Further, the grey plains are characterized by features of which instances may be seen with a very small instrument, though the more delicate specimens require considerable power--namely, the long winding ridges which either run concentrically with the margins of the plains, or cross their surface from side to side. Of these the most notable is the great serpentine ridge which traverses the Mare Serenitatis in the north-west quadrant of the moon. As it runs, approximately, in a north and south direction, it is well placed for observation, and even a low power will bring out a good deal of remarkable detail in connection with it. It rises in some places to a height of 700 or 800 feet (Neison), and is well shown on many of the fine lunar photographs now so common. Another point of interest in connection with the Maria is the existence on their borders of a number of large crater formations which present the appearance of having had their walls breached and ruined on the side next the mare by the action of some obscure agency. From consideration of these ruined craters, and of the 'ghost craters,' not uncommon on the plains, which present merely a faint outline, as though almost entirely submerged, it has been suggested, by Elger and others, that the Maria, as we see them represent, not the beds of ancient seas, but the consolidated crust of some fluid or viscous substance such as lava, which has welled forth from vents connected with the interior of the moon, overflowing many of the smaller formations, and partially destroying the walls of these larger craters. Notable instances of these half-ruined formations will be found in Fracastorius (Plate XIX., No. 78, and Plate XI.), and Pitatus (Plate XIX., No. 63, and Plate XV.). The grey plains vary in size from the vast Oceanus Procellarum, nearly 2,000,000 square miles in area, down to the Mare Humboldtianum, whose area of 42,000 square miles is less than that of England. 2. THE CIRCULAR, OR APPROXIMATELY CIRCULAR FORMATIONS.--These, the great distinguishing feature of lunar scenery, have been classified according to the characteristics, more or less marked, which distinguish them from one another, as walled-plains, mountain-rings, ring-plains, craters, crater-cones, craterlets, crater-pits, and depressions. For general purposes we may content ourselves with the single title craters, using the more specific titles in outstanding instances. [Illustration: PLATE XIV. Region of Maginus: Overlapping Craters. Paris Observatory.]