A curious and very common arrangement of this internal
plateau of lunar craters is its lying at a lower level than the external
plains, quite the contrary to a terrestrial crater, which generally has
its bottom much higher than the level of the surrounding country. It
follows therefore that the deep lying curve of the bottom of these ring
mountains would give a sphere with a diameter somewhat smaller than the
Moon's."
"What can be the cause of this peculiarity?" asked M'Nicholl.
"I can't tell;" answered Barbican, "but, as a conjecture, I should say
that it is probably to the comparatively smaller area of the Moon and
the more violent character of her volcanic action that the extremely
rugged character of her surface is mainly due."
"Why, it's the _Campi Phlegraei_ or the Fire Fields of Naples over
again!" cried Ardan suddenly. "There's _Monte Barbaro_, there's the
_Solfatara_, there is the crater of _Astroni_, and there is the _Monte
Nuovo_, as plain as the hand on my body!"
"The great resemblance between the region you speak of and the general
surface of the Moon has been often remarked;" observed Barbican, "but
it is even still more striking in the neighborhood of _Theophilus_ on
the borders of _Mare Nectaris_.
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