temperature had been uniform, and had been right, there would not have been left precipices nearly vertical and thousands of feet in height, such as we find scattered throughout the lunar surface, as they would in this case have sunk under their own weight.
Having presented the arguments for and against each of the other theories, let us now see in what respects the lunar craters resemble and in what respects they differ from those of our Earth. Terrestrial craters are of two types, those like Vesuvius and those like Kilauea. To most of us the former is the typical volcano. Volcanoes of this type, which are by far the most numerous upon the globe, emit thousands of tons of steam at every eruption. It is, indeed, mainly by the explosion of this steam that the other volcanic products are expelled.
A volcano is primarily a crack in the ground. Steam rushing out of this crack enlarges it at one or more separate places, while the rest of the crack gradually fills and closes up when the pressure which produced it is relieved. The solid material projected from the enlarged portion of the crack slowly builds up a volcanic cone. The crater at the top is always small as compared with the erupted matter of the cone, seldom reaching a mile in diameter, while it is usually only a few hundred yards. The cone itself is symmetrical, and sometimes reaches a height of three miles above its base.
In the case of the terrestrial volcano, after a long period of comparatively quiet eruptions, or more commonly of no eruptions whatever, a tremendous explosion will sometimes occur which completely blows away the top of the mountain. Such an explosion will scatter the debris in the form of fine dust and ashes over thousands or even millions of square miles of the Earth's surface. The dust caused by the explosion of Krakatoa in 1883 was scattered by our atmosphere over the whole surface of the globe. It is computed that the column of stones and ashes at the time of the eruption shot up into the air to a height of at least seventeen miles, while the sound of the explosion was distinctly heard in the island of Rodriguez, nearly 3,000 miles distant.
Sometimes nearly the whole mountain disappears, leaving only a large crater ring. Later a small volcanic cone sometimes forms at the centre. This may grow to large dimensions, nearly filling the original ring. Many volcanoes have this appearance. Part of such a ring, known as Monte Somma, appears about Vesuvius. Not infrequently the ring fills with water, forming a circular lake. These crater rings, especially where the central cone is small or lacking, strongly resemble the lunar volcanoes. The three
