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Chalk is a soft, white, powdery limestone consisting mainly of fossil shells of foraminifers
From old English cealc chalk, lime, plaster; pebble, from Greek khalix small pebble, in English transferred to the opaque, white, soft limestone
Fine Grained Rock, Opaque Rock
Very fine grained fault rock which is composed of glassy matrix that often contains inclusions of wall-rock fragments.
From pseudo- + tachylite, a glassy rock generated by frictional heat within faults.
Fine Grained Rock, Opaque Rock