the processes involved in the formation of igneous rocks cannot be successfully handled. But every day sees an increase in the amount of data available, and encourages us to believe that sooner or later some of the simpler igneous rocks at any rate will be completely explicable on physico-chemical principles. Postvolcanic Clastic. may also contain certain ingredients derived from the atmosphere Rock masses of igneous origin have no sooner consolidated than they begin to change. The gases with which the magma is charged are slowly dissipated, lava-flows cften remain hot and steaming for many years. These gases attack the components of the rock and deposit new minerals in cavities Changes. and fissures. The beautiful zeolites, so well known to collectors of minerals, are largely of this origin. Even before these " post-volcanic processes have ceased atmospheric decomposition begins. Rain, frost, carbonic acid, oxygen and other agents operate continuously, and do not cease till the whole mass has crumbled down and most of its ingredients have been resolved into new products. In the classification of rocks these secondary changes are generally considered unessential; rocks are classified and described as if they were ideally fresh, though this is rarely the case in nature. Epigenitic change (secondary processes) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than Secondary one of these alterations will be found in progress in the Changes. same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in acid rocks, such as rhyolite, but is also found in serpentine, &c. Kaolini-separating and removing these fine particles. They to a very large zation is the decomposition of the felspars, which are the commonest minerals of igneous rocks, into kaolin (along with quartz, muscovite, &c.); it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite); it is typical of peridotites, but occurs in most of the basic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally in diabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase felspar. The sedimentary rocks, which constitute the second great group, have many points in common that distinguish them from the igneous and the metamorphic. They have all originated on the surface of the earth, and at the period of their formation were exposed only to the temperature of the air and to atmospheric pressure (or the pressures which exist at the bottoms of seas and lakes). Their minerals are in most cases not susceptible to change when exposed to moist air or sea, and many of them are hydrated (chlorite, micas, &c.), or oxidized (iron ores), or contain carbonic acid (calcite, dolomite). The extent, however, to which this is the case depends largely on the rapidity with which they have accumulated; coarse rocks quickly piled up often consist of materials only partly weathered. When crystalline, the sedimentary rocks are usually soluble at low temperatures. The members of this group occur in beds or strata, hence they are often known as the stratified rocks; the upper beds are always of later formation than those which underlie them, except (as may happen when great disturbance has taken place) the whole series is inverted or overturned. Many of the stratified rocks have been formed by the agency of moving water (rivers, currents, &c.) and are grouped together as "aqueous" rocks; others have been deposited by the wind in deserts, on sandy beaches, &c. (these are "aeolian "). Others are the remains of animals or of plants, modified by the action of time, pressure and percolating water. Lastly, we find beds of crystalline nature, such as rock-salt and gypsum, which have been formed by the desiccation of saline waters; other crystalline stratified rocks, such as dolomite and many bedded iron-stones, are replacement products due to the introduction of mineral matter in solution, which replaced the original rock mass partially or wholly. When the rocks exposed at the earth's surface give way before the attack of the agencies of denudation, they crumble down and are resolved into two parts. One of these consists of solid material (sand, clay and angular débris) insoluble in carbonated waters; the other part is dissolved and washed away. The undissolved residues, when they finally come to rest, form clastic sedimentary rocks (sandstone, conglomerate, shale, &c.). The dissolved portions are partly transferred to the sea, where they help to increase its store of salts, and may again be precipitated as crystalline sedimentary rocks; but they are also made use of by plants and by animals to form their skeletal and vital tissues. From this latter portion the rocks of organic origin are built up. These The mechanical sorting by the transporting agencies is usually somewhat incomplete, and mixed types of sediment result, such as gravels containing sand, or clays with coarser arenaceous particles. Moreover, successive layers of deposit may not always be entirely similar, and alternations of varying composition may follow one another in thin laminae: e.g. laminae of arenaceous material in beds of clay and shale. Organic matter is frequently mingled with the finer-grained sediments. Gr. Vndos, pebble); psammitic (or