THERALITE, in petrology, a group of plutonic rocks built up of basic plagioclase (labradorite), nepheline, and a titaniferous augite. The name is derived from Onpav, to pursue, as its dis covery was looked forward to as completing the series of basic rocks containing nepheline as an essential constituent.
Olivine, an alkali-amphibole, biotite and orthoclase may be present as subordinate constituents. With the exception of nephe line and orthoclase, the minerals of theralites are usually in well shaped crystals. Nepheline itself may be largely represented by secondary zeolites. Theralites are of comparatively rare occur rence. They are known from the Bohemian alkali province, as at Duppau, in association with shonkinite, at the Katzenbuckel (South-east Tirol), together with pulaskite and foyaite in the Serra de Monchique (Portugal), at Umptek (on the White sea), and among the Carboniferous intrusions of Ayrshire (Scotland).
Closely related to the theralites are the teschenites (from Teschen, Moravia). In place of nepheline these rocks contain primary analcime, but types containing both nepheline and anal cime are known. In central Scotland, around Edinburgh and Glas gow, teschenites are abundant, forming thick sills intrusive into the Carboniferous rocks. Teschenites are sometimes ophitic, and show transitions to olivine-dolerites on the one hand and to picrites on the other. The rock known as (from Lugar, Ayrshire) is a related type containing small amounts of plagioclase but abundant analcime ; nepheline is present in subordinate amount.
Other rocks related to the theralites are the essexites and shonkinites. The former are characterized by dominant plagio clase, subordinate orthoclase, and green augite, hornblende, biotite and olivine. Nepheline also occurs by no means uncommonly. By increase in the proportion of nepheline the essexites pass into theralites. Essexites occur, together with nepheline syenite, in Essex county, Mass., at Mount Royal near Montreal, in southern Norway (Oslo district), at Rongstock, Bohemia, and among the carboniferous teschenites near Edinburgh and in the Campsie hills, Stirlingshire. The shonkinites are melanocratic rocks of
much rarer occurrence. Augite and orthoclase are the prime con stituents, but plagioclase, barkevikite, olivine, biotite and variable amounts of nepheline are present. At Shonkin Sag, in the High wood mountains of Montana, shonkinite forms the greater part of a stratified laccolith passing at the border into a peculiar basic rock described as a leucite-basalt porphyry.
Shonkinites are also known from Ontario, British Columbia, and the East Indies (Celebes, Timor). (C. E. T.) THERAMENES (d. 403 B.c.), Athenian statesman, was the adopted son of Hagnon, a prominent conservative who in 43o impeached Pericles, and after the Sicilian expedition became one of the ten probuli (rp6i3ovXoc, commissioners) appointed to devise economies in the administration. Theramenes formulated a new party-cry, "the constitution of our fathers." It was no doubt largely due to his advocacy that the probuli, strengthened by further members, were commissioned to draft new measures on behalf of the public safety and to examine Cleisthenes' "ancestral code." (See BOULE and GREECE: History.) Late in 405 Theramenes went as plenipotentiary to Lysander (q.v.) to obtain peace terms (see PELOPONNESIAN WAR) ; after long negotiations he proceeded to Sparta and arranged a settle ment which the Athenians ratified (April 404). In spite of this peace the disorder in Athens did not abate. At the instance of Critias, Theramenes was arraigned for treason and, although he successfully repelled the denunciation, he was led away by vio lence and forced to take poison.