NOTES ON THE COMPARATIVE GEOLOGY OF THE EARTH AND MOON. BY HENRY J. SLACK, F.G.S., MEMBER OF THE COUNCIL OF THE MICROSCOPICAL SOCIETY OF LONDON. THE classical scholar will, I am afraid, at first object to the collocation of such terms as "geology of the moon." He might accept selenology for the science meant, just as selenography has been accepted as the appropriate designation of the art of depicting lunar formation; but geology he will say is a discourse about this earth, and the Greek word from which the first half is derived cannot be made to comprehend the moon. It does, however, seem reasonable that we should consider all celestial bodies of the same class, such as primary and secondary planets under one generic appellation, "earths," and if so, we might speak of "comparative geology" as the science which considers them as members of the same group.* By and by, perhaps, we shall have "heliology" to designate the science which endeavours to unfold the structure of the sun, and if so, "comparative heliology" may be admissible to describe the science which seeks to elucidate the comparative structure of all bodies which may be collectively termed suns. It is impossible to look at the moon without instituting comparisons between the mountains, plains, and cavities of that body and those of our earth; but certain preliminary inquiries seem necessary before we can arrive at any conception of the extent to which such comparisons are likely to hold good. Suppose, for example, the nebular hypothesis is assumed as the basis of such investigations, we should thon refer both the moon and the earth back to some enormously remote period, when they were condensed from a vaporous condition to one of a fluidity and plasticity resulting from heat. Such may have been the origin of our globe, but vain would be the search for specimens of the first solidified crust. Probably no portions of the earth which we can get at, by ascending its mountains or mining into its depths, represent the original solids that were first formed by the cooling and contraction of the ignited fluids, which, according to the nebular hypothesis, were the parents of the earliest solid forms. Changes from water action, which must have been, in geologic time, long posterior to any incandescent state of the earth's surface; Sir J. Herschel uses the term "geology of the moon;" so it may be considered established. Astronomy, 7th Edit., p. 283. changes from chemical and electrical action rearranging the molecules of solids; these may have gone for unknown cycles of ages modifying and remodifying all the matter accessible to us, which may have originally resulted from the various steps and stages of nebular condensation. In the address recently delivered by the late President* of the Geological Society, he suggested the probability that the earliest plastic condition of the earth of which we find traces, may have resulted from aqueous and not from igneous causes. Were this proved to be the case, it would not follow, as he intimated, that the earth was not once an incandescent ball, the cooling of which proceeded from the circumference towards the centre. All that we should be entitled to affirm from the most convincing proof that many of the rocks now called igneous were of aqueous origin, would be, that the traces of the pre-existent state had disappeared through the successive operation of causes in which fire played a secondary, and not a primary part. The assumption that the earth was once fluid, or pasty, from heat, is quite consistent with a second assumption that the principal cause of plasticity in ages long posterior to those of fire, may have arisen from a partial solution of materials, in which water was the chief agent. If these theoretical views be provisionally accepted, there may have been two terrestrial periods of more or less plasticity, an igneous and an aqueous one, and our times may be so far down in the aqueous ages that we may not be able to trace any strata so far back as even to approach the commencement of the aqueous period. Looking from the earth to the moon, we must be struck with evidences of great plasticity having prevailed when numerous tracts of selenian scenery were shaped into a prodigious number of craters, with lofty walls bearing greater or less traces of subsequent change. Mere violence of explosion without a greater plasticity than we are acquainted with, through terrestrial formations, could scarcely account for what is observed. The volcanoes of the Sandwich Islands, and those of Central France, so ably elucidated by Mr. Scrope, bear the greatest resemblance in character to lunar craters; but even the former are pigmies in point of breadth, and were probably formed under different conditions of plasticity in the crust of the globe to which they belong. A volcanic cone might be formed under some circumstances as a "crater of elevation ;" and Von Buch, Elie de Beaumont, and others have conceived our terrestrial volcanoes to have had such an origin. A "crater of elevation" means a cone formed by raising up in a huge bubble, strata that were pre * Mr. Hamilton. viously horizontal, and throwing out any matter that might occupy the central part. As Sir C. Lyell and Mr. Scrope have shown, our volcanoes do not exhibit the appearances that would indicate such an origin, and the more probable opinion is that they were formed by outpourings of matter through volcanic vents. "In the Sandwich Islands," to which we have referred, Sir C. Lyell says, "we have examples of volcanic domes 15,000 feet high, produced by successive outpourings from vents at or near the summit."* "The usual slope of these sheets of lava is between 5° and 10°; but Mr. Dana convinced himself that owing to the suddenness with which they cool in air, some lavas may occasionally form on slopes equalling 25°, and still preserve a considerable compactness of texture. It is even proved he says, from what he saw in the great lateral crater of Kilauea, on the flanks of Mount Loa, that a mass of such melted rock may consolidate at an inclination of 30°, and be continuous for 300 or 400 feet. Such masses are narrow, he admits, but if the lava had been more generous, they would have had a greater breadth, and by a succession of ejections overspreading each cooled layer, a considerable thickness might have been obtained."