  The space velocities and Galactic orbital elements of stars calculated from
the currently available high-accuracy observations in our summary catalog of
spectroscopic magnesium abundances in dwarfs and subgiants in the solar
neighborhood are used to identify thick-disk objects. The relative magnesium
abundances in thick-disk stars are shown to lie within the range
0.0<[Mg/Fe]<0.5 and to decrease with increasing metallicity starting from
[Fe/H]=-1.0. This is interpreted as evidence for a longer duration of the star
formation process in the thick disk. We have found vertical gradients in
metallicity (grad_Z[Fe/H]=-0.13\pm 0.04 kpc^{-1}) and relative magnesium
abundance (grad_Z [Mg/Fe]=0.06\pm 0.02 kpc^{-1}), which can be present in the
subsystem only in the case of its formation in a slowly collapsing protogalaxy.
The large spread in relative magnesium abundance (-0.3<[Mg/Fe]<0.5) in the
stars of the metal-poor "tail" of the thick disk which constitute 8% of the
subsystem, can be explained in terms of their formation inside isolated
interstellar clouds that interacted weakly with the matter of a single
protogalactic cloud. We have found a statistically significant negative radial
gradient in relative magnesium abundance in the thick disk (grad_R
[Mg/Fe]=-0.03\pm 0.01 kpc^{-1}) instead of the expected positive gradient. The
smaller perigalactic orbital radii and the higher eccentricities for
magnesium-richer stars, which among other stars, are currently located in a
small volume of the Galactic space near the Sun are assumed to be responsible
for the gradient inversion. A similar but statistically less significant
inversion is also observed in the subsystem for the radial metallicity
gradient.
