in the data base; the history of plains formation almost certainly is more complex than our threefold division indicates, but we were not able to define consistent criteria of albedo, texture, and cratering for more than three plains units because of the highly variable quality of available pictures. Intercrater plains material About half of the intercrater area consists of material characterized by a very high density of small, mostly degraded craters, and an irregular to rough surface. Superposition relations suggest that this unit is about the same age as, or older than, all mappable craters and basins. The origin of intercrater plains material is enigmatic; some may be primitive crust, as implied by Trask and Guest, but more likely it is of mixed origin, dominated by breccias formed by now-unrecognizable ancient craters. Some of the more plainslike areas included within this unit may well have an origin similar to that of intermediate plains material. Cratered plains material Within the 5° overlap area with the Kuiper quadrangle to the south, an area has been mapped that displays moderately rough to rough terrain and a high density of mostly degraded craters. This unit is very similar to intercrater plains material, and cannot be distinguished from it anywhere else in the Victoria quadrangle. Most of the cratered plains material is probably volcanic in origin, but some of it may consist of impact breccias. Intermediate plains material Smooth to moderately irregular plains occupy most of the area between large craters not underlain by intercrater plains material. These plains superficially resemble the plains of the lunar maria; they generally have a relatively low albedo and are characterized by numerous elongate ridges. Like the lunar maria, the two younger mercurian plains units have been ascribed to volcanic activity, although this interpretation has been questioned. A volcanic origin seems most probable, but no compelling evidence exists in the Victoria quadrangle to support this opinion. The elongate ridges, though clearly associated with intermediate plains material, are not restricted to it. Locally, ridges extend into intercrater plains material adjacent to intermediate plains material, and large young (c4 and c3) craters superposed on the intermediate plains material commonly are transected by these ridges . Smooth plains material Partly filling most craters is plains material that is smoother and less densely cratered than intermediate plains material. Because most areas underlain by this unit are enclosed within craters, contacts between smooth plains