On the chemical composition of cosmic rays of highest energy

We present arguments aiming at reconciling apparently contradictory results concerning the chemical composition of cosmic rays (CRs) of highest energy, coming recently from the Auger and HiRes collaborations. In particular, we argue that the energy dependence of the mean value and root mean square fluctuation of shower maxima distributions observed by the Auger experiment are not necessarily caused by the change of the nuclear composition of primary CRs. They could also be caused by the change of distribution of the first interaction point in the cascade. A new observable, in which this influence is strongly suppressed, is proposed and tested.