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Rh will be visible. In gneisses these alternating folia are thicker and less regular than in schists; they are often lenticular, dying out rapidly. Gneisses also, as a rule, contain more felspar than schists do, and they are tougher and less fissile. Contortion or crumpling (Pl. IV. fig. 6) of the foliation is by no means uncommon, and then the splitting faces are undulose or puckered. The origin of schistosity or foliation is not perfectly understood, but it is clear that in many cases it is due to pressure, acting in a direction perpendicular to the banding, and to interstitial movement or internal flow arranging the mineral particles while they are crystallizing.

Rocks which were originally sedimentary and rocks which were undoubtedly igneous are converted into schists and gneisses, and if originally of similar composition they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine felspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, we may distinguish between sedimentary and igneous schists and gneisses. Often the metamorphism is progressive, and if the whole district occupied by these rocks be searched traces of bedding, of clastic structure, unconformability or other evidence may be obtained showing that we are dealing with a group of altered sediments in other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemist, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and, however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks, for example, as limestones, calc-schists, dolomites, quartzites and aluminous shales have very definite chemical characters which distinguish them even when completely recrystallized.

The schists and gneisses are classified according to the minerals they consist of, and this depends principally on their chemical composition. We have, for example, a group of metamorphic limestones, marbles, calc-schists and cipolins, with crystalline dolomites; man of these contain silicates such as mica, tremolite, diopside, scapolite, quartz and felspar. They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, felspar, zoisite and hornblende. These were once sandstones and arenaceous rocks The graphitic schists may readily be believed to represent sediments once containing coaly matter or plant remains; there are also schistose ironstones (haematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists of igneous origin we may mention the silky calc-schists, the foliated serpentine (once ultra basic masses rich in olivine), and the white mica-schists, porphyroids and banded halleflintas, which have been derived from rhyolites, quartz-porphyries and acid tuffs. The majority of mica-schists, however, are altered clays and shales, and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. As a special subgroup we have the andalusite-, staurolite- kvanite- and sillimanite-schists, together with the cordierite-gneisses, which usually make their appearance in the vicinity of gneissose granites, an have presumably been affected by contact alteration. The more coarsely foliated gneisses are almost as frequent as the mica-schists, and present a great variety of types differing in composition and in appearance. They contain quartz, one or more varieties of felspar, and usually mica, hornblende or augite, often garnet, iron oxides, &c. Hence in composition they resemble granite, differing principally in their foliated structure. Many of them have “augen” or large elliptical crystals, mostly felspar but sometimes quartz, which are the crushed remains of porphyritic minerals; the foliation of the matrix winds around these augen, closing in on each side. Most of these augen gneisses are metamorphic granites, but sometimes a conglomerate bed simulates a gneiss of this kind rather closely. There are other gneisses, which were derived from felspathic sandstones, grits, arkoses and sediments of that order; they mostly contain biotite and muscovite, but the hornblende and pyroxene gneisses are usually igneous rocks allied in composition to the hornblende-granites and quartz-diorites. The metamorphic forms of dolerite, basalt and the basic igneous rocks generally have a distinctive facies as their pyroxene and olivine are replaced by dark green hornblende, with often epidote, garnet and biotite. These rocks have a well developed foliation, as the prismatic hornblendes lie side by side in parallel arrangement. The majority of amphibolites, hornblende-schists, foliated epidiorites and green schists belong to this group. Where they are least altered they pass through chloritic schists into sheared diabases, flaser gabbros and other rocks in which remains of the original igneous minerals and structures occur in greater or less profusion.

