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Encyclopedia Britannica - Main :: POL-PRE |
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PORPHYRY (IlopcPisptos) (A.D. 233-c. 304) , Greek scholar, historian, and Neoplatonist, was born at Tyre, or Batanaea in Syria. He studied grammar and rhetoric under Cassius Longinus (q.v.). His original
Of his numerous works on a great variety of subjects the following are extant: Life of Plotinus and an exposition of his teaching in the 'Ad,oppat trpbs ra vomra (Sententiae ad intelligioitia ducentes, Aids to the study of the Intelligibles). The Life of Pythagoras
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original
Porphyry is well known as a violent opponent of Christianity and defender of Paganism; of his Kara Xpcarcavc;w (Adversus Christianos) in 15 books, perhaps the most important of all his works, only fragments remain. Counter-treatises were written by Eusebius of Caesarea, Apollinarius (or Apollinaris) of Laodicea, Methodius of Olympus
Porphyry 's view of the book of Daniel, that it was the work of a writer in the time of Antiochus Epiphanes, is given by Jerome. There is no proof of the assertion of Socrates, the ecclesiastical historian, and Augustine, that Porphyry was once a Christian.There is no complete edition of the works of Porphyry. Separate editions: Vita Plolini in R. Volkmann's edition of the Enneades of Plotinus (1883); Sententiae, by B. Mommert (1907); Vita Pythagorae, De antro nympharum, De abstinentia, Ad Marcellam, by A. Nauck (1885); Isagoge et in Aristotelis categorias commentarium," by A. Busse in Commentaria in Aristotelem graeca (1887), iv. 1, with the translation of Boetius (ed. with introd., S. Brandt, 1906); fragments of the Chronica in C. W. Miller, Frag. list
Leipzig
On Porphyry and his works generally see Fabricius, Bibliotheca raeca (ed. Harles), v. 725; Eunapius, Vita philosophorum; article in Suidas; Lucas Holstenius, De vita et scriptis Porphyrii (Cam-bridge, 1655) ; J. E. Sandys, Hist. of Classical Scholarship (1906), i. 343: W. Christ, Gesch. der griechischen Litteratur (1898), 621; M. N. Bouillet, Porphyre, son role dens l'ecole neoplatonicienne (1864); A. I. Kleffner, Porphyrius der Neuplatoniker and Christenfeind (Paderborn, 1896) ; on his philosophy, T. Whittaker, The Neo-Platonists ( Cambridge , 1901), and NEOPLATONISM.'PORPHYRY (Gr. irop4 peon, Lat. purpureus, purple), in petrology, a beautiful red volcanic rock which was much used by the Romans for ornamental purposes when cut and polished. The famous red porphyry (porfido rosso antico) came from Egypt, but its beauty and decorative value were first recognized by the Romans in the time of the emperor Claudius. It was obtained on the west coast of the Red Sea, where it forms a dike 8o or 90 ft. thick. For a long time the knowledge of its source was lost, but the original locality, marked by many ancient quarries, has been re-discovered at Jebel Dhokan, and the stone is again an article of commerce. In a dark red ground-mass it contains many small white or rose-red plagioclase felspars, black shining prisms of hornblende, and small plates of iron oxide. The red colour of the felspars and of the ground-mass is unusual in rocks of this group, and arises from the partial conversion of the plagioclase felspar into thulite and manganese-epidote. These minerals also occur in thin veins crossing the rock. Manyspecimens show effects of crushing and in extreme cases this has produced brecciation. Another famous porphyry, hardly less beautiful, is the verde antique, porfido verde antico, or marmor lacedaemonium viride of Pliny, which was obtained between Lebetsova and Marathonisi in Peloponnesus. It has the same structure as the red porphyry as it contains large white or green felspars in a fine ground-mass. The green colour arises from the abundant formation of chlorite and epidote in the large felspars and throughout the rock. In ancient times it was much used as an ornamental stone, these two varieties of porphyry making a fine contrast with one another. Green porphyries are not so rare as red. A similar rock is obtained at Lambay Island near Dublin. They are still used extensively, especially for small ornaments. Large pieces are difficult to obtain free from flaws, and marble is preferred for mural work, not only because of the