the Moon and Mercury. Most likely, large parts of the smooth plains are of volcanic origin, although in some areas they may be of impact-melt origin. The very smooth plains on Mercury were included in the smooth plains unit by Trask and Guest. Here the geologic units are mapped separately, because very smooth plains material is clearly younger than smooth plains material. The very smooth plains unit, which is featureless and has no resolvable superposed craters, is possibly ejecta fallback on the floors of craters. However not all craters contain this material; some are floored by material with a rugged surface mapped as crater floor material, because it is analogous to the floor material in younger lunar craters such as Copernicus or Aristarchus. One other possibility is that the very smooth plains are volcanic. Crater materials The craters on Mercury show various states of preservation, ranging from crisp-featured craters with bright rays to those that are almost totally obliterated and consist only of a subdued ring of heavily cratered hills. As on the Moon, the principal eroding process is likely impact; thus, a fresh crater will be degraded systematically over time. Craters of similar size that show similar states of preservation are therefore considered to be of about the same age. Craters are mapped according to a fivefold classification on the basis of their states of degradation (McCauley and others, 1981). The components used to define the crater ages are rays, secondary craters, ejecta facies, central peaks and rings, rim form, and inner terraces. As a crater ages, the number of superposed craters increases and each of the morphologic elements becomes more subdued. Volcanic activity also may bury or destroy certain crater components, but the crater may still be dated by the preservation of the remaining rim. On the basis of mapping in this quadrangle and in the adjacent Tolstoj quadrangle (Schaber and McCauley, 1980), the Caloris impact is considered to have occurred in late c3 time (McCauley and others, 1981). One problem with the above-mentioned crater-dating technique on Mercury is that secondary craters occur closer to the principal crater and are thus more clustered than on the Moon, where they are relatively widespread. In consequence, an older crater adjacent to a fresh one becomes strongly degraded as a result of heavy bombardment by secondary craters from the younger crater and appears much older than it is. Ghost craters are