Rock
3 of the 6 encyclopedias on this shelf carry an entry for Rock. All 3 are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
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 plubeen tonic. tonic. A fifth category has now 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.
Aiton's Encyclopedia (1910)
in geology any solid portion of the earth's crust. This definition includes sand, clay, and the other kinds of earth, as well as the various sorts of stone. From the viewpoint of the chemist, oxygen forms a half of all the rocks, and silicon, an element of sand, forms a fourth. The remaining fourth is composed chiefly of aluminum, iron, calcium (lime), sodium (salt), potassium, and magnesium. The remaining sixty elements, all taken together, contribute less than one per cent of the rock material of the globe. See Granite ; Quartz ; Feldspar; Mica ; Kaolin ; Salt ; Gypsum ; Clay ; Limestone ; Sandstone ; also articles on the several metals and precious stones.
The Encyclopedia of Founding (1892)
A stony substance which forms a great part of the earth's crust, sometimes loose and friable-like sand, and again compact, like granite and limestone. Rocks are classified as primitive, rocks of transition, stratified, alluvial depositions, and volcanic, and modifications resulting from the conditions to which they have been exposed.
What Has Changed in the Last Century
The three-, four- or five-fold schemes offered here — Collier's 1921 list of aqueous, volcanic, metamorphic, plutonic and aeolian, and the 1892 talk of "primitive" and "transition" rocks — have collapsed into the simple igneous/sedimentary/metamorphic triad taught today, with the old idea of a separate "primitive" or "plutonic" class abandoned. The far larger change is that geologists now know how old rocks are and why they move: radiometric dating, developed through the 20th century, puts the oldest intact rock (the Acasta gneiss of northern Canada) at about 4.03 billion years, and the plate tectonic revolution of the 1960s supplied the mechanism for mountain building, volcanism and metamorphism that these entries could only describe. Aiton's 1910 chemistry, though, has held up remarkably well — oxygen still makes up roughly half the crust by weight and silicon about a quarter. Since 1969 the subject has also grown beyond the Earth, with hundreds of kilograms of lunar rock returned by Apollo and Luna and rock analyses done in situ on Mars.
Written for this edition, 2026.