  The characteristics relating to the lateral and longitudinal development of
EAS in the energy region of 10^15-10^19eV have been analyzed in the framework
of the QGSJET model and of mass composition of primary cosmic rays. It is found
that at E(0) >= 5*10^15eV the mean mass composition of primary cosmic rays
begins to vary as indicated by a rise of <ln A> with increasing energy. The
maximum value of <ln A> is observed at E(0) ~ (5-50)*10^16eV. It is confirmed
by data of many compact EAS arrays and does not contradict an anomalous
diffusion model of cosmic ray propagation in our Galaxy. In the superhigh
energy region (>=10^18eV) the value <ln A> begins to decrease, i.e. the mass
composition becomes lighter and consists of protons and nuclei of He and C. It
does not contradict our earlier estimations for the mass composition and points
to a growing role of the metagalactic component of cosmic rays in the superhigh
energy region.
