"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 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. In the gloomy bottom of the crater, whose dimensions are vast enough to swallow Mont Blanc body and bones, could be distinguished a magnificent group of cones, at least half a mile in height and glittering like piles of crystal. Towards the north several breaches could be seen in the ramparts, due probably to a caving in of immense masses accumulated on the summit of the precipitous walls.
As already observed, the surrounding plains were dotted with numberless craters mostly of small dimensions, except Gay Lussac on the north, whose crater was about 12 miles in diameter. Towards the southwest and the immediate east, the plain appeared to be very flat, no protuberance, no prominence of any kind lifting itself above the general dead level. Towards the north, on the contrary, as far as where the peninsula jutted on Oceanus Procellarum, the plain looked like a sea of lava wildly lashed for a while by a furious hurricane and then, when its waves and breakers and driving ridges were at their wildest, suddenly frozen into solidity. Over this rugged, rumpled, wrinkled surface and in all directions, ran the wonderful streaks whose radiating point appeared to be the summit of Copernicus. Many of them appeared to be ten miles wide and hundreds of miles in length.
The travellers disputed for some time on the origin of these strange radii, but could hardly be said to have arrived at any conclusion more satisfactory than that already reached by some terrestrial observers.