  The mixed phase of extremely type-II layered superconductors in perpendicular
magnetic field is studied theoretically via the layered XY model with uniform
frustration. A partial duality analysis is carried out in the weak-coupling
limit. It consistently accounts for both intra-layer (pancake) and inter-layer
(Josephson) vortex excitations. The main conclusion reached is that
dislocations of the two-dimensional (2D) vortex lattices within layers drive a
unique second-order melting transition at high perpendicular fields between a
low-temperature superconducting phase that displays a Josephson effect and a
high-temperature ``normal'' phase that displays no Josephson effect. The former
state is best described by weakly coupled 2D vortex lattices, while the latter
state is best characterized by a decoupled vortex liquid. It is further argued
on the basis of the duality analysis that the second-order melting transition
converts itself into a first-order one as the perpendicular field is lowered
and approaches the dimensional cross-over scale. The resulting critical
endpoint potentially accounts for the same phenomenon that is observed in the
mixed phase of clean high-temperature superconductors.
