oldest eon of the Earth's history.Archipelago - a chain or cluster of islands.Arenaceous - Sediments consisting essentially of sand grains, that is, of quartz and rock fragments down to 0.005mm in size. Conglomerates, sandstones, grits and siltstones fall into this category Particle size 2mm to 1/16mmArenite - a) General term for consolidated sedimentary rock composed of sand sized fragments. b) clean sandstone, well sorted, <10% argillaceous matrix. Opposite to Wacke. (Etymology: Arena Latin sand)Arenitic - Pertaining to, having quality of, or resembling sandstone.Arkose - An arenaceous sedimentary rock. Like sandstone in its general character, but containing feldspar to at least 10%.Formed by the disintegration of the acid igneous rocks and gneisses.Argillaceous - Sedimentary rocks of the clay grade, i.e. composed of minute mineral fragments and crystals less than 0.005mm in diameter, also much colloidal material. Apart from finely divided detrital matter, they consist of the so-called clay minerals, such as montmorillonite, kaolinite, gibbsite and diaspore. Siltstones, mudstones, shales, clays etc.Ash - fragments less than 2 mm (0.079 inches) in diameter of pulverized rock, minerals and volcanic glass, created during volcanic eruptions.Asphalt - sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits.Asthenosphere - a region of the Earth's outer shell beneath the lithosphere. The asthenosphere is of indeterminate thickness and behaves plastically.Augite - A complex aluminous silicate of calcium, iron and magnesium, crystallising in the monoclinic system, and occurring in many igneous rocks, particularly those of basic composition. It is an essential component of basalt, dolerite and gabbro.Aureole - Zone surrounding igneous intrusion in which country rock shows effects of contact metamorphism.Autochthonous - Fossils, sediments, or rocks that were formed or produced in the location where they are now found. This term is widely applied to a coal or peat that originated at the place where the plants comprising it grew and decayed and to rocks that have not been displaced by overthrust faulting.B[edit]Banded Iron Formation - distinctive type of rock often found in primordial sedimentary rocks.Basalt - fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.Basement rock - the thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.Basic rocks - Igneous rock with low silica content (<54%). The groups ultrabasic, basic, intermediate and acid constitute a series with progressively increasing SiO2 content.Basin - rock formation scooped out by water erosion.Basin and Range Province - particular type of topography that covers much of the southwestern United States and northwestern Mexico that is typified by elongate north-south trending arid valleys bounded by mountain ranges which also bound adjacent valleys.Batholith - a large discordant pluton with an outcropping area greater than 100 square kilometers.Bedrock - native consolidated rock underlying the surface of a terrestrial planet, usually the Earth.Bioerosion - the erosion of hard ocean substrates by living organisms through a number of mechanisms.Biostratigraphy - branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages