregate of secondary products. Most serpentines (q.v.)
arise in this way. See also PICRITE. (J. S. F.) PERIER, CASIMIR PIERRE French states man, was born at Grenoble on Oct. 11, 1777, the son of a rich banker and manufacturer, Claude Perier (1742-1801), who was one of the first directors of the Bank of France ; of his eight sons, Augustin (1773-1833), Antoine Scipion (1776-1821), Casimir Pierre and Camille (1781-1844), all distinguished themselves in industry and in politics. Casimir joined the army of Italy in 1798. On his father's death he left the army and with his brother Scipion founded a bank in Paris. He opposed the ruinous methods by which the duc de Richelieu sought to raise the war indemnity demanded by the Allies, in a pamphlet Re flexions sur le pro jet d'emprunt (1817), followed in the same year by Dernieres reflex ions . . . in answer to an inspired article in the Moniteur. In the same year he entered the chamber of deputies for Paris, tak ing his seat in the Left Centre and making his first speech in defence of the freedom of the press. Re-elected for Paris in 1822 and 1824, and in 1827 for Paris and for Troyes, he sat for Troyes until his death. Under Louis Philippe Perier became president of the chamber of deputies, and sat for a few months in the cabi net without a portfolio.
On the fall of the ministry of Laffitte, Casimir Perier, who had drifted more and more to the Right, was summoned to power (March 13, 1831), and in a year he restored civic order in France and re-established her credit in Europe. Paris was only held in check by the premier's determination; revolts at Lyons and Grenoble were also put down. The minister refused to be dragged into armed intervention in favour of the revolutionary Govern ment of Warsaw, but he constituted France the protector of Belgium by the prompt expedition of the army of the north against the Dutch in Aug. 1831; French influence in Italy was asserted by the audacious occupation of Ancona (Feb. 23, 1832) ; and the refusal of compensation for injuries to French residents by the Portuguese Government was followed by a naval demon stration at Lisbon. In the spring of 1832 during the cholera out break in Paris, Perier visited the hospitals with the duke of Or leans. He fell ill the next day and died on May 16, 1832.
His Opinions et discours were edited by A. Lesieur (2 vols., 1838) ; C. Nicoullaud published in 1894 the first part (Casimir-Perier, depute de l'opposition, 1817-183o) of a study of his life and policy ; and his ministry is exhaustively treated by Thureau-Dangin in vols. i. and ii. (1884) of his Histoire de la monarchie de :millet.