  We find that the ratio r_{mu e} of the muon to the electromagnetic component
of an extended air shower at the ground level provides an indirect measure of
the depth X_{max} of the shower maximum. This result, obtained with the
air-shower code AIRES, is independent of the hadronic model used in the
simulation. We show that the value of r_{mu e} in a particular shower
discriminates its proton or iron nature with a 98% efficiency. We also show
that the eventual production of forward heavy quarks inside the shower may
introduce anomalous values of r_{mu e} in isolated events.
