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Webster 1913 Edition

Noun.

[F.

terrain

, from L.

terra

earth.]

(Geol.)

A group of rocks having a common age or origin; – nearly equivalent to formation, but used somewhat less comprehensively.

terrane ‎(plural terranes)

(geology) A block of the Earth's crust that differs from the surrounding material, and is separated from it by faults.
- 2004, Richard Fortey, The Earth, Folio Society 2011, p. 195:
  The mountain belt came together as an accretion of terranes.