An experimental study of the distribution of arrival time of energetic hadrons relative to associated air shower particles has been made at a mountain altitude of 730 g.cm−2. Monte Carlo simulations of the expermental observations have shown that these observations are sensitive to the composition of primary cosmic rays of energies 1013–1015eV. The energy spectra of primary protons and iron group nuclei required to understand these observations are (dN/dE)protons=1.5×104 E−2.71±0.06m−2sr−1sec−1Gev−1(dN/dE)Fegroup=1.27 E−2.36±0.06m−2sr−2sec−1Gev−1respectively, where E is energy per nucleon, indicating that iron group nuclei are the dominant component of primary cosmic rays at air shower energies of 1014–1016eV.