Estimate of the mass composition of cosmic rays above $4\times 10^{19}$ eV

The arrival directions of extensive air showers are analyzed on the basis of world data. It is found that the zenith-angle distributions for energies $E>10^{19}$ eV and $E>4\times 10^{19}$ eV differ from each other. According to our estimates, the SUGAR array detects no showers above $10^{20}$ eV. The mass composition of ultrahigh-energy cosmic rays is estimated. Cosmic rays with $E>4\times 10^{19}$ eV most likely consist of superheavy nuclei.