.—Most text-books of geology treat of petrology in more or less detail (see § Bibliography). Elementary books on petrology include F. H. Hatch, Petrology (5th ed., London, 1909), L. V. Pirsson, Rocks and Rock-minerals (New York, 1908); J. D. Dana, Handbook of Mineralogy and Petrography (12th ed, New York, 1908); A. Harker, Petrology for Students (4th ed., Cambridge, 1908); G. A. J. Cole, Aids to Practical Geology (6th ed, London, 1909). For rock minerals consult J. P. Iddings, Rock Minerals (New York, 1906); A. Johannsen, Determination of Rock-forming Minerals (New York, 1908); E. Hussak and E. G. Smith, Determination of Rock-forming Minerals (2nd ed., New York, 1893); N. H and A. N. Winchell, Optical Mineralogy (New York, 1909). On the classification and origin of rocks see A. Harker, Natural History of Igneous Rocks (London, 1909); J. P. Iddings, Igneous Rocks, (New York, 1909), Cross, Iddings, Washington and Pirsson, Quantitative Classification of Igneous Rocks (Chicago, 1902); C. Van Hise, Metamorphism (Washington, 1904); A. P. Merrill, Rocks, Rock-weathering and Soils (London, 1897); C. Doelter, Petrogenesis (Brunswick, 1906); J. H. L. Vogt, Silikatschmelzlösungen (Christiania, 1903); F. Fouqué and A. Michel Lévy, Synthèse des minéraux et des roches (Paris, 1882) The principal authorities on the analysis and chemical composition of rocks are J. Roth, Beiträge zur Petrographie (Berlin, 1873–1884), A. Osann, Beiträge zur chemischen Petrographie (Stuttgart, 1903); H. S. Washington, Manual of the Chemical Analysts of Rocks (New York, 1904) and Chemical Analyses of Igneous Rocks (Washington, 1904); F. W. Clarke, Analyses of Rocks (Washington, 1904); Max Dittrich, Anleitung zur Gesteinsanalyse (Leipzig, 1905); W. F. Hillebrand, Analysis of Silicate and Carbonate Rocks (Washington, 1907).

The great systematic treatises on Petrology are F. Zirkel, Lehrbuch der Petrographie (2nd ed., Leipzig, 1894, 3 vols); H. Rosenbusch, Mikroskopische Physiographie (4th ed, Stuttgart, 1909, 2 vols.)

Useful German handbooks include E. Weinschenk, Polarisationsmikroskop, Gesteinsbildende Mineralien and Gesteinskunde (2nd ed., Freiburg, 1907, &c); R. Reinisch, Petrographisches Praktikum (2nd ed., Berlin, 1907); H. Rosenbusch, Elemente der Gesteinslehre (3rd ed., Stuttgart, 1909); A. Grubenmann, Die krystallinen Schiefer (Berlin, 1907); F. Loewisson Lessing, Petrographisches Lexikon (1893 and 1898, also a Fr. ed., 1901); F. Rinne, Praktische Gesteinskunde (2nd ed, Hanover, 1905).

The principal French works are E. Jannettaz, Les Roches (3rd ed., Paris, 1900); F. Fouqué and A. Michel Lévy, Minéralogie micrographique (Paris, 1879); A Michel Lévy and A. Lacroix, Les Mineraux des roches (Paris, 1888); A. Lacroix, Minéralogie de la France (I., II., Paris, 1893); and Les Enclaves des roches éruptives (Macon, 1893).

British petrography is the subject of a special work by J. J. H. Teall (London, 1888). Much information about rocks is contained in the memoirs of the various geological surveys, and in ''Quart. Journ.'' ''of the Geol. Soc. of London, Mineralogical Magazine, Geological'' Magazine, Tschermak's Mineralogische Mittheilungen (Vienna), Neues Jahrbuch fur Mineralogie (Stuttgart), Journal of Geology (Chicago), &c.

PETRONEL, a 16th or 17th century fire-arm, defined by R. Barret (Theorike and Practike of Modern Warres, 1598) as a “horseman's peece.” It was the fire-arm which developed on the one hand into the pistol and on the other into the carbine. The name (Fr. petrinel for poitrinal) was given to the Weapon either because it was fired with the butt resting against the chest (poitrine, Lat. pectus) or because it was carried slung from a belt across the chest.

PETRONIUS (G. (?) Petronius Arbiter), Roman writer of the Neronian age. His own work, the Satirae, tells us nothing directly of his fortunes, position, or even century. Some lines of Sidonius Apollinaris refer to him and are often taken to imply that he lived and wrote at Marseilles. If, however, we accept the identification of this author with the Petronius of Tacitus, Nero's courtier, we must suppose either that Marseilles was his birthplace or, as is more likely, that Sidonius refers to the novel itself and that its scene was partly laid at Marseilles. The chief personages of the story are evidently strangers in the towns of southern Italy where we find them. Their Greek-sounding names (Encolpius, Ascyltos, Giton, &c.) and literary training accord with the characteristics of the old Greek colony in the 1st century The high position among Latin writers ascribed by Sidonius to Petronius, and the mention of him beside Menander by Macrobius, when compared with the absolute silence of Quintilian, Juvenal and Martial, seem adverse to the opinion that the Satirae was a work of the age of Nero. But Quintilian was concerned with writers who could be turned to use in the