From the foregoing sketch of the processes of rock-making it may easily be inferred that the varieties of rocks may be almost unlimited, and that they may be defined, named, and classified on many different bases; for example.
(1) If the mode of origin is chiefly in mind, rocks may be classed as igneous (lavas, tuffs, etc.); metamorphic (schists, gneisses, anthracite, magnetite, etc.); aqueous (water-laid sediments, stalactites, travertine, etc.); eolian (dunes, loess in part); glacial (till, moraines); clastic (mantle-rock, sandstone, conglomerate, etc.); organic (peat, lignite, coal, etc., and indirectly, limestone, chalk, infusorial earth, etc.); and so on.
(2) If the textural or structural characters are in mind, rocks are designated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure rocks); glassy (obsidian, tachylite); porphyritic (distinct crystals in obscure matrix); granitic (well-grained); compact, porous, earthy, arenaceous (sandy), schistose, etc.
(3) If the chemical composition is chiefly regarded, they may be classed as silicious, calcareous, carbonaceous, ferruginous, etc.; or, if the chemical nature is considered, they are grouped as acidic, basic, or neutral.
(4) If the crystalline character is made the basis, they are designated phanerocrystalline (distinctly crystallized), microcrystalline (minutely crystallized), cryptocrystalline (hiddenly crystallized), and amorphous (non-crystalline).
(5) If attention is fastened on certain ingredients, rocks are characterized as quartzose, micaceous, chloritic, talcose, pyritiferous, garnetiferous, etc.
(6) When rocks are regarded as mineral aggregates, if (a) the aggregates are simple, they are named from the dominant minerals, as dolomite, hornblendite, garnetite, anorthite, etc.; and if (b) the aggregates are complex they take special names, as syenite (orthoclase and hornblende), gabbro (plagioclase feldspar and pyroxene), etc.
(7) When the point of view is structure of the mass, they are classed as massive, stratified, shaly, laminated, slaty, foliated, schistose, etc.
(8) When physical state or genesis is considered, they are grouped as clastic, fragmental, or detrital (conglomeratic, brecciated, arenaceous, argillaceous, etc.); or pyroclastic (tufaceous, scoriaceous, agglomeratic); or massive, in a sense slightly different from that above (7).
As sometimes one of these characteristics and sometimes another is most important in a given rock, or in a given study, no one classification is satisfactory in all cases, yet each has its advantages in particular cases.