LIMBURGITE  , in

petrology, a dark-coloured volcanic rock resembling basalt in appearance, but containing normally no felspar . The name is taken from Limburg (Germany), where they occur in the well-known rock of the Kaiserstuhi . They consist essentially of olivine and augite with a brownish glassy ground mass . The augite may be green, but more commonly is brown or violet; the olivine is usually pale green or colourless, but is sometimes yellow (hyalosiderite) . In the ground mass a second generation of small eumorphic augites frequently occurs; more rarely olivine is present also as an ingredient of the matrix . The principal accessory minerals are titaniferous iron oxides and apatite . Felspar though sometimes present is never abundant, and nepheline also is unusual . In some limburgites large phenocysts of dark brown hornblende and biotite are found, mostly with irregular borders blackened by resorption; in others there are large crystals of soda orthoclase or anorthoclase . Hauyne is an ingredient of some of the limburgites of the Cape Verde Islands.• Rocks of this group occur in considerable numbers in Germany (Rhine district) and in Bohemia, also in Scotland, Auvergne, Spain, Africa (Kilimanjaro), Brazil, &c . They are associated principally with basalts, nepheline and leucite basalts and monchiquites . From the last-named rocks the limburgites are not easily separated as the two classes bear a very close resemblance in structure and in mineral composition, though many authorities believe that the ground mass of the monchiquites is not a glass but crystalline analcite . Limburgites may occur as flows, as sills or dykes, and are sometimes highly vesicular .

Closely allied to them are the augitites, which are distinguished only by the

absence of olivine; examples are known from Bohemia, Auvergne, the Canary Islands, Ireland, &c .