Petrography

Petrography is that branch of geological science which deals with rocks viewed as aggregates of mineral matter. It is a study, therefore, which is carried on chiefly indoors, its object being to ascertain the mineralogical composition, the texture, and other physical characters of rocks, for which various appliances and apparatus are required. Although petrography is properly only a description of rocks, it is hardly possible to describe rocks without reference to their geological relations and mode of origin. Hence by many geologists the term Petrology is preferred as a name for this branch of their science, while others use Lithology in a similar sense. For a general account of rocks from the point of view of their origin, reference may be made to the section Petrology under GEOLOGY.

Petrographers are hardly yet agreed on any particular classification of rocks, not certainly from the want of materials, for a very large number of so-called rock-species have been described. But in the case of the crystalline igneous rocks so many gradations exist between one kind and another that the definition of rock-species is often very difficult. As careful descriptions multiply and comparisons are made it is probable that many of the igneous rocks which flourish at present as species will come to be included as mere varieties of a few well-marked types.

In examining a rock the petrographer notes first those characters which can be seen by the naked eye (macroscopic characters), such as its structure (whether crystalline, vitreous, compact, or clastic, &c.); its state of aggregation or relative hardness; its colour; its composition; and specific gravity—which may vary from 0.6 among the hydro-carbon compounds to 3.1 among the basalts, the average specific gravity of rocks in general being about 2.5 or a little more. In the case of many coarse-grained rocks, especially those belonging to the derivative division, it is hardly requisite to go beyond a macroscopic examination. But when the rock appears to be homogeneous it is necessary to subject it to closer scrutiny. Thin slices are therefore prepared for microscopic study, after which it is frequently found that the apparently smooth compact mass turns out to be composed wholly or largely either of crystalline or of fragmental materials. Even in the case of rocks which are manifestly crystalline, and the mineral ingredients of which can be determined macroscopically, it is necessary that a microscopic examination should be made. When this is done the rock will sometimes be shown to contain minute crystalline granules and crystals, or small quantities of non-differentiated matter and glass which quite escape the unassisted eye. The minute structure of the various rock-forming minerals is likewise investigated by means of the microscope, and the chemical changes which they may have undergone since the time of their formation are carefully studied. In this way much light has been thrown on the genesis of rocks and the changes which these have subsequently experienced.

Of the minerals known to science comparatively few are rock-formers; the larger number of these are practically confined to the igneous and schistose rocks, very few entering into the formation of the derivative class. The mineral constituents of the igneous rocks are grouped as essential, accessory or adventitious, and secondary. The essential minerals are the most important, as it is upon their presence that the various species of rocks depend. Accessory minerals are accidental ingredients, the presence or absence of which does not affect the specific character of a rock; if sufficiently prominent or remarkable they merely give rise to varieties. Secondary minerals are the products of chemical changes subsequent to the formation of the rock in which they occur—the essential and accessory minerals being primary or original constituents. Among the more important essential minerals of igneous rocks are quartz, feldspar, nepheline, leucite, pyroxene, hornblende, mica, and olivine. All these also occur as accessory ingredients, and there are very many other adventitious minerals, but only the following need be named—magnetite, ilmenite (see IRON), apatite, schorl, titanite (sphene), haüyne (nosean), zircon, &c. Amongst the secondary minerals and decomposition products in igneous rocks are quartz, chaledony, calcite, oxides of iron (see IRON), zeolites, epidote, chlorite, serpentine, green-earth, &c. The chief mineral constituents of the Schistose rocks are the following: Quartz, mica, feldspar, talc, chlorite, hornblende, actinolite (see AMPHIBOLE), omphacite, smaragdite. Less prominent ingredients are andalusite and chiastolite, staurolite, ottrelite, kyanite, magnetite, schorl, sphene, epidote, pyrite, specular iron, &c. The derivative rocks having been formed out of the debris of pre-existing rock-masses, whether igneous, schistose, or sedimentary, it is obvious that they may contain many of the minerals already mentioned. Thus, there are some sandstones composed of quartz, feldspar, and mica—the debris of granite or gneiss. But most of the minerals which aqueous rocks have derived from crystalline igneous and schistose rocks are more or less altered—the felspars are kaolinised, the micas are reduced to non-elastic scales or folia of a dull gray colour and much diminished lustre, the pyroxenes, amphiboles, olivine, &c. are either unrecognisable or represented by decomposition products. Quartz, as might have been anticipated, owing to its resistance to the chemical action of water and its superior hardness, is the most common mineral constituent of derivative rocks. The clay-rocks consist in large measure of the insoluble residue of the various silicates of alumina and the alkalis and alkaline earths, of which igneous and schistose rocks are so largely composed. The readily soluble and readily precipitated minerals calcite, dolomite, rock-salt, and gypsum are also important rock-formers in certain groups of derivative rocks. As binding materials (i.e. the mineral cements which hold the grains of many sedimentary rocks together) we have quartz, chaledony, opal, &c., calcite, haematite, and limonite (see IRON), dolomite, siderite, &c. The rocks which are mainly composed of organic debris necessarily consist chiefly of calcareous and carbonaceous materials.

