areas in this quadrangle are Sobkou and Budh Planitiae. The surface of the smooth plains material is relatively sparsely cratered, and overlap relations indicate that these plains units are younger than the intercrater plains and intermediate plains. Smooth plains also embay units of the Caloris Group. Smaller patches of smooth plains occur in depressions and old crater floors. In many areas, especially those closer to the Caloris Basin, they exhibit mare ridges like those on the Moon and thus have a rolling appearance. The boundary between smooth plains and the Odin Formation is not everywhere clear, except at high resolution. Smooth plains are mapped in the Shakespeare quadrangle only where there is no clear evidence of small hills characteristic of the Odin Formation. Interpretation of the origin of the smooth plains is difficult but significant, because it bears directly on the internal constitution and thermal history of Mercury. Like the lunar maria, the smooth plains occur on the floors of large craters and basins, and the broad swath of plains around Caloris finds an analogy with Oceanus Procellarum around Imbrium on the Moon. However, the Caloris plains differ from the maria in having no observed positive-relief volcanic features such as those sparsely scattered on the lunar maria. The absence of sharp albedo differences between the smooth plains and the older terrain (Hapke and others, 1975), compared with the distinct difference in albedo between the lunar maria and highlands, may be more indicative of composition than of origin of the rocks. On the basis of distribution and volume, Strom and others argued that in most areas the smooth plains consist of extensive sheets of basic lava similar to the lunar maria. Schultz (1977), studying modified impact craters, also argued in favor of volcanism. On the other hand, Wilhelms pointed out that the lunar light plains could also serve as an analog of the mercurian smooth plains: Apollo 16 samples indicate that lunar light plains consist of cataclastic breccia and impact melt, interpreted as being emplaced by large impact events (James, 1977). Wilhelms, therefore, proposed that the smooth plains on Mercury may be related to the Caloris impact directly, as breccias and impact melts, rather than as lavas. However, the light plains on the Moon are nowhere so well developed or extensive as the plains around Caloris, and if Wilhelms’ explanation is correct, considerable differences must exist between large impact events on