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OLIGOCLASE
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OLIGOCLASE , a See also:rock-forming See also:mineral belonging to the See also:plagioclase (q.v.) See also:division of the felspars. In chemical See also:composition and in its crystallographical and See also:physical characters it is intermediate between See also:albite (NaAlSis08) and See also:anorthite (CaAl2Sis0s), being an isomorphous mixture of three to six molecules of the former with one of the latter. It is thus a soda-See also:lime See also:felspar crystallizing in the anorthic See also:system. Varieties intermediate between oligoclase and albite are known as oligoclase-albite. The name oligoclase was given by A. Breithaupt in 1826 from the Gr. 6Xiyos, little, and tcltnv, to break, because the mineral was thought to have a less perfect cleavage than albite. It had previously been recognized as a distinct See also:species by J. J. See also:Berzelius in 1824, and was named by him soda-See also:spodumene (Natron -spodumen), because of its resemblance in See also:appearance to spodumene. The hardness is 6z and the sp. gr. 2.65-2.67. In See also:colour it is usually whitish, with shades of See also:grey, See also:green or red. Perfectly colourless and transparent glassy material found at Bakersville in See also:North Carolina has occasionally been faceted as a See also:gem-See also: Sands and sandstones of See also:Lower sands of Septarian See also:Clay, Cyrena marls of Mainx. o
p 8 Ormoy, See also:Fontainebleau and Bolderberg. or Lignites of Haring, ;o
`d o Pierrefitte. Sands of Bergh Rupelton. Gypsiferous See also:limestone of See also:Aix, a
aW P0. Sands of Morigny, Faille. of with See also:Stettin sands. and 9998
Jeurre, See also:Oyster marls. Cl Lower marine Molasse of Z
Molasse of Etrechy. Clay of See also:Boom. See also:Basel.
:~•i,~ Bembridge Beds. Limestone of See also:Brie, Sands of Vieux-See also:
c a Headon Beds. "Glaises verses," and Sands of Grimmertingen. See also:Amber-bearing Lignites of Brunstatt.
iu ao See also:Cyrene marls. Sands of Wemmel. g Marls of Priabona,
6 0 Supra •ypseous marls, Glauconitic sands of limestones of Crosara.
b See also:Samland.
limestones of Champigny,
"First" and "Second"
masses of See also:gypsum.
The See also:land See also:flora of this See also:period was a See also:rich one consisting largely of evergreens with characters akin to those of tropical See also:India and See also:Australia and subtropical See also:America. Sequoias, sabal palms, ferns, See also:cinnamon-trees, See also:gum-trees, oaks, See also:figs, laurels and willows were See also:common. Chary is a common fossil in the fresh-See also:water beds. The most interesting feature of the Iand See also:fauna was undoubtedly the astonishing variety of mammalians, especially the See also:long See also:series from the See also: 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 See also:Arsinoitherium, from See also:Egypt are See also:early proboscidian forms which may have lived in this period; Anchitherium, Anchippus, &c., were forerunners of the See also:horse. See also:Palaeotherium, See also:Anthracotherium, Palaeogale, Steneofiber, Cynodictis, Dinictis, Ictops, Palaeolagus, Sciurus, Colodon, Hyopotamus, See also:Oreodon, Poebrotherium, Protoceras, Hypertragulus and the gigantic Titanotherids (Titanotherium, Brontotherium, &c.) are some of the important genera, representatives of most of the See also:modern See also:groups, including carnivores (Canidae and Felidae), insectivores, rodents, ruminants, camels. Tortoises were abundant, and the genus Rana made its appearance. Rays and See also:dog-See also:fish were the dominant marine fish; logoonal brackish-water fish are represented by Prolebias, See also:Smerdis, &c. See also:Insects abounded and arachnids were rapidly developing. Gasteropods were increasing in importance, most of the genera still existing (Cerithium, Potamides, Melania, large Nat ices, Pleurotomaria, Voluta, Turritella, Rostellaria, Pyrula). Cephalopods, on the other See also:hand, show a falling off. Pelecypods include the genera Cardita, .Pettunculus, See also:Lucina, Ostrea, Cyrena, Cytherea. Bryozoa were very abundant (Membranipora, Lepralia, Hornera, Idmonea). Echinoids were less numerous than See also:Norway; this presents a brilliant red metallic glitter, due to the presence of numerous small scales of See also:haematite or See also:gothite enclosed in the felspar. Oligoclase occurs, often accompanying See also:orthoclase, as a constituent of igneous rocks of various kinds; for instance, amongst plutonic rocks in See also:granite, See also:syenite, See also:diorite; amongst See also:dike-rocks in See also:porphyry and See also:diabase; and amongst volcanic rocks in See also:andesite and See also:trachyte. It also occurs in gneiss. The best See also:developed and largest crystals are those found with orthoclase, See also:quartz, See also:epidote and See also:calcite in See also:veins in granite at See also:Arendal in Norway. (L. J. Additional information and CommentsThere are no comments yet for this article.
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