Most publications on planetary interiors in the last 4 years have dealt with the moon, and the most popular topic has been the interpretation of lunar mascons (the nature and discovery of mascons are covered in the review ‘Selenodesy and Planetary Geodesy’ by W. M. Kaula). Urey  and Stipe  suggest that the mass concentrations are dense meteoritic material, remnants of the same meteoritic projectiles that excavated the mare basins on which the mascons are centered. Conel and Holstrom  attribute the gravitational anomalies to flat plates of dense rock, probably lava, that filled crater basins in less-dense lunar crustal material. O'Keefe , Baldwin [1968a], and Wood et al.  arrived at almost the same conclusion. Kaula  and Wise and Yates  propose that plugs of denser mantle material were pushed up into the mare basins. Kaula suggests the plug was driven past the position of isostatic equilibrium by excess lithostatic pressures generated in the moon by more rapid cooling of, and consequent shrinkage of, the outer layers of the moon; Wise and Yates halt the mantle plug at the isostatically compensated level but then fill the remaining topographic low with lava.