Petrology

PETROLOGY, the science of rocks, the branch of geology concerned with the investigation of the composition, structure and history of the rock masses building up the accessible portions of the earth's crust (Gr. 7r6-pos, rock). Rocks are usually defined as aggregates of minerals without reference to their state of cohe sion. Loose sands, clays, gravels or soils are included among rocks as being mineral masses playing an important role in field geology. On the other hand, the less soluble parts of the skeletons of animals and plants may form a considerable portion of rocks as, for example, coral-limestone, lignite and chalk. Lastly rocks may be built up almost wholly of non-crystalline material as obsidian, pitchstone or tachylyte, representing liquids so rapidly chilled that consolidation has occurred in the glassy state. Rocks are the units with which the geologist deals in investigating the structure of the crust, and some varieties cover enormous areas. Granite, sandstone and schist often form whole provinces and build up lofty mountains, while other rock types are of such rare occurrence that they are known only in one or two localities in distant parts of the earth's surface.

Treatment of the Subject.

Broadly speaking, rocks are divided into three great classes, igneous, sedimentary, and meta morphic rocks. In this paragraph the subject matter of that part of the science of petrology concerned with igneous rocks is briefly surveyed; the object is to point out the headings under which particular subjects are treated (there is a separate article on each italicized item). Here we confine ourselves to a discussion of the nature, composition, classification and origin of igneous rocks, and the methods by which they are examined (for the other groups see SEDIMENTARY ROCKS and METAMORPHISM).

Igneous rocks occur as intrusions or extrusions. The plutonic or deep seated intrusive rocks which cooled far below the surface occur as batholiths, bosses, laccoliths and veins, and include granite, syenite diorite, gabbro and peridotite: related to the granites are aplite greisen, pegmatite and schorl-rock; to the syenites, borolanite, monzonite, nepheline-syenite and ijolite; to the diorites, aphanite, napoleonite and tonalite; to the gabbros pyroxenite and theralite, and to the peridotites, picrite and ser pentine. The hypabyssal intrusive rocks occur as sills, veins, dikes, necks, etc., and are represented by porphyry and porphyrite,

dolerite and lamprophyre; to the porphyries belong felsite and quartz-porphyry. The extrusive rocks are volcanic and found typically as lava flows; they include obsidian, perlite, pitchstone and rhyolite, phonolite and trachyte, andesite, dacite and basalt (with the related spilite, tachylyte and variolite), nephelinite and tephrite.

Intermediate between the sedimentary and igneous rocks comes the group of rocks known as pyroclastic or fragmental volcanic rocks. These include agglomerate, some types of breccia and tuff (see also VOLCANOES). The agencies which affect igneous (among other) rocks and modify them are discussed under METAMORPHISM, METASOMATISM and PNEUMATOLYSIS.

Composition of Igneous Rocks.

Though the number of recognized minerals approximates to i,000, only a comparatively small number are important as rock-formers, more especially if the subdivisions into which the common species are broken up are disregarded. The vast majority of the igneous rocks are built up of less than a dozen groups of minerals. These are (I) quartz, (2) felspars, (3) pyroxenes, (4) amphiboles, (5) micas, (6) olivines, (7) nepheline, (8) leucite, (9) iron ores and (10) apatite. To this list, for sedimentary rocks, we may add chlorite, kaolin calcite, dolomite, and a few other minerals. Metamorphic rocks are built up largely of the same minerals as igneous rocks but include special species which are foreign to igneous rocks, partly on account of their composition and partly because they appear to require special physical conditions (stress) for their production. The mineralogical nature of a rock is primarily de pendent on its chemical composition, but other factors are of first importance in determining the paragenesis of rock-forming minerals, principally the mode of origin of the rock or the physical conditions obtaining during its formation. Two rock masses may have very much the same bulk composition and yet differ funda mentally in their mineralogical assemblages. Thus, certain lam prophyres rich in biotite have much the same composition as certain leucite-basalts, a hornblendite as a camptonite and so on. Igneous rocks of identical chemical composition but dissimilar mineralogical constitution are said to be heteromorphous. The governing factors are the physical conditions attending consolida tion of the rock.