The Collaborative International Dictionary of English v.0.48:

Perdurable \Per*dur"a*ble\ (p[~e]r*d[=u]r"[.a]*b'l; 277), n.
   [Cf. F. perdurable, OE. pardurable. See Perdure.]
   Very durable; lasting; continuing long. [Archaic] --Chaucer.
   --Shak.
   [1913 Webster] -- Per*dur"a*bly, adv. [Archaic]
   [1913 Webster] Perdurance

WordNet (r) 3.0 (2006):

perdurable
    adj 1: very long lasting; "less durable rocks were gradually
           worn away to form valleys"; "the perdurable granite of
           the ancient Appalachian spine of the continent" [syn:
           durable, indestructible, perdurable,
           undestroyable]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "perdurable":
   abiding, age-long, aged, ageless, ancient, antique, ceaseless,
   chronic, coeternal, constant, continual, continuing, continuous,
   dateless, diuturnal, durable, endless, enduring, eternal, eterne,
   ever-being, ever-durable, ever-during, evergreen, everlasting,
   everliving, hardy, illimitable, immemorial, immutable, incessant,
   indestructible, infinite, interminable, intransient, inveterate,
   lasting, long-lasting, long-lived, long-standing, long-term,
   longeval, longevous, macrobiotic, never-ceasing, never-ending,
   nonstop, nonterminating, nonterminous, of long duration,
   of long standing, olamic, perduring, perennial, permanent,
   perpetual, persistent, persisting, remaining, sempervirent,
   sempiternal, stable, staying, steadfast, steady, timeless, tough,
   unceasing, unending, unfading, unintermitting, uninterrupted,
   unremitting, vital, without end