The characteristics relating to lateral and longitudinal development of EAS in the energy region of 1015 - 1019 eV have been analyzed in the framework of the QGSJET model and of mass composition of primary cosmic rays. It is found that at E0 >= 5x1015 eV 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 E0 ~(5 - 50) x1016 eV. It is confirmed by data of many compact EAS arrays and does not contradict to an anomal diffusion model of cosmic ray propagation in our galaxy (Lagutin et al., 2001). In the superhigh energy region (>= 10**18 eV) the value <ln(a)> begins to decrease, i.e. the mass composition becomes lighter and consists from proton and nuclei of He and C. It does not contradict to 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 (Berezinsky et al., 2004).

Proceedings of the 30th International Cosmic Ray Conference; Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.); Universidad Nacional Autónoma de México, Mexico City, Mexico, 2008; Vol. 4 (HE part 1), pages 167-170