Adobe—a fine silty or loamy deposit formed by gentle wash from slopes and subsequent lodgment on flats; especially applied to silty accumulations in the basins and on the plains of the western dry region.
Agglomerate—an aggregate of irregular, angular, or subangular blocks of varying sizes, usually of volcanic origin, distinguished from conglomerate in which the constituents are rounded.
Alluvium—sediment deposited by streams.
Amygdaloid—a vesicular igneous rock whose cavities have become filled with minerals; the fillings are called amygdules, because sometimes almond-like in form.
Andesite—an aphanitic igneous rock consisting essentially of the plagioclase feldspar andesine (sometimes oligoclase) and pyroxene (or some related ferromagnesian mineral); sometimes cellular, porphyritic, or even glassy; usually rich in feldspar microlites.
Anorthosite—a rock consisting mainly of the feldspar labradorite.
Aphanite—a rock whose constituents are so minute as to be indistinguishable to the naked eye; rather a condition of various rocks than of any specific rock.
Aqueous rocks—a general term applied to rocks deposited through the agency of water.
Arenaceous rocks—either those which are mainly sand or those in which sand is a notable accessory.
Argillite—a clayey rock; usually applied to hard varieties only.