  We explore the feasibility of estimating primary cosmic ray composition at
ultra high energies from the sum of muon, electron and photon densities and the
depth of maximum of extensive air showers detected by the Auger Observatory.
  From the information of the Fluorescence ($X_{max}$) and water Cerenkov
detectors ($\rho(1000)$) we infere the most probable type of primary which
originated the shower. The method is tested simulating Extensive Air Showers
(EAS) at energies up to $10^{20}$ eV using a version of MOCCA/Sibyll adapted
for Dec Alpha Servers. We also discuss some results on the mass composition
using the depth of the shower maximum and the lateral distribution of showers
in the energy range $10^{18}$-$10^{20}$ eV.
