  The ratio of the muon flux density to charged particle flux density at
distances of 300 and 600 m from the shower axis ($\rhom(300)/\rhos(300)$ and
$\rhom(600)/\rhos(600)$) is measured. In addition, the energy dependence of
$\rhom(1000)$ is analysed for showers with energies above $10^{18}$ eV. A
comparison between the experimental data and calculations performed with the
QGSJET model is given for the cases of primary proton, iron nucleus and gamma-
ray. We conclude that the showers with $\E\ge3\times10^{18}$ eV can be formed
by light nuclei with a pronounced fraction of protons and helium nuclei. It is
not excluded however that a small part of showers with energies above $10^{19}$
eV could be initiated by primary gamma-rays.
