Cosmic-ray composition around<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>eV

We use the depth of maximum distribution as measured by the stereo Fly's Eye detector to study the chemical composition of the primary cosmic radiation between 3 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ eV and ${10}^{19}$ eV. The analysis depends on the use of simulations to study the response of the detector as well as sensitivity to assumptions about the properties of hadronic interactions that determine cascade development. We find that the incident particles include both heavy nuclei and protons in comparable amounts. The fraction of protons appears to increase at the highest energy explored.