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Rh  (Sententiae ad intelligibilia ducentes, Aids to the study of the Intelligibles). The Life of Pythagoras, which is incomplete, probably formed part of a larger history of philosophy (, in four books) down to Plato. His work on Aristotle is represented by the Introduction to and Commentary (, in the form of questions and answers) on the Categories. The first, translated into Latin by Boëtius, was extensively used in the middle ages as a compendium of Aristotelian logic; of the second only fragments have been preserved. His , a chronological work, extended from the taking of Troy down to 270; to it Eusebius is indebted for his list of the Macedonian kings. The treatise is called an (lecture) by Eusebius, who in his Praeparatio evangelica (x. 3) has preserved a considerable extract from it, treating of plagiarism amongst the ancients. Other grammatical and literary works are (Quaestiones homericae); and De antro nympharum, in which the description in the Odyssey (xiii. 102-112) is explained as an allegory of the universe. The (De abstinentia), on abstinence from animal food, is especially valuable as having preserved numerous original statements of the old philosophers and the substance of Theophrastus's (On Piety). It also contains a long fragment from the Cretans of Euripides. The is an exhortation to his wife Marcella to practise virtue and self-restraint and to study philosophy. The letter to the Egyptian priest Anebo, dealing with religious questions, was answered by a member of the school of Iamblichus, who called himself Abammon, in the De mysteriis. It is frequently referred to by Eusebius, Cyril and Augustine. Eusebius preserved fragments of the (De philosophia ex oraculis haurienda), in which he expressed his belief in the responses of the oracles of various gods as confirming his theosophical views. Porphyry is well known as a violent opponent of Christianity and defender of Paganism; of his (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, and Macarius of Magnesia, but all these are lost. 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 Plotini 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 Boëtius (ed. with introd., S. Brandt, 1906); fragments of the Chronica in C. W. Müller, ''Frag. hist. graec.'' (1849), iii. 688; Quaestiones homericae, by H. Schrader (1880, 1890); Letter to Anebo in W. Pharthey's edition of Iamblichus De mysteriis (1857); De philosophia ex oraculis haurienda, by G. Wolff (1856); fragments of the Adversus Christianos by A. Georgiades (Leipzig, 1891); English trans. of the De abstinentia, De antro nympharum and Sententiae, by Thomas Taylor (1823); of the Sententiae by T. Davidson in the Journal of Speculative Philosophy, iii. (1869); of the De abstinentia by S. Hibberd (1857), and of the Ad Marcellam by A. Zimmern 1896.

On Porphyry and his works generally see Fabricius, Bibliotheca graeca (ed. Harles), v. 725; Eunapius, Vita philosophorum; article in Suïdas; Lucas Holstenius, De vita et scriptis Porphyrii (Cambridge, 1655); J. E. Sandys, ''Hist. of Classical Scholarship (1906), i. 343; W. Christ, Gesch. der griechischen Litteratur (1898), § 621; M. N. Bouillet, Porphyre, son rôle dans l'école néoplatonicienne'' (1864); A. I. Kleffner, Porphyrius der Neuplatoniker und Christenfeind (Paderborn, 1896); on his philosophy, T. Whittaker, The Neo-Platonists (Cambridge, 1901), and.  PORPHYRY (Gr., 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 80 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. Many

specimens 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, 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, greenstone-porphyry, 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-porphyries and the elaeolite-porphyries. In all of them porphyritic orthoclase or alkali felspar is the characteristic mineral. The granite-porphyries and (q.v.) consist mainly of orthoclase, quartz and ferro-magnesian mineral, usually biotite but sometimes hornblende, augite or enstatite.

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-porphyries, like the syenites, are less common than the granite-porphyries and granites. They are characterized by an abundance of orthoclase and a scarcity or absence of quartz. The phenocrysts are orthoclase (and oligoclase), biotite, hornblende or augite; the ground-mass is principally alkali felspar with sometimes a little quartz. In many specimens the felspars of the second generation form a mosaic of ill-shaped grains, in others they are little rectangular crystals which may have a fluxion arrangement (orthophyric type of ground-mass). Some of the rocks formerly known as orthoclase-porphyries belong to this group; others are ancient trachytic lavas (orthophyres). Closely related to the syenite-porphyries is the rhomben-porphyry of south Norway and West Africa. In these the large felspars have rhomb-shaped sections owing to their peculiar crystalline development. Olivine, augite and biotite occur in these rocks, but there is no quartz or soda-lime felspar. The porphyritic felspars contain both soda and potash and belong to anorthoclase. Rhomben-porphyries occur as dikes connected with the syenites (laurvikites of southern Norway), and many ice-borne boulders of these rocks have been found among the drift deposits of the east of England.

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,