Hugoniot equation of state measurements have been performed on pure synthetic single-crystal forsterite (Mg2SiO4) in the pressure range 70--160 GPa (0.7--1.6 Mbar). These and earlier data for polycrystalline forsterite are compared with theoretical Hugoniots for the assemblages 2MgO (rocksalt) + SiO2 (stishovite) and MgO (rocksalt) + MgSiO3 (perovskite). The densities attained by single-crystal forsterite at pressures in excess of 120 GPa are greater than those expected in the event of shock-induced transformation to the isochemical oxide mixture. A similar test of the hypothesis of shock-induced transformation to the perovskite-bearing assemblage is sensitive to the choice of MgSiO3 (perovskite) bulk modulus. Recent static compression measurements of Yagi et al. (1978) yield a KoT of 286 GPa (for KoT' = 5), which, along with other elastic and thermodynamic parameters, suggests that shocked forsterite may be more dense than the perovskite-bearing assemblage. Crystalline phases of up to 5% greater zero-pressure density or equally dense short-range-order-only phases may well be involved. Alternatively, the use of an isentropic bulk modulus of 250 GPa (estimated by Liebermann et al., 1977) for MgSiO3 (perovskite) allows consistency between the data and the calculated MgO+MgSiO3 (perovskite) Hugoniot for a reasonable choice (~ 3.8) of Kos' for the latter phase. The new forsterite data along with high-pressure Hugoniot data for other olivines and olivinitic rocks define a smooth isobaric variation of Hugoniot density with composition. It is shown that an estimated pyrolite (Ringwood, 1975) Hugoniot density of 5.31 g/cm3 at 120 GPa is ~2% less dense than inferred from typical lower mantle density profiles.