GENERAL OBSERVATIONS.
The condition of the Moon's surface as a whole indicates that it has been a theater of extraordinary volcanic activity. In size and number its vulcanoids far exceed the volcanoes of the Earth. The largest terrestrial crater known is that of Kilauea in the Hawaiian Islands which is 2½ miles in diameter. Several craters of the Moon, however, exceed 50 miles in diameter and one measures 114¼ miles. While the absolute heights of the mountains of the Moon do not greatly exceed those of the Earth, proportionally they are much higher, since the Moon's diameter is only one-fourth that of the Earth. The vulcanoids of the Moon differ in other respects from the volcanoes of our globe. "On the Earth they are usually openings on the summits or sides of mountains—on the Moon, depressions below the adjacent surface even when it is a plain or valley; on the Earth the mass of the cone usually far exceeds the capacity of the crater — on the Moon they are much nearer equality; on the Earth they are commonly the sources of long lava streams—on the Moon, traces of such outpourings are rare." (Webb.)