  [Abridged abstract] We have performed an abundance analysis for 176 F- and G-
dwarfs of the Galactic thick disk component. Using accurate radial velocities
combined with $Hipparcos$ astrometry, kinematics (U, V, and W) and Galactic
orbital parameters were computed. We estimate the probability for a star to
belong to the thin disk, the thick disk or the halo.
  Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Y, Ba, Ce, Nd, and Eu have been obtained. The abundances for thick disk
stars are compared with those for thin disk members from Reddy et al. (2003).
The ratios of $\alpha$-elements (O, Mg, Si, Ca and Ti) to iron for thick disk
disk stars show a clear enhancement compared to thin disk members in the range
$-0.3 <$ [Fe/H] $ < -1.2$. There are also other elements -- Al, Sc, V, Co, and
possibly Zn -- which show enhanced ratios to iron in the thick disk relative to
the thin disk. The abundances of Na, Cr, Mn, Ni, and Cu (relative to Fe) are
very similar for thin and thick disk stars. The dispersion in abundance ratios
[X/Fe] at given [Fe/H] for thick disk stars is consistent with the expected
scatter due to measurement errors, suggesting a lack of `cosmic' scatter. The
observed compositions of the thin and thick disks seem to be consistent with
models of galaxy formation by hierarchical clustering in a $\Lambda$CDM
universe. In particular, the distinct abundance patterns observed in the thin
and thick disks, and the chemical homogeneity of the thick disk at different
galactocentric distances favor a scenario in which the majority of thick-disk
stars were formed {\it in situ}, from gas rich merging blocks.
