304 ANNUAL PtEPORT SMITHSONIAN INSTITUTION, 192 8 



solidation. On the other hand, Brown ^^ has determined that the 

 higher, southern limb i.s farther from the moon's center of mass 

 than the northern. In other words, the principle of isostasy applies 

 to the moon. There seems, then, to be sufficient analogy between 

 the moon's maria and the earth's ocean basins for the former to give 

 testimony in regard to the causes that have operated to make the 

 latter. 



There are several ways of determining the relative ages of the 

 features on the moon's .surface. The most positive time value is 

 the order of superposition of craters. Next to be noted is the fact 

 that the older craters have lower-angled slopes, as shown by the 

 deficiency of shadows cast by the slanting sunlight. The younger 

 craters, showing their youth by their sharpness of definition, as well 

 as by superposition, also have the ray systems that are so con- 

 spicuous under the high-angle light upon the full moon, as seen in 

 Plate 1. The maria, on the other hand, are younger than the 

 chaotic upland surface, as shown by the melting in of that surface 

 and the absence of ancient ruined craters. Some of the craters on 

 the margins of the maria are, however, partly melted relics of the 

 older surface, but most of them are clearl}^ younger than the lava 

 plains. The great crater Copernicus (near the top of pi. 3) is 

 conspicuous among these younger features, but in the older area of 

 the moon there are craters just as young, as seen in Tycho (near the 

 center of pi. 2), which has the most conspicuous ray system on the 

 moon. 



Do all the physical features of the moon belong to a period early 

 in its history; is it now a dead world? This is the view commonly 

 taught by astronomers and it has been accepted by geologists. In- 

 terpreting the moon's surface features by the methods of geology, 

 however, a quite different answer is obtained. Its internal igneous 

 forces work too slowly and intermittently to be seen by the human 

 eye, unless such small changes as those observed in the crater Linne 

 are indeed the result of recent igneous activity. 



The measure of the relative recency of events may be obtained from 

 a study of the progress of crumbling recorded on the walls of the 

 craters. In terrestrial deserts the cliief causes of disruption of the 

 rocks are (1) rapid change of temperature from night to day, (2) 

 freezing of water in joints, and (3) dashes of rain on heated rocks. 

 On the moon, however, rain is wholly absent, freezing in joints may 

 be a minor factor from volcanic waters; but the changes of tempera- 

 ture between midnight and midday, though slower between maxima, 

 are rapid at sunrise and sunset and the results are far more pro- 

 found than in the earth's deserts. During the lunar night, the tem- 



18 E. W. Brown, Address on Cosmical Physics. Biitisli Association, sec. A, Australia. 

 1914 Reference No. 201, p. 6. 



