_a._ MOUNTAIN CHAINS. These may have a length of 80 to 100 miles and heights of from 5,000 to 17,000 feet. As in the case with the mountains of the Earth, they are usually steeper on one side than on the other. The range called the Appenines, seen near the north pole of the Moon, is a good illustration of such mountain chains. Other ranges are the so-called Alps and Caucasus. These names were applied by Hevelius, an astronomer of Danzig, who made the first map of the Moon in 1647. He gave to the features of the Moon's surface names of localities similar to those on the Earth which they most resembled. His system was largely abandoned by later astronomers, however, the later method being to name the different features of the Moon after celebrated astronomers and philosophers. _b._ HIGHLANDS SURROUNDED BY MOUNTAINS. These are partly with and partly without well-determined directions. _c._ ISOLATED MOUNTAINS. These usually occur on the gray plains. They vary from 4,000 to 7,000 feet in height. _d._ VEIN MOUNTAINS. These occur only on the gray plains. They are long, narrow, contorted ridges, usually from 700 to 1,000 feet in height. _e._ CIRCULAR MOUNTAINS. These are the most characteristic and peculiar features of the Moon's surface. They vary in size from the so-called "Walled Plains," 150 to 15 miles in diameter, to crater mountains whose diameters range from 15 miles down to a few hundred feet. Thirty-three thousand of these crater mountains have been counted by one astronomer, the number increasing as the size diminishes.