It is shown that evidence on cosmic-ray showers of energy 3×${10}^{16}$ to ${10}^{20}$ eV indicates that scaling in the fragmentation region is valid up to the highest energies if (and only if) hadron-air inelastic cross sections continue to rise in the manner observed at lower energies. It is also shown, with use of additional air-shower evidence, that $〈\ln A〉$, the logarithmic mean primary mass number, changes from (4±2) at 1.6×${10}^{15}$ eV to ($0_{-0\mathrm{}}^{+0.6\mathrm{}}$) at and above 3×${10}^{16}$ eV.

• Received 31 July 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.459