  By means of local Hall probe ac and dc permeability measurements we
investigated the phase diagram of vortex matter for the HgBa$_2$CuO$_{4+\delta
}$ superconductor in the regime near the critical temperature. The second peak
line, $H_{\rm sp}$, in contrast to what is usually assumed, doesn't terminate
at the critical temperature. Our local ac permeability measurements revealed
pronounced hysteretic behavior and thermomagnetic history effects near the
onset of the second peak, giving evidence for a phase transition of vortex
matter from an ordered qausilattice state to a disordered glass.
