Composition Studies of Ultra-High Energy Cosmic Rays using $X_{max}$ by the High-Resolution Fly's Eye Observatory*

Both the mean value of airshower maximum $X_{max}$ and the width of the $X_{max}$ distribution are known to be sensitive to the chemical composition of cosmic rays initiating a set of extensive airshowers. Thus, studies of $X_{max}$ are an important part of efforts to understand the nature and origin of Ultra-High Energy Cosmic Rays, the most energetic elementary particles known in the universe. With the Northern Hemisphere's largest exposure to date to cosmic rays above $10^{18}$~eV, the two fluorescence detectors of the High-Resolution Fly's Eye (HiRes) Observatory are uniquely positioned to study this phenomenon. Here, we report the results of recent studies of the depth of airshower maximum $X_{max}$ in cosmic-ray induced extensive airshowers observed in stereo by HiRes.