tectonic and volcanic history of Mercury. Melosh (1977) and Melosh and Dzurisin (1978) proposed a planetary grid composed of conjugate northeastand northwest-trending shear fractures formed by the stresses of tidal despinning early in mercurian history. They thought that these fractures were later modified, and predicted that east-trending normal faults caused by tensional stresses would be found in the polar regions. In a later report, Pechmann and Melosh (1979, p. 243) stated that “the NE and NW trends become nearly N-S in the polar regions.” The northwest-trending component of the postulated global grid of fractures is markedly absent in the Borealis region. Northeast-trending scarps and troughs are conspicuous, however, across intercrater plains material and in crater fill (smooth plains material) between the 155° and 185° meridians, and from crater Van Dijck northward to crater Purcell and beyond. The scarps tend to be straight in intercrater plains material, but become notably lobate in crater fill (for example, within Saikaku). This set of northeast-trending scarps and troughs, and another set of north-trending scarps and troughs within and north of crater Van Dijck, probably follow zones of structural weakness in the mercurian crust. Ancient fractures that were reactivated by later impacts may have first provided the conduits for crater fill (smooth plains material) and later been propagated upward through the fill. That these ridges, scarps, and troughs are parts of a global grid of fractures cannot be stated conclusively because of their proximity to the terminator and the lack of photographic coverage beyond the 190° meridian. Some scarps probably were formed by normal faulting of the smooth plains material that covers some crater floors, as in the Kuiper quadrangle (Scott and others, 1980). We cannot, however, determine whether most lineaments are internal or are parts of a faulted and lineated facies associated with a nearby but unphotographed impact basin. Melosh (1977) predicted that normal east-trending faults would form in high Mercurian latitudes as a result of slight crustal shortening. His predicted faults may be represented by a generally east-northeast-trending scarp and a lineament that cut across intermediate plains material and the crater Jókai between the 125° and 155° meridians. The north pole is too close to the terminator to detect the presence or absence of a “polygonal arrangement without preferred orientation,” as predicted by Melosh and Dzurisin (1978, p. 233). Arcuate and radial lineaments that might result from tectonic adjustments of the Mercurian crust,