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Rh increased the demand for petroleum spirit. Petroleum has largely superseded other oils, and is still gaining ground, as a lubricant for machinery and railway rolling-stock, either alone or in admixture with fixed oils. The more viscous descriptions of mineral oils have also been found suitable for use in the Elmore process of ore-concentration by oil.

Legislation.—Since the inception of the petroleum industry, most civilized countries have prescribed by law a test of flash-point or inflammability, designed in most cases primarily to afford a definition of oils for lighting purposes which may be safely stored without the adoption of special precautions. In the United Kingdom the limit has, for the purpose in question, been fixed by the legislature at 73° F., by the “Abel test,” which is the equivalent of the former standard of 100° F. by the “open-test.” While the subject of the testing of petroleum for legislative purposes has been investigated in Great Britain by committees of both branches of the legislature, with a view to change in the law, the standard has never been raised, since such a course would tend to reduce the available supply and thus lead to increase in price or deterioration in quality. Moreover the chief object of the Petroleum Acts passed in the United Kingdom has hitherto been to regulate storage, and it has always been possible to obtain oils either of higher or lower flash-point, when such are preferred, irrespective of the legal standard, in addition to which it may be asserted that in a properly constructed lamp used with reasonable care the ordinary oil of commerce is a safe illuminant. The more recent legislation with regard to “petroleum spirit” relates mainly to the quantity which may be stored for use on “light locomotives.”

The more important local authorities throughout the country have made regulations under the powers conferred upon them by the Petroleum Acts, with the object of regulating the “keeping, sale, conveyance and hawking” of petroleum products having a flash-point below 73° F., and the Port of London authority, together with other water-way and harbour authorities in the United Kingdom, have their own by-laws relating to the navigation of vessels carrying such petroleum.

In other countries the flash-point standards differ considerably, as do the storage regulations. In France, the standard is 35° C. (Granier tester, equivalent to 98° F.), and according to their flash point, liquid hydrocarbons are divided into two classes (below and above 35° C), considered differently in regard to quantities storable and other regulations. In Germany, the law prescribes a close-test of 21° C, equal to about 70° F., whilst in Russia the standard is 28° C, equal to 84.4° F., by the close-test, in both these countries the weights of petroleum which may be stored in specified buildings are determined by law. In the United States, various methods of testing and various minimum standards have been adopted. In Pennsylvania, the prescribed limit is a “fire-test” of 110° F., equivalent to about 70° F., close-test, while in the State of New York it is 100° F., close-test.

See Sir Boverton Redwood's Petroleum and its Products (2nd ed., London, 1906), Beeby Thompson, Petroleum Mining (1910), L. C. Tassart, Exploitation du Pétrole (1908), C. Engler and H. Hofer, Das Erdöl 5 vols. (1909 seq.); A. B. Thompson, The Oil Fields of Russia (1908), and J. D. Henry, Oil Fields of the Empire (1910).

PETROLOGY, the science of rocks (Gr.  ), the branch of geology which is concerned with the investigation of the composition, structure and history of the rock masses which make up the accessible portions of the earth's crust. Rocks have been defined as “aggregates of minerals.” They are the units with which the geologist deals in investigating the structure of a district. Some varieties cover enormous areas and are among the commonest and most familiar objects of nature. Granite, sandstone, clay, limestone, slate often form whole provinces and build up lofty mountains. Such unconsolidated materials as sand, gravel, clay, soil are justly included among rocks as being mineral masses which play an important role in field geology. Other rock species are of rare occurrence and may be known in only one or two localities in distant parts of the earth’s surface. Nearly all rocks consist of minerals, whether in a crystalline or non-crystalline state, but the insoluble and imperishable parts of the skeletons of animals and plants may constitute a considerable portion of rocks, as for example, coral limestone, lignite beds and chalk.

