Metallicities and horizontal-branch population gradients are used to distinguish between two populations of Galactic halo globular clusters. The β population of clusters appears to be almost coeval and occurs mainly at R < R_sun_. Clusters of the β population were probably formed during a rapid Eggen, Lynden-Bell, & Sandage (ELS)-type collapse of the inner proto-Galaxy. Clusters of the α population all occur at R > 8 kpc. Many of these objects are found to lie on plunging retrograde orbits. This suggests that clusters of the α population formed during an extended Searle-Zinn-type merger and collapse phase. The relative ages, metallicities, and locations of the α and β populations indicate, as has also been suggested by Larson and by Lee, that the Galaxy formed inside out, with the dense proto-Galactic core forming stars and clusters before its lower density halo. It is very tentatively suggested that the metal-poor bulge cluster NGC 6287 might be the oldest Galactic globular cluster. In a massive galaxy the quasar phenomenon probably takes place after the bulk of its globular cluster system has formed. A quasar outburst may have guillotined cluster formation in the nearby S0 galaxy NGC 3115, thus preventing the formation of a thick-disk globular cluster population.
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Sidney van den Bergh (1993) studied this question.