  In this work we consider the eruption of a tenuous relativistic hydrodynamic
jet from a dense baryonic envelope. As the jet moves out and away, it carries
along and continues to accelerate a layer of baryonic material which we refer
to as the plug. We solve the relativistic equations of motion for the
trajectory of the plug, and verify it using a relativistic hydrodynamic
simulation. We show that under these conditions, the plug breaks up at a radius
larger by a factor of a few from the radius of the envelope, due to the onset
of the Rayleigh Taylor instability. After breakup the jet continues to
accelerate to higher Lorentz factors while the plug fragments maintain a
moderate Lorentz factor. The presence of slower moving ejecta can explain late
time features of GRBs such as X ray flares without recourse to a long lived
engine.
