M'Allister smiled a rather wintry smile, and then subsided. John was a bit too smart for him that time.
Passing on, we inspected the large cleft running parallel to the Straight Wall, and the small mountain close by named after Birt, the well-known selenographer. We then crossed the Sea of Clouds again, and had a long look at the great system of straight clefts near Campanus and Hippalus, together with the fine walled plain Gassendi, the floor of which is at some parts 2000 feet above the lunar surface. I had often studied this through the telescope, as it is a most interesting formation.
"Well, Professor," remarked M'Allister, "I have travelled nearly all over our own world, but in all my journeyings I have never seen such wild and rugged scenery as I have during the few hours we have been passing over the moon. The mountains seem to be split and rent in all directions, especially where there are volcanic craters in the neighbourhood—and, really, they seem to be everywhere; while landslips are very numerous, and the mountain passes are extremely rugged and gloomy."
"Yes," I replied, "my telescopic observations had prepared me for a great deal, but the weird ruggedness of the lunar scenery exceeds all my anticipations."
"What is the explanation of it all?" M'Allister inquired.
"I should think, M'Allister, that much of it was originally caused by the extreme violence of volcanic outbursts," I answered; "but the excessive expansion and contraction, resulting from the alternate spells of intense heat and intense cold to which the moon is continually exposed, will account for the formation of many of those tremendous chasms and precipices which we see everywhere around us, as well as for the huge mounds of dislodged rocks and débris, which are piled up in such chaotic confusion on the ledges of the mountains and round their bases.
"On the earth such débris would very soon have become smoothed by atmospheric erosion, the interstices would have been filled up with dust and soil, while the growth of vegetation would have added a new charm to the effect.
"You have seen the great landslip in the Isle of Wight! When it fell all was wild desolation, but it has become covered with such a luxuriant growth of vegetation that it now presents a scene of beauty.
"On the moon, however, there is neither atmosphere, rain, nor moisture to produce weathering of the rocks or to encourage the growth of vegetation; so the rocks remain just as sharp, rugged, and bare as they were ages ago when they were first split off from the mountains.
"No doubt very large masses of rocks are still frequently being dislodged, and if we could see them falling from the upper part of a mountain, rebounding along the spurs, with fragments flying in all directions and ultimately dashing to pieces at the base, it would seem to us most uncanny not to hear the slightest sound arising from all this apparent commotion. Without an atmosphere, however, no sound could be produced, no matter how many thousands of tons of rock might fall to the ground.