For purposes of description rocks may be grouped in the three following divisions: (I.) Igneous Rocks; (II.) Derivative Rocks; and (III.) Schistose Rocks.

I. Igneous Rocks (q.v.).—Of these there are two series—(a) crystalline and (b) fragmental or clastic. The crystalline series includes semi-crystalline and vitreous or glassy rocks, some of the more important characters of which may be noted. The vitreous rocks usually contain crystallites and microlites, and they often show perlitic and spherulitic structures. Some varieties are highly porous and froth-like (see PUMICE). Others are more or less homogeneous, closely compact, and smooth like bottle-glass; while yet others are markedly porphyritic, usually with crystals of sanidine (see FELSPAR). Such vitreous rocks are most usually acidic (i.e. highly siliceous), but basic glasses are also known. The semi-crystalline rocks are composed of crystalline minerals and glassy matter in very variable proportions. The crystalline ingredients often show minute inclusions of other minerals (endomorphs) or of glass, &c., which have been caught up while the crystal was growing in its molten magma. Frequently also the crystals contain minute cavities which may be empty or filled with some liquid or gas. The wholly crystalline rocks contain of course no glass or non-differentiated matter. It is in this class of rocks—many of which are of plutonic origin—that liquid cavities are of most common occurrence in the constituent minerals. In the minerals of semi-crystalline and crystalline rocks which have been erupted at or near the surface liquid cavities are less common. In vitreous, semi-crystalline, and crystalline rocks alike the mineral ingredients are not seldom disposed in lines or bands. This is called fluxion or fluidal structure—the ingredients having arranged themselves in this manner while the igneous rock was fluid and in motion. Although not unknown in some holocrystalline rocks, it is a structure which is more characteristic of the vitreous and semi-crystalline rocks which have been poured out at the earth's surface as lavas. The appearance of the original mineral constituents of many igneous rocks shows that their crystallisation cannot have taken place contemporaneously. In most cases it can be shown that they belong to two stages in the consolidation of the rock of which they form a part. Thus, in many crystalline and semi-crystalline rocks we readily distinguish a crystalline or semi-crystalline ground-mass, scattered through which occur larger crystals, many of which may be broken and corroded. These latter are believed to have crystallised while the molten rock was still at a considerable depth below the surface. Afterwards, when the fluid mass was poured out at or near the surface, and cooled rapidly, the smaller minerals and glassy matter of the ground-mass were formed. Although these two classes of minerals can be seen most clearly in rocks of a trachytoid and porphyritic aspect, yet even in granitoid rocks evidence of two stages or periods of consolidation can often be detected. The general character of fragmental igneous rocks is discussed under AGGLOMERATE, TUFF, and IGNEOUS ROCKS. Most of the rocks mentioned in the following tables have separate articles assigned to them. (a) Vitreous and Crystalline Series:

