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PHONOLITE (Gr. ~vi7, sound, and Moos,...

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Originally appearing in Volume V21, Page 471 of the 1911 Encyclopedia Britannica.
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PHONOLITE (Gr. ~vi7, See also:sound, and Moos, See also:stone)  , in See also:petrology, a See also:group of volcanic lavas containing much See also:nepheline and sanidine See also:felspar . The See also:term "clinkstone " was formerly given by geologists to many See also:fine grained compact lavas, which split into thin tough plates, and gave out a ringing See also:sound when struck with the See also:hammer . Some of these clinkstones were phonolites in the See also:modern sense, but as the name clinkstone was used for a large variety of rocks, many of which have no See also:close See also:affinities with one another, it has been discarded and "See also:phonolite " is substituted for it . The group includes rocks which are See also:rich in alkalis with only a moderate percentage of See also:silica; hence they contain no See also:free See also:quartz but much See also:alkali felspar (sanidine and anorthoclase) and nepheline . Large plates of sanidine are often visible in the rocks; the nepheline is usually not obvious to the unaided See also:eye . Most phonolites show fluxion structure, both in the See also:orientation of their phenocrysts and in the smaller crystals which make up the ground-See also:mass; and this determines to a large extent the platy jointing . Although vitreous and pumiceous forms are known they are rare, and in the See also:great See also:majority of cases these rocks are finely crystalline with a dull or shimmering lustre in the ground-mass . Marked characteristics are the readiness with which they decompose, and the frequency of See also:veins and cavities occupied by See also:natrolite, See also:analcite, See also:scolecite and other See also:zeolites . Small See also:black grains of See also:augite or See also:hornblende and sometimes See also:blue specks of hauyne may be seen in the rocks when they are fresh . The dominant minerals are sanidine, nepheline, See also:pyroxene, See also:amphibole, various felspathoids and See also:iron oxides .. The sanidine is usually in two generations, the first consisting of large crystals of flattened and See also:tabular shape, while the second See also:generation is represented by small rectangular prisms arranged in parallel streams in the ground-mass; these felspars are nearly always simply twinned on the See also:Carlsbad See also:plan . They contain often as much soda as potash .

The nepheline takes the See also:

form of hexagonal prisms with See also:flat ends, and may be completely replaced by fibrous zeolites, so that it can only be recognized by the outlines of its pseudomorphs . In some phonolites it is exceedingly abundantmagnetite and See also:zircon occur in the phonolites, and See also:sphene is often rather See also:common . Another See also:mineral which is more frequent in phonolites than in many other rocks is See also:brown melanite See also:garnet . The majority of the rocks of this group are of See also:Tertiary or See also:Recent See also:age, but in See also:Scotland Carboniferous phonolites occur in several localities, e.g . Traprain in See also:Haddingtonshire, also in the Eildon Hills and in See also:Renfrewshire . In See also:Brazil phonolites belonging to the same See also:epoch are also known . There are several districts in See also:Europe where Tertiary or Recent phonolites occur in considerable See also:numbers, as in See also:Auvergne (Mont See also:Dore), the See also:Eifel, and Bohemia . The See also:Wolf See also:Rock which lies off the See also:south See also:coast of See also:Cornwall, and is the site of a well-known lighthouse, is the only mass of phonolite in See also:England; it is supposed to be the remains of a Tertiary See also:lava or intrusion . The See also:Canary Islands, Cape Verde Islands, See also:Sardinia, See also:Aden, See also:British See also:East See also:Africa and New See also:Zealand contain many types of phonolites; they are known also in New South See also:Wales, while in the See also:United States phonolites occur in See also:Colorado (at Cripple See also:Creek) and in the Black Hills of South Dakota . See also:Leucite occurs in See also:place of nepheline in a small group of phonolites (the leucite-phonolites), known principally from Rocca Monfina and other places near See also:Naples . Blue hauyne is rather a conspicuous mineral in some of these rocks, and they also contain a See also:good See also:deal of sphene . When sanidine, nepheline and leucite all occur together in a volcanic rock it is classed among the leucitophyres (see PETROLOGY, See also:Plate III. fig .

2) . The chemical analyses of phonolites given below show that these rocks are very rich in alkalis and alumina with only a moderate amount of silica, while See also:

lime, See also:magnesia and iron oxides are See also:present only in small quantity . They have a close resemblance in these respects to the nepheline-syenites of which they provide the effusive types . (J . S . F.) PHORCYS•(PHORCUS, PHORCYN), in See also:Greek See also:mythology, son of See also:Pontus (See also:Sea) and Gaea (See also:Earth), See also:father of the Graeae, the Gorgons, Scylla, and Ladon (the See also:dragon that guarded the See also:golden apples of the See also:Hesperides) . In See also:Homer (Odyssey, xiii . 96) he is an aged sea-deity, after whom a See also:harbour in See also:Ithaca was named . According to See also:Varro (quoted by Servius in Aeneid, V . 824) Phorcys was a See also:king of See also:Corsica and Sardinia, who, having been defeated by King See also:Atlas in a See also:naval engagement in the course of which he was drowned, was subsequently worshipped as a marine divinity .

End of Article: PHONOLITE (Gr. ~vi7, sound, and Moos, stone)
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