  We analyze the relations between the relative magnesium abundances in stars
and their metallicities, Galactic orbital elements, and ages. The relative
magnesium abundances in metal-poor ([Fe/H] < -0.4) thin-disk stars have been
found to systematically decrease with increasing stellar orbital radii in such
a way that magnesium over abundances ([Mg/Fe]>0.2 dex) are essentially observed
only in the stars whose orbits lie almost entirely within the solar circle. At
the same time, the range of metallicities in magnesium-poor stars is displaced
from (-0.5<[Fe/H]<+0.3 dex) to (-0.7<[Fe/H]<+0.2 dex) as their orbital radii
increase. This behavior suggests that, first, the star formation rate decreases
with increasing Galactocentric distance and, second, there was no star
formation for some time outside the solar circle while this process was
continuous within the solar circle. The decrease in the star formation rate
with increasing Galactocentric distance is responsible for the existence of a
negative radial metallicity gradient (grad_{R}[Fe/H]=(-0.05 \pm 0.01) kpc^{-1})
in the disk, which shows a tendency to increase with decreasing age. At the
same time the relative magnesium abundance exhibits no radial gradient. We have
confirmed the existence of a steep negative vertical metallicity gradient
(grad_Z [Fe/H]=(-0.29 \pm 0.06) kpc^{-1}) and detected a significant positive
vertical gradient in relative magnesium abundance (grad_Z [Mg/Fe]=(0.13 \pm
0.02) kpc^{-1}); both gradients increase appreciably in absolute value with
decreasing age. We have found that there is not only an age-metallicity
relation, but also an age-magnesium abundance relation in the thin disk.