1. 1. ORTHOCLASE ROCKS: obsidian, pitchstone (structural varieties of vitreous rocks, as perlite, spherulite rock, pumice), phonolite, trachyte, liparite, orthoclase-porphyry, quartz-porphyry, syenite, granite.
2. 2. PLAGIOCLASE ROCKS: andesite and porphyrite, diorite, basalt, gabbro.
3. 3. FELSPATHOID ROCKS: nepheline-basalt, leucite-basalt.
4. 4. OLIVINE ROCKS or PERIDOTITES: pierite, lherzolite, dunite, &c.
5. 5. ALTERED IGNEOUS ROCKS: various serpentine rocks.

(b) Fragmental or Clastic Series:

Volcanic agglomerate, volcanic breccia, tuff; volcanic sand, ashes, dust, blocks, lapilli, and bombs.

II. Derivative Rocks.—As water has played a very prominent part in the formation of this great division of rocks, these are frequently termed aqueous or sedimentary. Such being the origin of by far the greater number, we find that they generally occur in layers or beds, hence the name by which they are also widely known—stratified rocks. Some of the members of this division, however, are not of aqueous origin, while others do not occur in beds. But they are all alike in so far as the materials of which they consist have been derived by epigene agents from the degradation of pre-existing rocks, minerals, and organic bodies.

1. 1. GRAVEL AND SAND ROCKS: rock-debris and breccia; rain-wash and brick-earth; soil and subsoil; shingle and gravel, conglomerate; sand, sandstone, and grit; greywacke.
2. 2. CLAY ROCKS: kaolin, pipeclay, fireclay, brick-clay, fuller's earth, boulder-clay or till, loam, mudstone, argillaceous shale.
3. 3. CALCAREOUS ROCKS: limestone and its many varieties (such as calc-sinter, chalk, oolite, marl, coral-rock, &c.), dolomite or magnesian limestone.
4. 4. IRONSTONE ROCKS: limonite, hematite, spathic iron ore (sphero-siderite, blackband ironstone), magnetic iron ore.
5. 5. SILICEOUS ROCKS: siliceous sinter, flint (chert, hornstone, lydian stone, jasper, &c.), tripoli and radiolarian ooze.
6. 6. PHOSPHATIC ROCKS: bone-breccias, guano, coprolites.
7. 7. CARBONACEOUS ROCKS: peat, lignite, coal and its varieties, anthracite, oil-shale, petroleum, asphalt.
8. 8. GYPSUM AND HALITE GROUP: anhydrite, gypsum, and rock-salt.

III. Schistose Rocks.—The more representative rocks of this division are more or less crystalline and schistose or foliated (see FOLIATION). Some, however, show faint traces either of crystalline or foliated structure; while others, although distinctly crystalline, are not schistose. Again, some of the rocks are fragmental, with more or less of super-induced crystalline structure. Many schistose rocks are clearly of metamorphic origin. They are altered igneous and derivative rocks. The origin of others is still obscure. See ARCHÆAN SYSTEM, GEOLOGY.

Quartz-rock, quartz-schist, hälleflinta, schistose conglomerate, clay-slate and its varieties, phyllite, mica-schist, talc-schist, chlorite-schist, amphibole-schist (actinolite-schist, hornblende-schist), gneiss, granulite, eclogite, garnet rock, marble.

See Rutley, The Study of Rocks (1879); Hatch, Petrology (1891); Cole, Aids in Practical Geology (1891); Lasaulx, Einführung in die Gesteinslehre (1886); Kalkowsky, Elemente der Lithologie (1886); Jannettaz, Les Roches (1884). More advanced works are Teall, British Petrography (1888); Fouqué and Lévy, Microrlogie Micrographique (1879); Rosenbusch, Mikroskopische Physitographie d. Mineralien u. Gesteine (1885); Zirkel, Mikroskopische Beschaffenheit d. Mineralien u. Gesteine (1873).