  Interpretation of the energy spectrum and arrival distribution of cosmic rays
is complicated by lack of knowledge of the nature of the primaries. We review
claims for the mass composition above 1017 eV where it can be determined only
indirectly from air-shower observables. Difficulties in comparisons between
data arise because, inevitably, a set of measurements is interpreted using the
best model of hadronic interactions available at the time of analysis. We
discuss the situation and conclude that the evidence for a proton-dominated
mass composition, even at the highest energies, is unconvincing. However, it
may be that there are consistent differences between mass measurements from
optical techniques and those based upon other shower observables. We also find
that iron nuclei of ultra high energy can probably escape from the galaxies
that host GRBs, possible cosmic ray accelerators. The accelerators must lie
nearby.