sandy, Gr. auμos, sand), and rare. The crystalline sedimentary rocks have been deposited from solution in water. The commonest types, such as rock-salt, gypsum, anhydrite, carnallite, are known to have arisen by the evaporation of enclosed saline lakes exposed to a dry Crystalline. atmosphere. They occur usually in beds with layers of red clay and marl; some limestones have been formed by calcareous waters containing carbonate of lime dissolved in an excess of carbonic acid; with the escape of the volatile gas the mineral matter is precipitated (sinters, Sprudelstein, &c.). Heated waters on cooling may yield up part of their dissolved mineral substances; thus siliceous sinters are produced around geysers and hot springs in many parts of the world. There seems no reason to separate from these the veinstones which fill the fissures by which these waters rise to the surface. They differ from those above enumerated in being more perfectly crystallized and in having no definite stratification, but only a banding parallel to the more or less vertical walls of the fissure. Another subdivision of this class of rocks is due to recrystallization or crystalline replacement of pre-existing sediments. Thus limestones are dolomitized or converted into ironstones, flints and cherts, by percolating waters which remove the lime salts and substitute for them compounds of iron, magnesia, silicon, and so on. This may be considered a kind of metamorphism; it is generally known as metasomatism (q.v.). The rocks of organic origin may be due to animals or plants. They are of great importance, as limestones and coals belong to this group. They are the most fossiliferous of all Organic. rocks; but clastic sediments are often rich in fossils though crystalline sediments rarely are. They may be subdivided, according to their dominant components, into calcareous, carbonaceous, siliceous, ferruginous, and so on. The calcareous complex and less clearly understood; it is evident that pressure organic rocks may consist principally of foraminifera, crinoids, and interstitial movement have had a powerful influence, corals, brachiopoda, mollusca, polyzoa, &c. Most of them, however, contain a mixture of organisms. By crystallization and metaso- possibly assisted by rise of temperature. In thermal or contact matic changes they often lose their organic structures; metamorphism alteration the rocks are baked, indurated, and often in large of any kind has the same effect. The carbonaceous rocks are measure recrystallized. In regional metamorphism recrystalessentially plant deposits; they include peat, lignite and coal. lization also goes on, but the final products are usually schists The siliceous organic rocks include radiolarian and diatom oozes; in the older formations they occur as radiolarian cherts. Flint and gneisses. It is as a rule not difficult to distinguish the nodules owe their silica to disseminated fossils of this nature which two classes of metamorphic rocks at a glance, and they may have been dissolved and redeposited by concretionary action. conveniently be considered separately. Some kinds of siliceous sinter may be produced by organisms inhabiting hot silicated waters. Calcareous oolites in the same way may have arisen through the agency of minute plants. Bog iron ores also may be of organic rather than of merely chemical origin. The phosphatic rocks so extensively sought after as sources of fertilizing agents for use in agriculture are for the most part of organic origin, since they owe their substance to the remains of certain varieties of animals which secrete a phosphatic skeleton; but most of them no longer show organic structures but have been converted into nodular or concretionary forms. All sediments are at first in an incoherent condition (e.g. sands, clays and gravels, beds of shells, &c.), and in this state they may remain for an indefinite period. Millions of years have Cementaelapsed since some of the early Tertiary strata gathered tion. on the ocean floor, yet they are quite friable (e.g. the London Clay) and differ little from many recent accumulations. There are few exceptions, however, to the rule that with increasing age sedimentary rocks become more and more indurated, and the older they are the more likely it is that they will have the firm consistency generally implied in the term rock." The pressure of newer sediments on underlying masses is apparently one cause of this change, though not in itself a very powerful one. More efficiency is generally ascribed to the action of percolating water, which takes up certain soluble materials and redeposits them in pores and cavities. This operation is probably accelerated by the increased pressure produced by superincumbent masses, and to some extent also by the rise of temperature which inevitably takes place in rocks buried to some depth beneath the surface. The rise of temperature, however, is never very great; we know more than one instance of sedimentary deposits which have been buried beneath four or five miles of similar strata (e.g. parts of the Old Red Sandstone), yet no perceptible difference in condition can be made out between beds of similar composition at the top of the series