586 



MOON", EEOENT OBSERVATIONS AND STUDY OF THE. 



of distributing water over the surface for the 

 especial convenience of subaerial fauna and 

 flora. 



Many other rings or circular basins are dis- 

 tributed over the moon's surface, of sizes va- 

 rying from those we have just been consider- 

 ing, to diameters of over one hundred and fifty 

 miles ; but none of the larger ones present any 

 appearance suggestive of the presence of wa- 

 ter. The peculiar markings over the surface 

 about many of them make it unmistakably evi- 

 dent that, in times past, they were the sources 

 of water-flows, some to an extent that makes 

 them the centers of ray-like forms, which pro- 

 ceed from them in every direction. These ray- 



Islands. That mountain has been chosen be- 

 cause its crater is the widest known, and a 

 glance at both formations will show that the 

 ring-mountains of the moon have no feature in 

 common with terrestrial volcanic craters; one 

 is a depression below the general level, while 

 the other is, by comparison, a slight depression 

 in the top of a mountain elevated far above it. 

 In fact, there is no such thing as a volcano upon 

 the moon. 



Some of the radiating ridges from Tycho and 

 other centers are not turned aside by encount- 

 ering the mountain-rims of other circular ba- 

 sins ; but the water entered these and evident- 

 ly filled them up to the general level of the 



systems decrease from this complete surround- 

 ing through all degrees of incompleteness, un- 

 til many ring-basins are without any appear- 

 ance of radiates. 



An illustration of the basin named Tycho is 

 here presented, because it is the center of the 

 greatest ray-system on the lunar surface; one 

 of its radiates can be traced to the distance of 

 eighteen hundred miles from it; other "ring- 

 mountains," "basins," or "walled circular 

 plains " without any rays, may be seen in the 

 picture. This basin shows the general con- 

 struction of the " ring-mountains of the moon." 

 They are, aa a rule, quite circular, though there 

 are occasional modifications of this form. The 

 mountain-rim of the larger basins frequently 

 rises five thousand feet above the general level, 

 but the inclosed plain is often depressed below 

 the apex three times more than that altitude ; 

 and many of them have mountain elevations at 

 their centers. To the left of the basin has been 

 introduced, upon the same scale, an outline of 

 the volcanic crater Mona Keo of the Sandwich 



moon's surface, for the ray continues its origi- 

 nal direction across the bottom of the basin 

 entered. Its appearance, however, changes to 

 a smooth ridge across the lake thus formed, 

 depositing, as it went, the sediment it held in 

 suspension, still continuing its course over the 

 surface on the opposite side, and there again 

 assuming its original characteristics of form. 

 These resemble very much those constructed by 

 rivers flowing through their own deltas, which 

 deposited enough sediment to build their own 

 banks above the general level of the delta (like 

 the Nile or the Mississippi), and upon encount- 

 ering a broader expanse of water deposit a 

 sedimentary drift across its bottom. These 

 water-flows seem to have subsided gradually 

 by slowly decreasing in quantity, filling up 

 their channels with sediment, and most fre- 

 quently present the appearance of rounded, 

 elevated ridges, hardly high enough to be con- 

 sidered hills. But one of the rays that project 

 from Tycho is a long, deep valley, with banks 

 on either side, which a section of a river flow^ 



