Page:EB1911 - Volume 20.djvu/106

 In Belgium a sandy series (Wemmelian, Asschian, Henisian), mainly of brackish-water origin, is succeeded by the marine sands of Bergh (with the clay of Boom), which pass up through the inferior sands of Bolderberg into the Miocene. In Switzerland, beyond the limits of the Flysch, nearer the Alpine massif, is a belt of grits, limestones and clays in an uncompacted condition, to which the name “molasse” is usually given; mixed with the molasse is an inconstant conglomeratic littoral formation, called Nagelfluh. The molasse occurs also in Bavaria, where it is several thousand feet thick and contains lignites. Oligocene deposits occur in the Carpathian region and Tirol; as Flysch and brackish and lacustrine beds with lignite in Klausenburg, lignites at Haring in Tirol. In the Spanish Pyrenees they are well developed; in the Apennines the scaly clays (“argille scagliose”) are of this age; while in Calabria they are represented by thick conglomerates and Flysch. Flysch appears also in Dalmatia and Istria (where it is called “tassello”) and in North Bosnia, where it contains marine limestones. Lignites are found at Sotzka and Styria, marine beds in the Balkan peninsula, glauconitic sands prevail in South Russia, Flysch with sands and grits in the Caucasus, while marine deposits also occupy the Aral-Caspian region and Armenia, and are to be traced into Persia. Oligocene rocks are known in North Africa, Algeria, Tunis and Egypt, with the silicified trees and basalt sheets north of the Fayum. In North America the rocks of this period have not been very clearly differentiated, but they may possibly be represented by the White river beds of S. Dakota, the white and blue marls of Jackson on the Mississippi, the “Jacksonian” white limestone of Alabama, the limestone of Ocala in Florida, certain lacustrine clays in the Uinta basin, and by the ribband shales with asphalt and petroleum in the coastal range of California. In South America and the Antilles upper Oligocene is found, and the lignite beds of Coronel and Lota in Chile and in the Straits of Magellan may be of this age; in Patagonia are the lower Oligocene marine beds (“Patagonian”) and beds with mammalian remains. In New Zealand the Oamaru series of J. Hutton is regarded as Oligocene; at its base are interstratified basic volcanic rocks.

A correlation of Oligocene strata is summarized in the following table:—

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 Arsinoitherium, from Egypt are early proboscidian forms which may have lived in this period; Anchitherium, Anchippus, &c., were forerunners of the horse. Palæotherium, Anthracotherium, Palaeogale, Steneofiber, Cynodictis, Dinictis, Ictops, Palaeolagus, Sciurus, Colodon, Hyopotamus, Oreodon, Poebrotherium, Protoceras, Hypertragulus and the gigantic Titanotherids (Titanotherium, Brontotherium, &c.) 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, &c. Insects abounded and arachnids were rapidly developing. Gasteropods were increasing in importance, most of the genera still existing (Cerithium, Potamides, Melania, large Naticas, Pleurotomaria, Voluta, Turritella, Rostellaria, Pyrula). Cephalopods, on the other hand, show a falling off. Pelecypods include the genera Cardita, Pectunculus, Lucina, Ostrea, Cyrena, Cytherea. Bryozoa were very abundant (Membranipora, Lepralia, Hornera, Idmonea). Echinoids were less numerous than in the Eocene seas (Coelopleurus, Echinolampus, Clypeaster, Scutella). Corals were abundant, and nummulites still continued till near the close of the period, but they were diminished in size.

.—“Geology of the Isle of Wight,” ''Mem. Geol.'' Survey (2nd ed. 1889); A. von Koenen, ''Abhand. geol. Specialkart Preuss''. X. (1889–1894); M. Vollest, Der Braunkohlenbergbaum (Halle, 1889); E. van den Brocek, “Matériaux pour l’étude de l’Oligocène belge,” ''Bull. Soc. Belg. Géol.'' (1894); also the works of O. Heer, H. Filhol, G. Vasseur, H. F. Osborn, A. Gaudry, H. Douvillé, R. B. Newton, H. Dall, M. Cossmann, G. Lambert, &c., and the article.

OLIGOCLASE, a rock-forming mineral belonging to the (q.v.) division of the felspars. In chemical composition and in its crystallographical and physical characters it is intermediate between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), 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 oligoclase-albite. The name oligoclase was given by A. Breithaupt in 1826 from the Gr. , little, and  , 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. Berzelius 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 Bakersville in North Carolina has occasionally been faceted as a gem-stone. Another variety more frequently used as a gem-stone is the aventurine-felspar or “” (q.v.) found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red metallic glitter, due to the presence of numerous small scales of haematite or göthite 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.

OLIPHANT, LAURENCE (1829–1888), British author, son of Anthony Oliphant (1793–1859), was born at Cape Town.