J. D. Dana on the Volcanoes of the Moon. 345 
weigh but one-sixth what it does here. 'The lavas would there- 
fore not only be specifically lighter, but would become ‘more - 
blown up with the vapors, or more spongy. On the same ground 
too, we understand why the moon’s great craters should so gen- 
_ erally terminate in a raised’ rim, while those of the earth, like 
Mount Loa, have very gently sloping sides and ‘summit. This 
raised rim is fully illustrated about the Kilauea pools, as we have 
already stated: but in large overflowings, the earth’s lavas, ow- 
ing to their weight, flow far away by gravity, and this feature is 
therefore never exemplified on the earth on the same grand scale 
as in the moon.* , 
We may therefore say unhesitatingly, without fearing an im- 
peachment of our sobriety, that the moon’s volcanoes are in fact 
- Volcanoes, either extinct or active, although the craters would re- 
ceive comfortably more than a score of Etnas. We also com- 
prehend the important fact, that a cooling globe would become 
at first a scene of great boiling lakes from the hardening of some 
portions ;—that on a farther diminution of heat, these lakes would 
partially cool, excepting points or areas of greatest heat, and thus 
a subdivision of them would result: or else, they would grad- 
ually contract their overflowings, and so, as gradually, contract 
the size of the vent, obliterating all evidence of the former size : 
or again, they would more abruptly contract, and consequently 
form an inner ledge concentric with the outer walls, and perhaps 
also other concentric ledges still smaller. ae 
~ This is well illustrated in the figures, and nothing could better 
indicate the mode of action which characterizes the moon's 
craters, for we may trace out the successive diminutions. In 
Heinsius, (figure 3,) which is forty-eight miles in its longer 
diameter, this is beautifully shown ; there is one low ledge within 
another nearly concentric, and finally a smaller circular pit, of 
twelve miles breadth,—no mean size, though we call it small. 
An Cuter concentric ridge is also apparent through part of in 
citenit, which may have béen a still earlier outline ; and’ being 
wer than the ridge next within, it illustrates the statement, if 
the hypothesis be true, that the larger craters have lower walls. 
It is however possible that it may have resulted from a subsi- 
| * The elevation theory of Von Buch has been supported from facts in the moon. 
i We offer nothing here on that subject. 
_ Scoxp Srrizs, Vol. I, No. 6.—Nov., 1846. 45 