and near its base. The redeposited cementing material is most commonly calcareous or siliceous. Limestones, which were originally a loose accumulation of shells, corals, &c., become compacted into firm rock in this manner; and the process often takes place with surprising ease, as for example in the deeper parts of coral reefs, or even in wind-blown masses of shelly sand exposed merely to the action of rain. The cementing substance may be regularly deposited in crystalline continuity on the original grains, where these were crystalline; and even in sandstones (such as Kentish Rag) a crystalline matrix of calcite often envelopes the sand grains. The change of aragonite to calcite and of calcite to dolomite, by forming new crystalline masses in the interior of the rock, usually also accelerates consolidation. Silica is less easily soluble in ordinary waters, but even this ingredient of rocks is dissolved and redeposited with great frequency. Many sandstones are held together by an infinitesimal amount of collcid or cryptocrystalline silica; when freshly dug from the quarry they are soft and easily trimmed, but after exposure to the air for some time they become much harder, as their siliceous cement sets and passes into a rigid condition. Others contain fine scales of kaolin or of mica. Argillaceous materials may be compacted by mere pressure, like graphite and other scaly minerals. Oxides and carbonates of iron play a large part in many sedimentary rocks and are especially important as colouring matters. The red sands and limestones, for example, which are so abundant, contain Coloration. small amounts of ferric oxide (haematite), which in a finely divided state gives a red hue of all rocks in which it is present. Limonite, on the other hand, makes rocks yellow or brown; oxides of manganese, asphalt and other carbonaceous substances are the cause of the black colour of many sediments. Bluish tints result sometimes from the presence of phosphates or of fluorspar; while green is most frequently seen in rocks which contain glauconite or chlorite. Metamorphic Rocks.-The metamorphic rocks, which form the third great subdivision, are even more varied than the igneous and the sedimentary. They include representatives of nearly all kinds of the other two classes, their common characteristic being that they have all undergone considerable altuations in structure or in mineral composition. The agencies of metamorphism (q.v.) are of two kinds-thermal and regional. In the former case contact with intrusive igneous masses, such as granite, laccolites or dikes, have indurated and recrystallized the original rock. In the second case the actions are more Thermo metamor phism. When a rock is contact altered by an igneous intrusion it very frequently becomes harder, more crystalline and more lustrous, | owing to the development of many small crystals in its mass. Many altered rocks of this type were formerly called hornstones, and the term hornfelses (Ger. Hornfels) is often used by geologists to signify those fine grained, compact, crystalline products of thermal metamorphism. A shale becomes a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone changes to a grey, yellow or greenish lime-silicate-hornfels, tough and splintery, with abundance of augite, garnet, wollastonite and other minerals in which lime is an important component. A diabase or andesite becomes a diabase hornfels or andesite hornfels with a large development of new hornblende and biotite and a partial recrystallization of the original felspar. A chert or flint becomes a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close-fitting grains of quartz. If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact altered lavas the steam cavities are still visible, though their contents have usually entered into new combinations to form minerals which were not originally present. The minute structures, however, disappear, often completely, if the thermal alteration is very profound; thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed. By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thus shales may pass into cordierite rocks, or may show large crystals of andalusite (and chiastolite, Pl. IV., fig. 9), staurolite, garnet, kyanite and sillimanite. A considerable amount of mica (both muscovite and biotite) is simultaneously formed, and the resulting product has a close resemblance to many kinds of schist. Limestones, if pure, are often turned into coarsely crystalline marbles (Pl. IV., fig. 4); but if there was an admixture of clay or sand in the original rock such minerals as garnet, epidote, idocrase, wollastonite, will be present. Sandstones when greatly heated may change into coarse quartzites composed of large clear grains of quartz. These more intense stages of alteration are not so commonly seen in igneous rocks, possibly because their minerals, being formed at high temperatures, are not so easily transformed or recrystallized. In a few cases rocks are fused and in the dark glassy product minute crystals of spinel, sillimanite and cordierite may separate out. Shales are occasionally thus altered by basalt dikes, and felspathic sandstones may be completely