greater variety of patterns but also because it is much softer and more easily cut and polished. Many igneous rocks possess the structure which characterizes these porphyries (see PETROLOGY, Plate III.) : the presence of scattered crystals of larger size in a fine-grained ground-mass. Most lavas, and many of the rocks which occur as dikes and sills, have porphyritic structure. These may be called porphyries and this term has consequently been applied to a great variety of rocks, e.g. diorite-porphyry, granite-porphyry, greenstoneporphyry, augite-porphyry, liebenerite-porphyry, &c. More recently the use of the term has been restricted to a series of rocks which are of intrusive origin and contain much porphyritic felspar (with or without quartz or nepheline). The porphyritic intrusive rocks with large crystals of augite, olivine, biotite, and hornblende are for the most part grouped under the lamprophyres; while the term porphyry is rarely now applied to any of the effusive rocks or lavas. Furthermore, it has become usual to subdivide the intrusive porphyries into two classes; in one of these the phenocrysts are mainly orthoclase, in the other mainly plagioclase felspar. The first series is known as the " porphyries," while the second group is called " porphyrites." There are porphyries which correspond chemically and mineralogically to granites, syenites, and nepheline-syenites; while the porphyrites form a parallel series to the diorites, norites and gabbros. In each case the porphyritic type occurs generally as dikes and thin sheets which consolidated beneath the surface but probably at no great depth (hypabyssal rocks); while granite, gabbro and the other holocrystalline non-porphyritic rocks belong to the plutonic or abyssal group which cooled very slowly at great depths and under enormous pressure. The principal subdivisions of the group are the granite-porphyries, the syenite
mineral
mineral
Granite-porphyries are exceedingly common in all regions where acid intrusive rocks occur. Many granite masses are surrounded by dikes of this kind, and in some cases the chilled margin of a granite consists of typical porphyry. The syenite
Elaeolite- and leucite- (syenite) porphyries form apophyses and dikes around nepheline- and leucite-syenite intrusions. The former contain porphyritic nepheline which is often weathered to soft, finely crystalline aggregatesof white mica and other secondary in all respects except in their being less coarsely crystalline. products as in the well-known liebenerite-porphyry of Tirol and Norite-porphyrites have porphyritic plagioclase (labradorite gieseckite-porphyry of Greenland. The felspars of these rocks are albite, orthoclase and anorthoclase. and they often con- usually) with hypersthene or bronzite, often altered to bastite. tain soda-augite and amphiboles. Elaeolite-porphyries occur They accompany norite masses in Nahe (Prussia) and Tirol, along with nepheline-syenites in such districts as the Serra de They have vitreous forms which are described as andesitic-Monchique, south Norway, Kola, Montreal. Allied to them are pitchstones or hypersthene-andesftes. the tinguaites (so called from the Serra de Tingua, Rio de Janeiro, Brazil), which are pale green rocks with abundant alkali felspar nepheline, needles of green aegirine, and sometimes biotite and cancrinite. As a rule, however, these. are not porphyritic. Some authors group the tinguaites with the aplites rather than the porphyries. Grorudites are quartz-tinguaites free from nepheline, and solvsbergites are tinguaitic rocks in which neither quartz nor nepheline occur. The two last varieties have been described from the Christiania district in Norway, but tinguaites are known with nepheline-syenites in many parts of the world, e.g. Norway, Brazil, Portugal, Canada, Sweden, Greenland. The following analyses of porphyries of different types will show the chemical composition of a few selected examples:- Si02 Al203 Fe2O3 FeO MgO CaO K2O Na2O H2O I. 72.51 13.31 tr. 3.87 1.50 0.60 6.65 0.43 0.60 II. 67.18 16.65 0.55 2.15 I.54 2.35 2.91 4.03 0.75 IV. 58.82 2Io6 3.26 0.70 1.38 3.03 3.70 6. 3 I.26 V. 45.18 23.31 6.11 I.45 4.62 11.16 5.94 1.14 End of Article: PORPHYRY (IlopcPisptos) (A.D. 233-c. 304) If you wish, you can link directly to this article.
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