284 



♦ KNOWLEDGE ♦ 



[July 1, 1886. 



]iistory. Doubtless the earth had large craters like those 

 ^vliicli still remain in the moon. But then in the earth's 

 case there has been a much more thorough wearing down of 

 those upraised regions. There has been more water to do 

 the work by sea action, by river action, and by glacial 

 action ; there has been more air to help in the work by the 

 action of rain, wind, snow, and storm ; aud there has been 

 much more time for all these processes to take effect. 

 Similar considerations apply, indeed, to the earlier stages 

 when our atmosphere was denser, more complex, and more 

 destructive alike by its own action and by tlie action of the 

 various iluids formed by condensation out of the unpleasant 

 vapours present in it. Now even a phvnet cannot at once 

 eat its cake and have its cake. If our eaith has h.ad water 

 enough, air enough, and above all time enough, to wear 

 down in great degree tlie records of tlie first stages of its 

 vulcanian career, the immense outbursts of molten matter, 

 the vast craters, the mighty wrinkles and corrugations, 

 then naturally these records can no longer exist in their 

 original dimensions on the earth. But it is equally clear 

 that the coiresponding records will remain on the moon, 

 where there has been much less water, a far larer air, and 

 a much shorter time during which denuding forces of air 

 and water could work. 



We can at once see a reason then why on the moon the im- 

 mense craters which are much earlier vulcanian products than 

 mountain ranges, remain still extant. Many of the larger 

 and older ones show signs of denudation, which we may 

 probably i-egard as having been brought about when the 

 moon's atmosphere was still in its early state, loaded with 

 active acids, still intensely hot, and capable of producing 

 marked effects on the intensely heated crust. But far the 

 greater number of the lunar craters are manifestly in the 

 state in which they were first formed. The denuding forces 

 on the moon died out long before there had been time to 

 wear any but the earliest craters down, or wholly to wear 

 down any. 



On the contrary, there ai'e naturally few mountain ranges 

 of great size on the moon. The mounfcxin ranges are pro- 

 ducts formed out of the materials of previous formations ; 

 and if the great craters remain, or have been very little re- 

 <luced by denuding action, it follows that there has been very 

 little material availalde for mountain-making, and few 

 mountain ranges therefore have been formed. 



Btill, the great range of mountains called the Lunar 

 Apennines shows that we must consider mountain-making 

 on the moon in some degree, unimportant though this part 

 of the moon's vulcanian history may be, compared with the 

 corresponding part of the eai-th's. We may expect to find 

 that the great craters have in some degree been worn away 

 to provide materials for that lofty range of mountains. 

 And if we assume, as we fairly enough may, that the for- 

 mation of the Lunar Apennines corresj)onded with the 

 formation of terrestrial mountain ranges, then we may 

 reasonably look for the traces of just such j)rocesses as 

 geologists have recognised in the development of the Alps, 

 Rocky Mountains, and Himalayas on the earth. 



It will be found that the search for such evidence leads to 

 some very curious results, not only justifying the belief that 

 the Lunar Ajiennines were formed like the terrestrial moun- 

 tain ranges, but enabling us to interpret features of the 

 moon's globe which hitherto had x-emained unexplained. 



In the first place we find the Lunar Apennines between 

 two immense plains, the so-called Sea of Serenity on tlie 

 east (or what would be the east to a lunarian), the so-called 

 Sea of Showers on the west, each having an area of many 

 hundreds of square miles. These great plains are mani- 

 festly great tracts on which finely divided matter has been 

 deposited, as on a desert region like Sahara, or — more prob- 



ably — as on the floor of a great sea. Regarding them as 

 originally oceans, we see that the mountain range occupies 

 such a position as would correspond with the formation of 

 mountain ranges, as this process has taken place on the 

 earth. We may supposs then that the matter out of which 

 the Lunar Apennines were formed was deposited in a great 

 troughlike depression, formed along a region where the 

 crust had yielded in a far earlier stage, and whence molten 

 matter had been extruded. Judging from the height of the 

 Lunar Apennines, we see that the depth of sedimentary 

 matter deposited in that depression, as it slowly sank below 

 the level of an ancient Lunar Sea (originally extending over 

 the whole space occupied by the Se.is of Serenity and 

 Showers), must have been several miles. This range then 

 assuredly speaks to us of many hundreds of thousands of 

 year.s, during which the waters of a vast sea beat on the 

 shore lines of ancient lunar continents, receiving from lunar 

 rivers the matter worn from those continents by lunar winds, 

 and washed down by lunar rivers. (Of one of these rivers, 

 the immense mouth can still be recognised not far from the 

 great crater Plato on the east of that great lake.*) 



The .steady sinking of the sea floor all round the immense 

 seam thus formed gradually press.^d the matter deposited in 

 a troughlike depression into the form of a ridgelike elevation. 

 (The reader anxious for technical expressions — which have 

 the advantage certainly of giving the appearance of much 

 learning at a very cheap rate — may speak here, if he likes, 

 of subclinal and anticlinal ; but the actual shapes are 

 perhaps as well indicated by speaking of troughs and i-idges.) 

 We may well believe that intense heat was developed in the 

 process, and much of the sedimenfciry matter metamorphosed 

 into crystalline rock. That the original summits of the 

 elevated ridge were domed or rounded seems altogether 

 probable. That the summits are now no longer lounded 

 is clear from the shapes of the shadows cast on the floor of 

 the surrounding plane when the sun is rising in the lunar 

 east. Here, then, is clear evidence of denuding action con- 

 tinuing long after the ridge had been, as it were, shouldered 

 out of the seas by the side pressures of the sinking sea floors 

 on either side. 



On this occasion, then, or rather during this special stage 

 of the moon's history, the denuding processes went on long 

 enough, and with sufficient tuergy, to do such work as took 

 place many times over during the past history of our earth. 

 '■ The hills are shadows, our poet sings, and they flow from 

 form to form and nothing stands; like mists they molt, the 

 solid lands, like clouds they shape themselves and go." On 

 the moon a poet would hardly have used such a com[)arison, 

 even if any poet on the moon ever came to know so much 

 of the past of his world as we have just indicated. For 

 unquestionably the lands on the moon did not melt away 

 like mists, but once only gave enough of their substance up 

 to form one set of mountain ranges. The lunar poet could 

 not echo the terrene poet's exclamation, " Oh earth, what 

 changes hast thou seen ; " once, and once onl^y, he would have 

 to admit, " hast thou, oh moon, seen change." Yet for the 

 rest lie might have sung (millions of years ago) with the 

 Tenny.son of our later world, '• There rolls the deep where 

 grew the tree ; there where the loud street roars " (if ever 

 they had loud streets in lunar cities), "hath been the still- 

 ness of the central sea." 



But oven of one such change the traces should remain. 



* We use the words " east " and " west " the reverse way from that 

 employed in lunar maps. Picturing the moon as a planet, and com- 

 paring the processes taking place on her surface with those taking 

 place on the earth, it seems reasonable to speak of east and west on 

 the moon as we speak of them on the earth, not (as our maps of the 

 moon do) as we speak of them on the skies — for example, in speak- 

 ing of a constellation. 



