areas may be secondaries from the large craters. They also noted that the interiors of these large craters are filled with material that is less cratered, smoother, and therefore younger than the intercrater plains material. Two types of ghost craters occur in the Borealis region; both are nearly obliterated by smooth plains material. In one type found along the northwest border of Suisei Planitia (Guest and Greeley, 1983), only the uppermost parts of walls and rims protrude above smooth plains material. Ghost craters of this type display rounded rim crests that are densely cratered with secondaries, a feature typical of the rough surface of intercrater plains material. These craters are floored by smooth plains material and are therefore older than it; a similar relation occurs on the Moon, where the crater Archimedes is seen to be older than the mare material it contains. Another type of ghost crater, common in Borealis Planitia, is recognized only by an irregular or thin outline of a rim crest under a thin mantle of smooth plains material; the buried rim crest is shown on the map. The polygonal ghost crater centered at lat 82.5° N., long 100° W., northwest of Depréz, is a transitional form between these two types. Polar darkening is generally lacking on Mercury (Hapke, 1977), but darkening in restricted areas may be due to vapor-phase deposition accompanying micrometeorite impacts (Hapke, 1977). In the Borealis region, surface darkening affects some crater floors, and low-albedo areas are mapped in both intermediate plains and smooth plains materials. The low-albedo plains are marginal to the borders of Borealis and Suisei Planitiae, which suggests that darkening may be due to internal volatile materials escaping along the fractured margins of unrecognized buried or very degraded basins. Structure One of the major differences between the mercurian and lunar surfaces is “the widespread distribution [on Mercury] of lobate scarps which appear to be thrust or reverse faults resulting from a period of crustal compression...” These scarps are unique structural landforms that were noted soon after the acquisition of Mariner 10 photographs. Murray and others (1974) described them as having a sinuous outline, a slightly lobate front, and a length of more than 500 km. A more detailed description is given by Strom and others. Dzurisin (1978) classified these scarps, differentiating between intercrater and intracrater scarps (a scheme adopted in mapping the Borealis region) in an attempt to understand the