We find that the ratio r_μe 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_μe 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_μe in isolated events.Facultad de Ciencias ExactasInstituto de Física La Plata