superposition
  重疊,疊合,重合
  Su·per·po·si·tion n.  The act of superposing, or the state of being superposed; as, the superposition of rocks; the superposition of one plane figure on another, in geometry.
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  superposition
       n 1: (geology) the deposition of one geological stratum on
            another
       2: (geology) the principle that in a series of stratified
          sedimentary rocks the lowest stratum is the oldest [syn: principle
          of superposition, superposition principle]
       3: (geometry) the placement of one object ideally in the
          position of another one in order to show that the two
          coincide
       4: the placement of one thing on top of another