meters would require formation by extraordinarily viscous lavas. Numerous faint lineaments are visible within the quadrangle, especially in the area between the Tolstoj Basin and the large crater Zeami to the northeast. Many of these lineaments may be faint secondary-crater chains or gouges; others may represent traces of an ancestral structural pattern that partly controlled the excavation of the craters and basin. The lineaments may have been enhanced or preserved by the gentle upwarping of this region of Tolstoj ejecta discussed above. The largest lineament, which marks the northwest limit of recognizable Tolstoj ejecta, is a subdued scarp some 450 km long. Rejuvenation of earlier faults or fractures by subsequent impacts probably occurred throughout the history of the planet. Thus, except for the lobate compressional scarps, it is difficult to separate internally produced structures from those of the complex impact history of Mercury. The azimuthal trends of all lineaments mapped within the quadrangle are, however, dominantly northwest (315°) and northeast (35°–40°). A minor, almost north-south trend is also observed. This situation is reminiscent of the so-called lunar grid on the Moon, which is generally ascribed to planetwide internal causes. Geologic history The interpretable geologic history within the Tolstoj quadrangle begins with the period of formation of the intercrater plains, which persisted until shortly after impact of the asteroid that created the Tolstoj Basin. After this event was a period of only slightly less intense bombardment. This period was followed by impact of the asteroid that created the Caloris Basin and deposits of the Caloris Group. Although the intermediate plains were resurfaced at the time of the Caloris impact, their formation actually extended from the end of the intercrater plains-forming period through the end of formation of c3 craters. At about the time the last c3 craters and the first c4 craters were being formed, the upper surface of the smooth plains and Caloris floor plains were being emplaced. Part of the smooth plains and Caloris floor plains materials may have been deposited during or immediately after the Caloris event. After the emplacement of most of the smooth plains, some late c3 craters and all c4 and c5 craters, including the large crater Mozart, were superposed on all previous deposits. The recognizable geologic history of the quadrangle ends with these events probably several billion years ago. A summary of the generalized geology history of Mercury was given by Guest and O’Donnell