vitrified. Similar changes may be induced in shales by the burning of coal seams or even by an ordinary furnace. There is also a tendency for interfusion of the igneous with the sedimentary rock. Granites may absorb fragments of shale or pieces of basalt. In that case hybrid rocks arise which have not the characters of normal igneous or sedimentary rocks. Such effects are scarce and are usually easily recognized. Sometimes an invading granite magma permeates the rocks around, filling their joints and planes of bedding, &c., with threads of quartz and felspar. This is very exceptional, but instances of it are known and it may take place on a large scale. Regional Metamor phism. The other type of metamorphism is often said to be regional; sometimes it is called dynamic, but these terms have not strictly the same connotation. It may be said as a rule to make the rock more crystalline and at the same time to give it a foliated, schistose or gneissic structure. This latter consists in a definite arrangement of the minerals, so that such as are platy or prismatic (e.g. mica and hornblende, which are very common in these rocks) have their longest axes arranged parallel to one another. For that reason many of these rocks split readily in one direction (schists). The minerals also tend to aggregate in bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral. These seams are called folia (leaflets), and though never very pure or very persistent they give the rock a streaked or banded character when they are seen edgewise (Pl. IV. figs. 6, 7, 8). Along the folia composed of the soft or fissile minerals the rocks will sever most readily, and the freshly split specimen will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at face wise might be supposed to consist entirely of shining scales of mica. On the edge of the specimen, however, the white folia of granular quartz FIG. 7.-SLATE, WADEBRIDGE, CORN- FIG. 8.-MICA-SCHIST, BLAIR-ATHOLL. WALL (× 20). A fine-grained clay rock with small clear spots of quartz and minute scales of mica, chlorite, &c. The parallel arrangement of the latter is the cause of cleavage. Obscure dark lines cut across the rock and indicate the development of a secondary cross-cleavage or slip-cleavage. PERTHSHIRE, SCOTLAND (x32). A clay rock like the preceding one, but more metamorphic and coarsely crystalline. The clear spots are quartz and the bladed mineral between them is brown and white mica (biotite and muscovite). FIG. 9.-CHIASTOLITE-SLATE, SKIDDAW, CUMBERLAND (× 17). A clay rock affected by contact metamorphism attended by the production of needles of chiastolite, which have in transverse section a diamond-shape with dark enclosures at their centres and a dark cross radiating to their corners. will be visible. In gneisses these alternating folia are thicker and | J. D. Dana, Handbook of Mineralogy and Petrography (12th ed., 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. 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 Analysis 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 für Mineralogie (Stuttgart), Journal of Geology (Chicago), &c. (J. S. F.) 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; many 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 serpentines (once ultrabasic 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 PETRONEL, a 16th or 17th century fire-arm, defined by phyllite and mica-slates. They are among the most common meta- R. Barret (Theorike and Practike of Modern Warres, 1598) as morphic rocks; some of them are graphitic and others calcareous. horseman's peece." It was the fire-arm which developed The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abunon the one hand into the pistol and on the other into the carbine. dance of black and white micas and their thin, foliated, schistose The name (Fr. petrinel for poitrinal) was given to the weapon character. As a special subgroup we have the andalusite-, stauro- either because it was fired with the butt resting against the chest lite-, kyanite- and sillimanite-schists, together with the cordierite-(poitrine, Lat. pectus) or because it was carried slung from a belt gneisses, which usually make their appearance in the vicinity of gneissose granites, and 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. BIBLIOGRAPHY.-Most text-books of geology treat of petrology in more or less detail (see GEOLOGY: 8 Bibliography). Elementary books on petrology include F. H. Hatch, Petrology (5th ed., London, 1909); L. V. Pirsson, Rocks and Rock-minerals (New York, 1908); 64 a across the chest. PETRONIUS (G. (?)1 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 that he lived and wrote at Marseilles. If, however, we accept of Sidonius Apollinaris refer to him and are often taken to imply, 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 A.D. 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 1 The MSS. of the Satirae give no praenomen. Tacitus's Petronius is Gaius, though the elder Pliny and Plutarch call him Titus. The name Arbiter, given him by later writers, is not an ordinary cognomen; it may have been bestowed on him by contemporaries from the fact that his judgment was regarded as the criterion of good taste. |