15,800 feet in height, and eighteen miles distant on all sides, with no possibility of egress. There is no gap in the wall, no outlet by which you may escape. On the summit of the central cone on which you stand, there is a lesser cavity, through which the ashes and lava, of which the cone consists, were ejected. But all activity is past, and eternal silence reigns. You stand on volcanic ashes, but you do not suffer the inconvenience of ascending the cone of Vesuvius. Thanks to the weak attraction of the moon, you can tread on the treacherous slope without sinking.
Now this singular formation is not singular in the moon. It is the grand feature of the lunar surface. That surface is divided into the dark plains and the bright alpine regions, and, in the latter, the grand characteristic is the circular form with the central cone. These craters are of various dimensions. Some are fifty or sixty miles across, others are only a few hundred yards. Now, do we find anything corresponding to this on the earth? It is plain that our active volcanoes only feebly represent the lunar craters. The volcanic apertures on the earth are only small craters at the top of volcanic mountains of no great dimensions, such as Vesuvius. The terrestrial approximations to the typical form are only like the undeveloped limbs of animals, pointing to the more perfect. Among the older formations, however, we find indications of the lunar circularity. In Auvergne, for example, there are some illustrations, and probably,
