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_." "That's _Mare Nectaris_, the gray spot over there on the southwest, isn't it ?" asked M'Nicholl; "is there any likelihood of our getting a better view of it ?" "Not the slightest," answered Barbican, "unless we go round the Moon and return this way, like a satellite describing its orbit." By this time they had arrived at a point vertical to the mountain centre.

_Copernicus's_ vast ramparts formed a perfect circle or rather a pair of concentric circles.

All around the mountain extended a dark grayish plain of savage aspect, on which the peak shadows projected themselves in sharp relief.