  The Pierre Auger Observatory infers the chemical composition of
ultra-high-energy cosmic rays through two independent detection techniques. The
Fluorescence Detector (FD) measures the longitudinal profile of high energy air
showers and can determine the depth of the shower maximum $X_{max}$, which is
sensitive to the chemical composition of the primary cosmic rays. Additionally,
measurements by the Surface Detector (SD) provide independent experimental
observables based on the muonic shower component to analyze the chemical
composition. We present the results for the $X_{max}$ distributions and the
mass composition results measured by the FD and the SD for the energies $E \geq
10^{18}$\,eV. The data will be compared with the expectations for proton and
iron primaries according to different hadronic interaction models.

High Energy Astrophysical Phenomena;Instrumentation and Methods for Astrophysics