even hypothesized that the source basin for material of the extensive plains of Borealis Planitia “could well be lurking in the darkness beyond the terminator.” A fuller discussion of the problem is given by Strom. Crater materials In the Borealis region, craters are mapped according to the fivefold classification proposed by McCauley and others (1981), which determines Mercurian crater ages on the basis of crater diameter and morphologic degradation. Craters less than about in diameter are not mapped. All basins between and in diameter (including those that have central peaks and peak rings) are mapped as craters. Criteria used to determine impact structures are morphologic crater components such as rays, secondary rays, hummocky rims, various facies of crater ejecta, crater geometry and structure, or a combination of these. No rayed craters ≥ in diameter were observed in the mapped area, but many moderately bright and diffuse rays extend across smooth plains material or occur as halos around very small craters in Borealis Planitia. A train of northeast-trending discontinuous rays, which extends across Borealis Planitia as far as the Goethe Basin, may radiate from small unnamed and unmapped rayed craters near the south edge of the map area. The relative scarcity of small bright-halo craters on intermediate plains material, perhaps due to unique physical properties of this material, was first noted in the Tolstoj quadrangle (Schaber and McCauley, 1980); this scarcity is also characteristic of the unit in the Borealis region. The reduced ballistic range on Mercury compared to the Moon is caused by Mercury's stronger gravitational field (McCauley and others, 1981). This phenomenon, which results in a reduced dispersion of ejecta and secondary craters, is best observed within the Borealis region around craters Verdi and Depréz. Slight differences between mercurian and lunar crater morphologies are unrelated to differences in the Mercurian and lunar gravitational fields (Cintala and others, 1977; Malin and Dzurisin, 1977, 1978;). Instead, the morphologic components of crater interiors and the abundance of central peaks and terraces on both bodies seem to be related to the physical properties of the target material (Cintala and others, 1977; Smith and Hartnell, 1978). The clusters of closely packed and overlapping large craters west of crater Gauguin and east of crater Mansart, together with nearby isolated craters and surrounding material, were mapped by Trask and Guest as heavily cratered terrain. According to them, many of the small craters superposed on the intercrater