  We consider a hybrid array composed of scintillation and water Cherenkov
detectors designed to measure the cosmic ray primary mass composition at
energies of about 1 EeV. We have developed a simulation and reconstruction
chain to study the theoretical performance of such an array. In this work we
investigate the sensitivity of mass composition observables in relation to the
geometry of the array. The detectors are arranged in a triangular grid with
fixed 750 m spacing and the configuration of the scintillator detectors is
optimized for mass composition sensitivity. We show that the performance for
composition determination can be compared favorably to that of Xmax
measurements after the difference in duty cycles is considered.
