The average mass composition of cosmic rays with primary energies between $10^{17}$eV and $10^{18}$eV has been studied using a hybrid detector consisting of the High Resolution Flys Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum, $X_{max}$, and in the change in the muon density at a fixed core location, $rho_mu(600m)$, as a function of energy. The composition has also been evaluated in terms of the combination of $X_{max}$ and $rho_mu(600m)$. The results show that the composition is changing from a heavy to lighter mix as the energy increases.
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