THERALITE (Gk. Orjpav, to pursue), in petrology, a group of plutonic holocrystalline rocks consisting of nepheline, basic plagioclase, augite and olivine, and so called because it is of rare occurrence, and its discovery was looked forward to with interest as completing the series of basic rocks containing nepheline as an essential constituent. The felspars are mostly of basic character and are often zonal; the nepheline is of later crystallization, rarely idiomorphic and often decomposed. Pyroxene in these rocks may be of green colour or purplishbrown and rich in titanium; olivine is usually abundant. Among the accessories may be mentioned apatite and iron oxides, biotite and dark brown hornblende, the latter often surrounding the purple augite. The rocks have rarely ophitic structure, but their minerals tend to have good crystalline form, except in the case of nepheline and orthoclase (if that be present). By decomposition the nepheline yields zeolites such as natrolite and analcite. The theralites are rarely crossgrained and have much resemblance to dolerites in hand specimens. Among localities for these rocks are Duppau in central Bohemia, Pridazzo (W. Alps), Umptek (on the White Sea), Madagascar and the Crazy Mountains in Montana. A variety of theralite occurs also at Montreal in Canada, and rocks from Crawford John in Lanarkshire and from Paisley in Renfrewshire have recently been ascribed to this group.
Very close to the theralites is a series of rock types known as the teschenites (from Teschen in Moravia). Instead of nepheline these rocks usually contain analcite, and from their microscopic characters it is by no means likely that the analcite is secondary after nepheline in this case; it appears, in fact, to be either primary or of pneumatolytic origin. Nepheline, however, has been found in teschenites from Portugal and from Moravia, so that the distinction between the two series practically vanishes. In central Scotland, around Edinburgh and Glasgow, teschenites are abundant, forming thick sills intrusive into the Carboniferous rocks, and some are also known from Leicestershire (Whit wick) and from Arran. These teschenites are sometimes ophitic and present transitions to olivine-diabases on the one hand and to picrites on the other. They are the deep-seated representatives of the basaltic lavas which were emitted in great numbers in the early part of the Carboniferous period. Other localities for teschenite are the Caucasus and the coast of California (Cuyama Valley, etc.).
The essexites are an allied series containing a larger amount of alkali felspar. Nepheline also occurs by no means uncommonly; the augite is sometimes green, but in other specimens is of a rich purple colour with well-marked zonal structure. Qlivine is by no means uncommon, and brown hornblende and biotite occur rather frequently. The type rock is from Essex (Massachusetts) and other examples have been described from Rongstock on the Elbe, from Mount Royal (Montreal), from S. Norway, near Christiania, and from St Vincent in the Cape Verde Islands. A few essexites have been found in Britain, accompanying the Carboniferous teschenites near Edinburgh and in the Campsie Hills of Stirlingshire. As they contain both orthoclase and plagioclase felspar they have a certain affinity to the olivine-monzonites and kentallenites.
The shonkinites are dark grey rocks consisting of olivine, green augite, dark brown biotite, nepheline and orthoclase, which are found at Shonkin Sag in the Highwood Mountains of Montana. They are basic variations of sodalite-syenite and have some resemblance to theralites, especially in the association of nepheline with large amounts of augite and olivine. They are of exceedingly rare occurrence. (J. S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)