Page:The Moon (Pickering).djvu/44

24 largest crater rings found upon the Earth are a nameless one in northern Kamchatka, Mount Asosan in the Island of Kiushiu, Japan, and Lake Bombon in the Island of Luzon, Philippines. Each of these rings measures about fifteen miles in diameter. Two other rings of ten and seven miles, known as Lake Bolesna and Monte Cavo, are found in Italy. Still others, somewhat smaller, exist in the western part of the United States. On the Moon, owing to the smaller force of gravity, we should expect such craters to have about six times the diameter they would have here; therefore, allowing for this, the difference in appearance between our own crater rings and what we actually find upon the Moon is not very great. The chief difference in their appearance is that on the Moon the crater floor is always lower than the surrounding country, while on the Earth it is usually higher.

There is an objection, however, to this theory, which lies not so much in the difference in the appearance of the craters as in the fact that we find no great symmetrical crater cones upon the Moon. Frequently a small irregular peak appears inside of a lunar crater, but it is very seldom surmounted by a crater of its own, and even if it is, it is always insignificant compared to the great ring that surrounds it. In fact, the features that are most prominent in our ordinary terrestrial volcanic regions are never seen upon the Moon.

Turning now to the other type of terrestrial volcano, two specimens of which are found lying close together on the Island of Hawaii, namely, Kilauea and Matma Loa, we find something much more closely resembling the lunar type than do our ordinary crater rings. From these craters there is no large eruption of steam, no tremendous explosions destroying the mountain summits, and no towering volcanic cones. While their extensive crater floors, measuring from two to three miles in diameter and from 500 to 1,000 feet in depth, are considerably elevated above the sea level, yet the outer slopes of the volcanoes themselves are so gradual that it is readily seen that if many of them occurred, arranged as closely together as those formed upon the Moon, the floors would be placed decidedly below the general surface level.

According to Captain Dutton, the openings in these craters are constantly enlarging when in action by the faulting and slipping in of their sides. Numerous cracks occur around them and across the craters themselves, while some follow parallel to their inner edge, in these various respects strongly resembling similar cracks found upon the