3. THE MOUNTAIN RANGES.--These are comparatively few in number, and are never of such magnitude as to put them, like the craters, beyond terrestrial standards of comparison. The most conspicuous range is that known as the Lunar Apennines, which runs in a north-west and south-east direction for a distance of upwards of 400 miles along the border of the Mare Imbrium, from which its mass rises in a steep escarpment, towering in one instance (Mount Huygens) to a height of more than 18,000 feet. On the western side the range slopes gradually away in a gentle declivity. The spectacle presented by the Apennines about first quarter is one of indescribable grandeur. The shadows of the great peaks are cast for many miles over the surface of the Mare Imbrium, magnificently contrasting with the wild tract of hill-country behind, in which rugged summits and winding valleys are mingled in a scene of confusion which baffles all attempt at delineation. Two other important ranges--the Caucasus and the Alps--lie in close proximity to the Apennines; the latter of the two notable for the curious Alpine Valley which runs through it in a straight line for upwards of eighty miles. This wonderful chasm varies in breadth from about two miles, at its narrowest neck, to about six at its widest point. It is closely bordered, for a considerable portion of its length, by almost vertical cliffs thousands of feet in height, and under low magnifying powers appears so regular as to suggest nothing so much as the mark of a gigantic chisel, driven by main force through the midst of the mountain mass. The Alpine Valley is an easy object, and a power of 50 on a 2-inch telescope will show its main outlines quite clearly. Indeed, the whole neighbourhood is one which will well repay the student, some of the finest of the lunar craters, such as Plato, Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity (Plates XIII. and XVII.). [Illustration: PLATE XVI. Region of Theophilus and Altai Mountains. Yerkes Observatory.]