OLIGOCLASE, a rock-forming mineral belonging to the plagioclase (q.v.) division of the felspars. In chemical composition and in its crystallographical and physical characters it is intermediate between albite (NaAlSiaOg) and anorthite (CaAljSioOs), being an isomorphous mixture of three to six ; molecules of the former with one of the latter. It is thus a soda- ; lime felspar crystallizing in the anorthic system. Varieties \ intermediate between oligoclase and albite are known as o Mgoclase-albite. The name ohgoclase was given by A. Breithaupt in 1826 from the Gr. 6X1705, Httle, and K\i.v, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct species by J. J. ; Berzehus in 1824, and was named by him soda-spodumene ( (Natron -spodumen), because of its resemblance in appearance, to spodumene. The hardness is 6| and the sp. gr. 2-65-2-67. In colour it is usually whitish, with shades of grey, green or red. Perfectly colourless and transparent glassy material found at BakersviUe in North Carolina has occasionally been faceted as a gem-stone. Another variety more frequently used as a gemstone is the aventurine-felspar or " sun-stone " (q.v.) found as reddish cleavage masses in gneiss at Tvedestrand in southern Oligocene System 8.
North German Region.
Alps and S. Europe.
Sands and sandstones of Ormoy, rontainebieau and Pierrefltte.
Sands of MoriKny, Falun of Jeurre, Oyster marls. Molasse of Etrechy.
Lower sands of Bolderberg.
Sands of Bergh with Clay of Boom.
Septarian Clay, or Rupelton.
Cyrena marls of Mainz.
Lignites of Haring.
Gypsiferous limestone of Aix, and Lower marine Molasse of Basel.
Bembridge Beds. Osborne Beds.
Limestone of Brie, marine beds of Sannois, ''Glaises vertes." and Cyrene marls.
Supragj-pseous marls, limestones of Champigny, '"First" and "Second" masses of gypsum.
Sands of \'ieu-x-Jones.
Clays of Henis.
Sands of Grimmertingen.
Sands of Wemrael.
Clays of Egeln and Latdorf.
Amber-bearing Glauconitic sands of Samland.
Lignites of Celas (Languedoc).
Lignites of Brunstatt.
Marls of Priabona, limestones of Crosara.
The land flora of this period was a rich one consisting largely of evergreens with characters akin to those of tropical India and Australia and subtropical America. Sequoias, sabal palms, ferns, cinnamon-trees, gum-trees, oaks, figs, laurels and willows were common. Chara is a common fossil in the fresh-water beds. The most interesting feature of the land fauna was undoubtedly the astonishing variety of mammalians, especially the long series from the White river beds and others in the interior of North America. Pachyderms were very numerous. Many of the mammals were of mixed types, Hyaenodon (between marsupials and placentals), Adapis (between pachyderms and lemurs), and many were clearly the forerunners of living genera. Rhinocerids were represented in the upper Oligocene by the hornless Aceratherium; Palaeomastodon and Arsinoitheriuvi, from Eg\'pt are early proboscidian forms which may have lived in this period; Anchitherium, Anchippus, etc., were forerunners of the horse. Palaeotherium, Anthracotherium, Palaeogale, Sleneofiber, Cytwdiclis, Dinictis, Ictops, Palaeolagus, Sciurus, Colodoii, Hyopotamus, Oreodon, Poehrotheriiim, Protoceras, Hypertragulns and the gigantic Titanotherids (Titanotherhim, Bronlotherium, etc.) are some of the important genera, representatives of most of the modern groups, including carnivores (Canidae and Felidae), insectivores, rodents, ruminants, camels. Tortoises were abundant, and the genus Rana made its appearance. Rays and dogfish were the dominant marine fish; logoonal brackish-water fish are represented by Prolebias, Smerdis, etc. Insects abounded and arachnids were rapidh' developing. Gasteropods were increasing in importance, most of the genera still existing (Cerithium, Potamides, Melania, large Naticas, Pleurolomaria, Valuta, Turritella, Rostdlaria, Pyrula). Cephalopods, on the other hand, show a falling off. Pelecypods include the genera Cardila, Pectunctdus, Lucina, Ostrea, Cyrena, Cytherea. Bryozoa were very abundant (Membranipora, Lepralia, Hornera, Idmonea). Echinoids were less numerous than Norway; this presents a brilliant red metalhc ghtter, due to the presenceof numerous small scales of haematiteor gothite enclosed in the felspar.
Oligoclase occurs, often accompanying orthoclase, as a constituent of igneous rocks of various kinds; for instance, amongst plutonic rocks in granite, syenite, diorite; amongst dike-rocks in porphyry and diabase; and amongst volcanic rocks in andesite and trachyte. It also occurs in gneiss. The best developed and largest crystals are those found with orthoclase, quartz, epidote and calcite in veins in granite at Arendal in Norway. (L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)