Of these different chains, the most important is that of the _Apennines_, about 450 miles long, a length, however, far inferior to that of many of the great mountain ranges of our globe. They skirt the western shores of the _Mare Imbrium_, over which they rise in immense cliffs, 18 or 20 thousand feet in height, steep as a wall and casting over the plain intensely black shadows at least 90 miles long. Of Mt. _Huyghens_, the highest in the group, the travellers were just barely able to distinguish the sharp angular summit in the far west. To the east, however, the _Carpathians_, extending from the 18th to 30th degrees of east longitude, lay directly under their eyes and could be examined in all the peculiarities of their distribution. Barbican proposed a hypothesis regarding the formation of those mountains, which his companions thought at least as good as any other. Looking carefully over the _Carpathians_ and catching occasional glimpses of semi-circular formations and half domes, he concluded that the chain must have formerly been a succession of vast craters. Then had come some mighty internal discharge, or rather the subsidence to which _Mare Imbrium_ is due, for it immediately broke off or swallowed up one half of those mountains, leaving the other half steep as a wall on one side and sloping gently on the other to the level of the surrounding plains. The _Carpathians_ were therefore pretty nearly in the same condition as the crater mountains _Ptolemy_, _Alpetragius_ and _Arzachel_ would find themselves in, if some terrible cataclysm, by tearing away their eastern ramparts, had turned them into a chain of mountains whose towering cliffs would nod threateningly over the western shores of _Mare Nubium_. The mean height of the _Carpathians_ is about 6,000 feet, the altitude of certain points in the Pyrenees such as the _Port of Pineda_, or _Roland's Breach_, in the shadow of _Mont Perdu_. The northern slopes of the _Carpathians_ sink rapidly towards the shores of the vast _Mare Imbrium_. Towards two o'clock in the morning, Barbican calculated the Projectile to be on the 20th northern parallel, and therefore almost immediately over the little ring mountain called _Pytheas_, about 4600 feet in height. The distance of the travellers from the Moon at this point could not be more than about 750 miles, reduced to about 7 by means of their excellent telescopes.