younger but dose to the same age as the Caloris Basin materials; thus parts of the plains are probably Caloris ejecta, either impact melt or very fluid debris flows. No obvious secondary craters from Caloris have been recognized on the smooth plains. The presence of large patches of smooth plains in the floor of the Tolstoj Basin and in irregular depressions in the extreme southeastern part of the map indicates that at least some of these materials may be volcanic. However, the absence of unequivocal lava-flow fronts and well-defined volcanic vents such as those of the lunar maria prevents a firm conclusion regarding volcanic origin. Small patches of very smooth plains material occur in the floors of many of the youngest craters. The patches may consist of fallback and impact melt related to the formation of individual craters and therefore may not represent late- stage volcanic fill or volcanic modification of the more youthful mercurian craters. Schultz suggested compositional differences or endogenic modification as possible causes of the color contrasts among the floor, wall, and rim areas of the dark-halo craters Zeami (129 km diameter), Tyagaraja (97 km diameter), and Balzac (67 km diameter). The dark ejecta and floors plains of these craters are distinctly redder than the surrounding plains, whereas their anomalously bright floor patches, central peaks, and wall areas are distinctly bluer. None of these dark- halo craters has associated bright rays, although secondary craters are well preserved. Compositional implications of contrasting color differences for mercurian crater and plains materials have been discussed by Hapke and others. The bright, bluish areas in these craters are now known to be hollows. Structure The circumbasin scarps around Caloris, Tolstoj, and Mozart are the most prominent structural features in the quadrangle. The main Caloris Montes scarp is thought to approximate the edge of the basin of excavation of Caloris and is probably a structural and stratigraphic counterpart of the Montes Rook scarp around the Orientale Basin on the Moon. A subdued outer scarp is present around most of the visible part of Caloris, better seen in the Shakespeare quadrangle to the north. This scarp is generally coincident with the transition between the massifs of the Caloris Montes Formation and the lineated facies of the Van Eyck Formation. The roughly rectilinear outlines of massifs within the Caloris Montes suggest structural control by a prebasin fracture pattern. The much lower, discontinuous outer scarp