Source : Webster's Revised Unabridged Dictionary (1913)

Longmynd rocks \Long"mynd rocks"\ (Geol.)
   The sparingly fossiliferous conglomerates, grits, schists,
   and states of Great Britain, which lie at the base of the
   Cambrian system; -- so called, because typically developed in
   the Longmynd Hills, Shropshire.