+ Mr. Darwin, likewise, describes in the Galapagos Islands, hills composed of tuff which must have flowed like mud, and yet consolidated at inclination of 20° or 30°. If a volcanic cone were formed by elevation of hard horizontal strata, they would be cracked in directions radiating from the centre of the elevating force; and Sir J. Herschel says, that in Lord Rosse's telescope the exterior of the lunar mountain Aristillus is all hatched over by deep gullies radiating towards its centre. Our terrestrial craters are of various ages, but none date back to the remoter periods of the world's history, which preceded the deposition of sedimentary rocks. But while all our terrestrial volcanoes belong to the period of aqueous stratified rocks, what are we to say concerning those of the moon? Did they originate in a period preceding that of aqueous stratification, if such a period as this last ever existed in our satellite, or are they referable to a period of aqueous stratification that has passed away? If they do not belong to any period of aqueous stratification, they cannot admit of any close comparison with our own volcanoes. Supposing the nebular and condensation. theory of planetary formation to be true, a period of igneous stratification may have existed in the earth or moon before the period of aqueous stratification arrived. Melted matter in cooling tends to arrange in layers corresponding with the specific * Principles of Geology, 9th Edit., p. 419. + Ibid., p. 383. gravities of the various substances in fusion, and it is plain that strata might be formed and dislocated by igneous as well as by mixed aqueous and igneous means. Many of the terraced crater walls in the moon correspond with what we should expect if they originated from successive ejections through volcanic vents, but if they were formed under general conditions similar to those of terrestrial volcanic activity, it is very remarkable that all should now seem extinct, or that we should have only faint and scarcely conclusive evidence that any of them are still in active operation. If the Selenians have watched our earth for the last thirty or forty years, through tolerable telescopes, they must have witnessed considerable volcanic change, as about twenty volcanic eruptions are estimated to occur in each year, and yet their globe or the half of it we see has been as quiet (in appearance) as if all its energies were dead. The inquiry whether the moon has now or has ever had an epoch of aqueous stratification is by no means easy. Many of the "water tool marks," as Mr. Campbell would call them, that can be discerned on our globe could scarcely be discriminated on the moon with present optical means. The Sinus Iridum looks much like a hollow worn by water on a rocky coast, but similar general appearances might result from fusion, irregular cooling, and explosive escape of gaseous matter. Near Aristarchus is a wriggling, precipitous valley that puts us in mind of the Cañons of the Colorado, but how shall we distinguish this and similar formations from the cracks that occur in castings, and which are sometimes curved ? Sir J. Herschel observes concerning lunar craters that "their enormous and vast depth are easily reconcileable with what we know of the special conditions which prevail on the moon's surface. For while on the one hand the force of volcanic explosion and ejection is nowhere dependant on the total mass of the planetary body on which the volcano may subsist, the repressing power to prevent an outbreak, which is the weight of the incumbent matter, is only about one-sixth of what an equal mass of overlying matter would exert on the earth." He likewise mentions the greater distance to which erupted matter might be carried in the moon, and the absence of the resistance afforded by our atmosphere. Such considerations are highly important; but does not the regularity of hundreds of adjacent craters require us to believe that when they were formed, the moon's surface was not in a hard, brittle state? The question of the possible existence of masses of fluid on the moon is usually narrowed to the consideration of whether water is there. The lunar climate forbids the idea that water, or snow, or glacial ice exists there now,* whatever may have been the case in past times. Sir J. Herschel says, "The climate of the moon must be very extraordinary; the alternation being that of unmitigated and burning sunshine, fiercer than an equatorial noon, continued for a whole fortnight, and the keenest severity of frost, far exceeding that of our polar winters for an equal time." If water in any of the forms in which we know it, existed on our satellite, the heat of the lunar day would abundantly evaporate it, and the first touch of the cold of the lunar night would precipitate it in clouds and tremendous floods of rain, or lumps of ice. At the moment of a lunar region passing into night or day, these changes would be violent, and would surely be seen to some extent by terrestrial observers. They would, moreover, produce enormous abrasions and degradations of surfaces, of which evidence would appear. The supposition, that although water in its various forms does not now exist in the moon, it may have been there in former times, and may, by river and glacial action, have modified the surface, is very difficult to deal with. When we can get no specimens of alluvial soil, can the general aspect of a country imperfectly seen, by reason of great distance, do more than suggest an hypothesis to be lightly held? And if certain lunar formations are ascribed to glacial action, how shall the theory be satisfactorily tested? On the earth, such action is presumed from a number of indications, many of which must elude our investigation when directed to a distant body. Scratches and grooves made in hard rocks by glacial ice could not be detected, nor could the polish of surfaces, from a similar cause. The spread of gravel and drift we could not hope to discern; but large moraines, containing a great quantity of large blocks, might be visible, but no special water-marks upon them would be within reach of our largest instruments. Who, for example, could tell a rounded boulder from a sharp-angled rock 240 miles off? And few observers can see the moon as near as that, or anything like it. Mr. Webb, in his Celestial Objects, well observes, "The Moon Committee of the British Association have recommended a power of 1000; few indeed are the instruments or the nights that will bear it; but, when employed, what will be the result? Since increase of magnifying power is equivalent to decrease of distance, we shall see the moon as large (though indistinct) as if it were 240 miles off." What should we think of a geologist who discussed the action of extinct glaciers in Wales, or the parallel roads of Glenroy from the appearance they presented through This remark refers to the half of the moon which we see. No reference is made to the speculation concerning the character of the invisible side. |