The Collaborative International Dictionary of English v.0.48:

Taconic \Ta*con"ic\, a. (Geol.)
   Designating, or pertaining to, the series of rocks forming
   the Taconic mountains in Western New England. They were once
   supposed to be older than the Cambrian, but later proved to
   belong to the Lower Silurian and Cambrian.
   [1913 Webster]