  The composition of Ultra High Energy Cosmic Rays (UHECR) is measured with the
High Resolution Fly's Eye cosmic ray observatory (HiRes) data using the Xmax
technique. Data were collected in stereo between 1999 November and 2001
September. The data are reconstructed with well-determined geometry.
Measurements of the atmospheric transmission are incorporated in the
reconstruction. The detector resolution is found to be 30 g cm^-2 in Xmax and
13% in Energy. The Xmax elongation rate between 10^18.0 eV and 10^19.4 eV is
measured to be 54.5 +/- 6.5 (stat) +/- 4.5 (sys) g cm^-2 per decade. This is
compared to predictions using the QGSJet01 and SIBYLL 2.1 hadronic interaction
models for both protons and iron nuclei. CORSIKA-generated Extensive Air
Showers (EAS) are incorporated directly into a detailed detector Monte Carlo
program. The elongation rate and the Xmax distribution widths are consistent
with a constant or slowly changing and predominantly light composition. A
simple model containing only protons and iron nuclei is compared to QGSJet and
SIBYLL. The best agreement between the model and the data is at 80% protons for
QGSJet and 60% protons for SIBYLL.
