  We report studies of ultra-high energy cosmic ray composition via analysis of
depth of airshower maximum (Xmax), for airshower events collected by the High
Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a
constant elongation rate d<Xmax>/d(log(E)) of 47.9 +- 6.0 (stat.) +- 3.2
(syst.)g/cm^2/decade for energies between 1.6 EeV and 63 EeV, and are
consistent with a predominantly protonic composition of cosmic rays when
interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction
models. These measurements constrain models in which the
galactic-to-extragalactic transition is the cause of the energy spectrum
"ankle' at 4 EeV.
