Rocks, in geology, the solid mineral masses which make up the earth's crust. These may be considered both geologically and minera-logically; mineralogy is the natural history of all such bodies as do not belong to the organic kingdoms of nature. In the geological investigation of rocks two questions arise: first, as to their structure and attitude and the mode of their arrangement in the earth's crust, whether stratified or unstratified, whether occurring in beds, veins, or intruded masses; and second, their origin and mode of formation. The geognostical relations of rocks, and the distinctions of crystalline and uncrystalline, of stratified and unstratified, of indigenous, exotic, and endogenous rocks, have been defined in the article Geology. Mineralogically rocks may be homogeneous or heterogeneous; that is to say, they may consist of one or of two or more mineral species. Thus a pure white marble is made up entirely of calcite, a form of carbonate of lime, and quartzite consists of the mineral quartz, both homogeneous; while granite is heterogeneous, consisting of a mixture of quartz and feldspar, sometimes with the addition of mica or of hornblende. (See Granite.) These component mineral species are sometimes so arranged as to show that the rock has resulted from an original crystallization, as in the case of granite or vitreous quartzite; and at other times, as in the case of sandstones and conglomerates, the aggregate is seen to be composed of the ruins of such rocks rearranged and cemented together.
Hence the great distinction between original and derived rocks, the former including all exotic or eruptive rocks and all endogenous rocks or veinstones, as well as the crystalline indigenous rocks. Certain rocks owe their origin directly to the accumulations of organic structures; such are coal, which consists of vegetable remains, many limestones, which are made up of corals, shells, or encrinites, and certain silicious beds composed of the shields of diatoms. We have thus a distinction, which is sometimes made, of chemically, mechanically, and organically formed rocks. The chief mineral species of original rocks are calcite, dolomite, gypsum, anhydrite, rock salt, quartz, orthoclase, albite and the related tri-clinic feldspars, nepheline and certain zeolites, the micas, chlorites, talc, serpentine, olivine, pyroxene, hornblende, garnet, epidote, stauro-lite, kyanite, andalusite, tourmaline, graphite, magnetite, hematite, corundum, and pyrite. A few of these form rocks by themselves; others are essential ingredients of composite rocks; while others occur as accessory though characteristic minerals in certain rock masses.
The compound or heterogeneous rocks can be accurately defined only by describing the component minerals, their proportions and mode of arrangement, and the texture and structure of the mass. Arbitrary names have been given to certain types of composite rocks, but the student soon learns that there are many intermediate varieties and admixtures which it is difficult to name or to classify. In describing rock masses the geognostical distinctions of indigenous, exotic, and endogenous are to be disregarded, as in very many cases it is impossible from the study of a specimen to say to which division it belongs. Thus we have indigenous and endogenous crystalline limestones, and in the case of granitic rocks the characters of indigenous, endogenous, and exotic are often so similar that it is only by study of the rock in situ that it can be determined to which class it belongs. The structure of original rocks is not always crystalline; some, like pearlstone and obsidian, being glasslike and amorphous. Others, though crystalline, are so finely grained as to be compact, and are designated as crypto-crystalline. To rocks in which distinct crystals are imbedded in a compact or crypto-crystalline base the name of porphyry is given, and this is sometimes extended to rocks in which the base enclosing the crystals is not compact.
The terms gneissic and granitic, or gneissoid and granitoid, are sometimes employed to designate rocks which, although unlike in composition, resemble gneiss or granite in structure and texture. The principal homogeneous original rocks are those composed of quartz, of carbonate of lime, and of dolomite. Gypsum, the ores of iron, and occasionally certain silicates, such as labradorite, serpentine, talc, and chlorite, form by themselves considerable rock masses. The most important indigenous rocks are heterogeneous, and foremost among these may be named those essentially made up of quartz and orthoclase feldspar, constituting the granitic rocks, which generally include mica or hornblende as an accessory mineral. The indigenous banded rocks of this composition take the name of gneiss, and are either horn-blendic or micaceous, the latter passing into mica schist, so common with gneiss in the Montalban or White mountain series. By the admixture in the hornblendic varieties of a triclinic feldspar (albite, oligoclase, or labra-dorite), which finally replaces the orthoclase, and the disappearance of the quartz, we get the rock known as diorite; and the substitution in such a rock of some form of pyroxene for hornblende produces what are known as dolerite and diabase.
To these last named three rocks belong most of the so-called traps, basalts, and greenstones, which are generally intrusive or exotic rocks, although indigenous rocks, composed of triclinic feldspars with a greater or less admixture of hornblende or of pyroxene, are abundant. Chlorite often accompanies the hornblende of these rocks, or replaces it, especially in the Green mountain or Huronian series, where such rocks are associated with strata in which a soft hydrous mica prevails, forming the so-called talcose slates, which seldom contain talc. Rocks composed chiefly of labradorite, sometimes without admixture, and at other times with small portions of hypersthene or of pyroxene, predominate in the Norian series. Under the title Volcano will be considered the history of volcanic rocks, and there and under Water will be discussed the chemical agencies which have produced the various rocks, the genesis of which cannot be well understood without a reference to the chemico-geological effects of fire and of water.
Much has been done of late in the minute study of the crystalline rocks, and here the microscope has been used with much success. - Among the best works on the subject of lithology are those of Bern-hard von Cotta, Senft, and especially Zirkel.