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A set of 76 open clusters with abundances based upon DDO photometry and/ormoderate dispersion spectroscopy has been transformed to a common [Fe/H] scaleand used to study the local structure and evolution of the galactic disk. Themetallicity distribution of clusters with R_GC is best described by twodistinct 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 ashallow abundance gradient over this distance range. Beyond R_GC = 10 kpc, themetallicity distribution has a dispersion between 0.10 and 0.15 dex, but with amean [Fe/H] = -0.3, implying a sharp discontinuity at R_GC = 10 kpc. Aftercorrecting for the discontinuity, no evidence is found for a gradientperpendicular to the plane. Adopting the clusters interior to 10 kpc as arepresentative sample of the galactic disk over the last 7 Gyr, the clustermetallicity range is found to be about half that of the field stars. Whencoupled with the discontinuity in the galactocentric gradient, the discrepancyin the metallicity distribution is interpreted as an indication of significantdiffusion of field stars into the solar neighborhood from beyond 10 kpc. Theseresults imply that the sun is NOT atypical of the stars formed in the solarcircle 4.6 Gyr ago. It is suggested that the discontinuity reflects the edge ofthe initial galactic disk as defined by the disk globular cluster system andthe so-called thick disk; the initial offset in [Fe/H] created by thedifferences in the chemical history on either side of the discontinuity hascarried through to the current stage of galactic evolution. If correct,diffusion coupled with the absence of an abundance gradient could make theseparation of field stars on the basis of galactocentric origin difficult.

Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk