  A substantial increase of the mean logarithmic mass $<\ln A>$ of galactic
cosmic rays {\em vs} energy has been observed . We study three effects that
could explain this trend i) different source spectra for protons and heavy
nuclei ii) a selective nuclear destruction in flight of heavies iii) a gradient
of the source number and chemical composition in the galactic disk.
  We take advantage of the diffusive cosmic ray propagation model developed at
LAPTH to study specifically the geometrical aspects of the propagation and
extend it to high energy. Using a simple modeling of the spectral knee around
$10^{15}$ eV, a bump in $<\ln A>$ appears. This feature is smoother when the
spectral index of protons is steeper than Fe's.
  We analyze the effects of the rigidity dependence of the diffusion
coefficient and the scale height of the confinement halo and we show that $<\ln
A>$ is most sensitive to the first parameter. Pure geometrical effects are less
determining than the diffusion coefficient spectral index. Subsequently, we
conclude that the physics of cosmic ray confinement is the essential cause of
the heavy nuclei enrichment until $\sim 10^{15}$ eV.
