  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 Fly's 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.
