utterly
into the shade everything like it on our Earth. The crater of Etna is at
most little more than a mile across. The crater of _Copernicus_ has a
diameter of at least 50 miles. Within it, the travellers could easily
discover by their glasses an immense number of terraced ridges, probably
landslips, alternating with stratifications resulting from successive
eruptions. Here and there, but particularly in the southern side, they
caught glimpses of shadows of such intense blackness, projected across
the plateau and lying there like pitch spots, that they could not tell
them from yawning chasms of incalculable depth. Outside the crater the
shadows were almost as deep, whilst on the plains all around,
particularly in the west, so many small craters could be detected that
the eye in vain attempted to count them.

"Many circular mountains of this kind," observed Barbican, "can be seen
on the lunar surface, but _Copernicus_, though not one of the greatest,
is one of the most remarkable on account of those diverging streaks of
bright light that you see radiating from its summit. By looking
steadily into its crater, you can see more cones than mortal eye ever
lit on before. They are so numerous as to render the interior plateau
quite rugged, and were formerly so many openings giving vent to fire and
volcanic matter. 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 Barbi