Treatment of the Subject.—In this paragraph the subject matter of the science of petrology is briefly surveyed, the object is to point out the headings under which particular subjects are treated (there is a separate article on the terms printed in italics). General questions as to the nature, origin and classification of rocks and the methods of examination are discussed in the present article, mineralogy comprises similar matter respecting the component minerals; metamorphism, metasomatism, pneumatolysis and the formation of concretions are agencies which effect rocks and modify them. Three classes of rocks are recognized the igneous, sedimentary and metamorphic. The plutonic, or deep-seated rocks, which cooled far below the surface, and occur as batholites, bosses, laccolites, and veins, include the great classes granite, syenite, diorite, gabbro and peridotite; related to the granites are aplite, greisen, pegmatite, schorl rock and micropegmatite, to the syenites, borolanite, monzonite, nepheline-syenite and ijolite, to the diorites, aphanite, napoleonite and tonalite; to the gabbros, pyroxenite and theralite, and to peridotites, picrite and serpentine. The hypabyssal intrusive rocks, occurring as sills, veins, dikes, necks, &c., are represented by porphyry and porphyrite (including bostonite, felsite and quartz-porphyry), diabase and lamprophyre; some pitchstones belong to this group and contain crystallites and spherulites. The volcanic rocks, found typically as lava flows, include rhyolite and obsidian (with sometimes perlite), trachyte and phonolite (and leucitophyre which is treated under leucite), andesite and dacite, basalt (with the related dolerite, variolite and tachylyte), nephelinite and tephrite. Among sedimentary rocks we recognize a volcanic group (including tuff, agglomerate and some kinds of pumice), an arenaceous series such as sand (some with glauconite), sandstone, quartzite, greywacke and gravel; an argillaceous group including clay, firebrick, phyllite, laterite shale and slate; a calcareous series with chalk, limestone (often forming stalactites and stalagmites), dolomite and marls or argillaceous limestones (flint occurs as nodules in chalk); the natural phosphates may be mentioned here. The metamorphic rocks are commonly gneisses and schists (including mica-schist), other types are amphibolite, charnockite, eclogite, epidiorite, epidosite, granulite, itacolumite, hornfels, mylonite and the scapolite rocks.

Composition.—Only the commonest minerals are of importance as rock formers. Their number is small, not exceeding a hundred in all, and much less than this if we do not reckon the subdivisions into which the commoner species are broken up. The vast majority of the rocks which we see around us every day consist of quartz, felspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, haematite, limonite and a few other minerals. Each of these has a recognized position in the economy of nature. A main determining factor is the chemical composition of the mass, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is commonest in limestones, as these consist essentially of carbonate of lime; quartz in sandstones and in certain igneous rocks which contain a high percentage of silica. Other factors are of equal importance in determining the natural association or para genesis of rock-making minerals, principally the mode of origin of the rock and the stages through which it has passed in attaining its present condition. Two rock masses may have very much the same bulk composition and yet consist of entirely different assemblages of minerals. The tendency is always for those compounds to be formed which are stable under the conditions under which the rock mass originated. A granite arises by the consolidation of a molten magma (a fused rock mass; Gr. , from , to knead) at high temperatures and great pressures and its component minerals are such as are formed in such circumstances. Exposed to moisture, carbonic acid and other sub aerial agents at the ordinary temperatures of the earth's surface, some of these original minerals, such as quartz and white mica are permanent and remain unaffected, others “weather” or decay and are replaced by new combinations. The felspar passes into kaolin, muscovlte and quartz, and if any black mica (biotite) has been present it yields chlorite, epidote, rutile and other substances. These changes are accompanied by disintegration, and the rock falls into a loose, incoherent, earthy mass which may be regarded as a sand or soil The materials thus formed may be washed away and deposited as a sandstone or grit. The structure of the original rock is now replaced by a new one, the mineralogical constitution is profoundly altered; but the bulk chemical composition may not be very different. The sedimentary rock may again undergo a metamorphosis. If penetrated by igneous rocks it may be recrystallized or, if subjected to enormous pressures with heat and movement, such as attend the building of folded mountain chains, it may be converted into a gneiss not very different in mineralogical composition though radically different in structure to the granite which was its original state.

Structure.—The two factors above enumerated, namely the chemical and mineral composition of rocks, are scarcely of greater