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eruptions. And yet the existence of central cones in many ofthem, and even in some of the largest, favours this hypothesis.It is well, therefore, to remember, that the attraction of theforce of gravity upon the surface of the Moon is not quiteMil of its value upon the Earth , and that an eruption of matterfrom a central cone, with such a velocity and at such anangle that it would descend upon a level plain on the Earth at a distance of five miles, would, if similarly projected on theMoon , he carried rather more than 30 miles before it wouldfall. It is therefore a not altogether unreasonable supposi-tion that such a centre of eruption may have deposited matterin a ring of 60 to 70 miles, or even more, in diameter. But itmay nevertheless be urged in reply to this argument that it isby no means likely that such violent eruptive forces would begenerated in the comparatively small mass of the Moon as inthe much larger mass of the Earth , and that the diameter ofthe largest crater upon the Earth , which is, we believe, in theSandwich Islands, does not exceed 10 miles.*
Other theories have consequently been elaborated as to theorigin of these huge ring-shaped lunar formations. It has, forinstance, been imagined that they may have been produced bythe exudation of molten matter at different times during thecooling and solidifying of the Moon ’s crust, a process whichwould involve frequent changes of internal level and pressure,and vast subsidences and derangements of the surface.
On the other hand, if we maintain the previously mentionedtheory, viz., that the circular ridge has originated in thedischarge of matter from a central cone, we need not besurprised if such cones nevertheless seem in many cases tobe wanting ; for a subsequent eruption of less energy mightdischarge matter sufficient to fill up the surrounding circle tothe level of the cone, or nearly so, which would cause it to beno longer visible.
We cannot, however, afford further space to the discussion ofthese theories. We will sum up our remarks by saying thatthere is every reason to believe, from the present appearanceof the Moon, that in ages long past it has been the scene of* See Miss Bird’s account of her recent tour in these islands.