Makch 1, 1886,] 



♦ KNO\VLEDGE ♦ 



147 



imd the eartli are so unlike that the imifonnitarian theory 

 seems to fail. 



But so soon as we applj' to the mcon the two lines of 

 reasoning which were touched on above, we at once find 

 reason, for expecting just such differences as actually 

 exist. Made of the same materials, proportioned pro- 

 bably in much the same way, the moon and the earth 

 would have very different histories. To begin with, as 

 we have just seen, the stages of the moon's life would be 

 very much shorter than those of the lifetime of oui" earth, 

 and therefore, even under the same conditions, the power 

 of producing great changes of contour would be far less 

 in the moon's case. But the conditions would not be the 

 same. With 81 times as much matter, and presumably 

 about 81 times as much water — spread over a surface 

 only 13.V times as large — the earth would have had six 

 times as much water per square mile as the moon : this 

 would have made a great difference in the efficiency 

 of all those forms of denuding action (and they are 

 altogether the most important) which depend on the 

 action of water in its various forms : it would also have 

 greatly increased the dui-ation of the action of these 

 forces upon the earth, as compared with that of the 

 action, of the similar but much feebler forces on the 

 moon. But an even more remarkable difference appears 

 in regard to the probable density of the lunar air when 

 she was passing through that stage of her life which 

 corresponded with the present state of our earth's life. 

 For, while isrecisely the same reasoning would appdy to 

 the air, so that the quantity over each square mile of the 

 moon would probably have been but one-sixth of the 

 quantity over each square mile of the earth, that 

 smaller quantity would have been compressed only by 

 the small force of lunar gravity, about one-sixth of ter- 

 restrial gravity. Thus the densilj- of the lunar air would 

 have been but one-thirty-sixth of the density of the air 

 we breathe. Air so tenuous as this would not only be 

 unfit to support life, it would have had very small efficiency 

 as a denuding agent, whether we consider its direct action, 

 or its power in conjunction with water. 



On the whole, subaerial denudation on the moon must 

 in all probability have been so exceedingly slow in its 

 action, and the time during which it acted so exceedingly 

 short, that the wonder is how any denudation at all can 

 have taken place on the moon's surface. We may pro- 

 bably ascribe such denudation as is indicated hj the con- 

 dition of the crater-covered regions, and by the aspect of 

 the mountain ranges — indeed by the very existence of 

 any mountain ranges at all — to the time when the lunar 

 atmosphere, like the earth's air in past ages, was laden 

 with carbonic acid (carbon dioxide), sulphurous acid, sul- 

 phuretted hydrogen, boracic acid, and so forth. The 

 earth's air when so constituted was immensely more dense 

 than it is at present, and all the jirocesses of denudation 

 went on far more rapidly than they have done since. 

 The lunar atmosphere at that stage of the moon's history 

 was probably about as dense as our own atmosphere is 

 now ; but even if less dense, its constitution and its high 

 temperature in connection with the high temperature of 

 the crust, would lead to changes at least as rapid and 

 efEective (while they lasted) as those which have taken 

 place on the earth since the earliest ages of which geolo- 

 gical records remain. 



Thus we should not expect to tiud the great 

 craters belonging to the early stages of the moon's 

 vulcauian history converted into such wrecks as aloue 

 attest on this earth the former existence of similar ter- 

 restrial crater-mountains. But we may yet fairl}- look for 

 evidence of considerable denudation during the time. 



short-lasting though it may have been, when the moon's 

 atmosphere and oceans were capable of doing effective 

 denuding work. Accordingly we find that while the 

 immense craters remain still the most striking features 

 of the moon's surface, they attest the action of subaerial 

 denudation during a period which, though it may have 

 been short comjiared with the corresponding period of 

 our earth's history, must still be measured by hundreds of 

 thousands oi years. 



Now the great craters, grand though their re- 

 mains are compared with the mere wrecks which (as 

 in Mull and Skye) remain to show on our earth how large 

 terrestrial craters once were, show yet signs of denuda- 

 tion. And we see in lunar mountain ranges the products 

 of such denuding work. The great range called the 

 Lunar Apennines attests, for example, on the moon the 

 long- continued action of denuding forces by which the 

 whole tract now occupied by the Sea of Serenity and the 

 Sea of Showers was covered with matter worn by the 

 action of sea and storm, river, rain, snow, and wind, from 

 the surface of the lunar continents around. In a vast 

 trough-like depression running athwart what was once the 

 floor of an immense sea (covering both the great regions 

 just named) the products of denudation were deposited 

 in greater quantity than on either side. Then as the 

 region thus heavily laden with deposited matter sank 

 more and more deejily, the matter was collected (to the 

 depth of many miles) out of which the future mountain 

 range was to be formed. When at length this process 

 ceased, and the shrinking of the moon's crust compelled 

 this seam of sedimentary deposit to rise, forced upwards 

 by side pressure, the range of mountains rose, rounded and 

 dome-shaped then, but presently to be wrought into the 

 precipitous pinnacled forms now recognised in the Lunar 

 Apennines. The side pressures no doubt generated 

 enormous heat, converting the sedimentary strata into 

 various kinds of crystalline rock, the harder materijils 

 resisting best the still active denuding foi-cesof the lunar 

 atmosphere, and forming the higher peaks of the range 

 as we know it now. 



But we see in the evidence of such denuding action, 

 the last important traces of subaerial denudation in the 

 moon. Not there as on this earth have lauds and seas 

 interchanged after the manner described by Tennyson 

 when he says — 



Tl ere rolls the deep where grew the tree: 



Oh earth what changes hast thou seen ! 



There where the loud street roars, hath been 

 The stillness o£ the central se;i. 

 The hills are shadows, and they flow 



From form to form, and nothing stands. 



Like mists they melt, the solid lands, 

 Like clouds they shape themselves and go. 



One interchange of land and water, and one only, can 

 be recognised on the moon. The floors of the great seas 

 tell of buried continents. The very shapes of sand-covered 

 craters, as large as any now remaining uncovered, can be 

 recognised — shadowy and ghost-like, but still clearly 

 recognisable — in the broad dark tracks called seas, which 

 doubtless are the floors of what were once great lunar 

 oceans. 



Thus do the most characteristic features of the moon's 

 surface (except the immense ray-systems which belong 

 to an earlier stage yet) find satisfactory interpretation in 

 the comparative shortness of the stages of the moon's 

 volcauian history. There are many great craters because 

 there was not time or power to wear them down. There 

 are few great mountain ranges because there was not time 

 or power to fashion many. But some work was done in 



