Rock
any portion of the earth's crust, coherent or incoherent, any sedimentary stratum or any dyke or overlying mass of volcanic or plutonic mineral matter.
The older writers drew a distinction between rocks and soils. Both are now regarded as rocks. Most rocks, originally soft, have become hard and compact by losing their moisture and being subjected to pressure. As a rule, a rock is not a bed of some simple mineral. In most cases there are crystals cemented together by imperfectly crystalline or amorphous matter, or there is a mixture of angular and rounded grains, also bound together by mineral matter. See MINERALOGY. Viewed as to composition, there are three leading classes of rock:
Siliceous or arenaceous, some formed of loose sand, others of hard sandstone, with all intermediate grades; argillaceous rocks, i. e., rocks of clay, or more specifically having one-fourth alumina rocks composed chiefly of carbonate of lime, some of them proved and most of the others suspected, to be originally composed of various organisms. Viewed as to their origin, Lyell long recognized four kinds of rocks. Aqueous or sedimentary, volcanic, metamorphic, and plutonic. tonic. A fifth category has now been superadded, viz., aërial or colian, formed by the action of wind. Aqueous, æolian, and metamorphic rocks are, as a rule, stratified; volcanic and plutonic rocks generally unstratified; the last two are Some stratified rocks called igneous. are unfossiliferous, others fossiliferous.
For the stratigraphical or chronological order of the latter, see FOSSILS. See GEOLOGY.