  A set of 76 open clusters with abundances based upon DDO photometry and/or
moderate dispersion spectroscopy has been transformed to a common [Fe/H] scale
and used to study the local structure and evolution of the galactic disk. The
metallicity distribution of clusters with R_GC is best described by two
distinct zones. Between R_GC = 6.5 and 10 kpc, the distribution has a mean
[Fe/H] = 0.0 and a dispersion of 0.1 dex; there is only weak evidence for a
shallow abundance gradient over this distance range. Beyond R_GC = 10 kpc, the
metallicity distribution has a dispersion between 0.10 and 0.15 dex, but with a
mean [Fe/H] = -0.3, implying a sharp discontinuity at R_GC = 10 kpc. After
correcting for the discontinuity, no evidence is found for a gradient
perpendicular to the plane. Adopting the clusters interior to 10 kpc as a
representative sample of the galactic disk over the last 7 Gyr, the cluster
metallicity range is found to be about half that of the field stars. When
coupled with the discontinuity in the galactocentric gradient, the discrepancy
in the metallicity distribution is interpreted as an indication of significant
diffusion of field stars into the solar neighborhood from beyond 10 kpc. These
results imply that the sun is NOT atypical of the stars formed in the solar
circle 4.6 Gyr ago. It is suggested that the discontinuity reflects the edge of
the initial galactic disk as defined by the disk globular cluster system and
the so-called thick disk; the initial offset in [Fe/H] created by the
differences in the chemical history on either side of the discontinuity has
carried through to the current stage of galactic evolution. If correct,
diffusion coupled with the absence of an abundance gradient could make the
separation of field stars on the basis of galactocentric origin difficult.
