"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. "Tremendous changes of this nature may be happening on the moon, but our telescopes are not powerful enough to enable us to see the results. They would have to cover an area of miles to be noticeable, unless they presented some particularly striking configuration." "Professor," exclaimed M'Allister, "how is it that all the shadows on the moon are such a dense black and so sharply defined at the edges?" "That," I exclaimed, "is entirely owing to the absence of the atmosphere. On the earth, even at night time, some light is diffused by our atmosphere, and shadows are never dense black even when thrown by a bright sun. On the moon it is black darkness everywhere outside the direct rays of the sun, and there is no gradual diminution of the darkness about the edges of shadows such as we see on the earth. The only mitigation of the blackness is seen where some light is reflected across from the rocky walls on which the sun is shining.