Owing to the sinking of the convex surface of the mare a compression will arise and ridges be formed. These are usually concentric, as in Maria Crisium, 3E [1.1, 6.1], and Nectaris, 6E [1.4, 2.7], but sometimes they pass through or near the centre, as in Mare Foecunditatis, 4E [0.7, 1.3]. When these ridges have sharp crests, owing to bending, they become less able to resist the internal compression, and craterlets often force their way through in such places. This compression, as we have seen, is the probable cause of the central ridge or peak found in many of the unaltered lunar craters, the weight of the surrounding mountain wall being an efficient aid in producing the requisite compression. As in the case of the ridges in the maria, a craterlet is sometimes found in the summit of the central peak, as occurs in Kilauea. Under these circumstances, as in the case of Timocharis, 11A [1.5, 4.7], the appearance is occasionally similar to that of the truncated cone of an explosive terrestrial volcano.
Among the most important of the smaller formations upon the Moon are the rills. These are simply gigantic cracks in the lunar surface, sometimes several hundred miles in length by one or two miles in breadth. They were formed at a time when the surface was too small for the interior and the fluid contents were so far removed from the surface that but little of it was able to escape and relieve the pressure. The rills are therefore of comparatively recent formation. They are rarely if ever found among the primary formations, unless these have been melted or softened by the subsequent application of heat. They are never found near the poles where the tidal action is small, although seen near other parts of the limb. Only three minute rills, therefore, are found in the Mare Frigoris, although they abound in the other maria. Nearly all the rills lie between latitude 57° North and 35° South. They are particularly numerous in the southeastern quadrant of the Moon and comparatively rare in the southwestern. The great rill west of Sirsalis, 16A [1.9, 1.8], the longest on the Moon, is supposed to be about 400 miles in length. Like most of the fine detail described in the rest of this chapter, it does not show on the photographs, but for the benefit of those wishing to look the matter up on the Moon itself the position of the various formations will be indicated by brackets, as before. The rill of Hyginus and its extension, which passes south of Julius Cæsar, are much the most conspicuous upon the Moon, and are shown in Plate 7E [2.0, 7.0 and 1.2, 7.1]. About a thousand of these rills are known to astronomers. They are frequently concentric with some mare or partially submerged crater. Striking illustrations of this are found just to the west of Mare Humorum, Frontispiece [5.5, 5.3], where three concentric systems
