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Online Encyclopedia
Originally appearing in Volume V22, Page 105 of the 1911 Encyclopedia Britannica.
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VIIVI.. 7554.20 54.46 1219.96.65 1.53 2.34 3.33 0.61 2.12 8.68 2.76 5.20 0.28 0.26 0.60 4.14 5.67 0.12 I., Elvan or granite porphyry (with pinite after cordierite)-Prah sands, Cornwall. II., Granophyre-Armboth, Cumberland. III., Granophyre-Carrock Fell, Cumberland. IV., Rhomben-porphyry -Tonsterg, Norway. V., Elaeolite porphyry-Beemerville, New Jersey. VI., Tinguaite-Kola. VII., Grorudite-Assynt, Scotland. Porphyrites.-The porphyrites as above mentioned are intrusive or hypabyssal rocks of porphyritic texture, with phenocryste of plagioclase felspar and hornblende, biotite or augite (sometimes also. quartz) in a fine ground-mass. The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous age and older. Both the red porphyry and the green porphyry of the ancients are more properly classified in this group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or no primary quartz. Porphyrites occur as dikes which accompany masses of diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure. The phenocrysts are plagioclase, often much zoned with central kernels of bytownite or labradorite and margins of oligoclase or even orthoclase. In a special group there are corroded blebs or porphyritic quartz: these rocks are called quartz-porphyrites, and are distinguished from the granite-porphyries by the scarcity or absence of orthoclase. The hornblende of the porphyrites is often green but sometimes brown, resembling that of the lamprophyres, a group from which the porphyrites are separated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres. Augite, when present, is nearly always pale green; it is not so abundant as hornblende. Dark brown biotite is very common in large hexagonal plates. Muscovite and olivine are not represented in these rocks. The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely absent. The Alpine dike rocks known as ortlerites and suldenites are porphyrites containing much green or brown hornblende and augite; these, however, hardly require a distinctive designation. Diorite-porphyrites have almost as wide a distribution as granite-porphyries, and occur in all parts of the world where intrusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks. To gabbros and norites certain types of porphyrite correspond which have the same mineral and chemical, composition as the parent rocks but with a porphyritic instead of granitic structure. Gabbro-porphyrites are not numerous; or rather most of these rocks are described as porphyritic basalts and dolcrites. The beerbachites are finely granular, dike rocks resembling gabbros SiO2 Al20a Fe20a FeO CaO MgO K2O Na2O H2O I. 64.94 1 17.50 0.69 3.94 2.59 2.83 3.11 3.44 I.36 II. 61.58 18.84 4.68 - 6.59 2.04 1.49 4.27 I.61 I , Quartz-porphyrite-Lippenhof, Schwarzwald. II., Porphyrite-Esterel, France. III., Norite-porohyrite-Klausen, Tirol. (J. S. F.)
End of Article: VIIVI
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