FRAGMENTATION" — BAREELL 305 



perature of the surface, which is not blanketed as on the earth by an 

 «itmosphere, must sink to near absolute zero. But the night endures 

 for two weeks, and in this time cold and resultant contraction must 

 penetrate many feet in depth. During the two weeks of lunar mid- 

 day the temperature of the moon's surface in its torrid zone has been 

 measured as being- above the boiling point of water on the earth. 

 This rhythmic monthly change of temperature of perhaps 500° F. 

 must have somewhat the same disintegrative effect as the exposure 

 of rocks to fire. Blocks large enough to be cold on the inside while 

 hot outside, and vice versa, will be riven by the rhythmic expansive 

 and contractive forces. The recurrent changes in volume must tend, 

 therefore, to work the loose material down to flatter and flatter 

 slopes, although the individual fragments will still stand up with 

 steep faces. There are, however, limits to these actions, dependent 

 upon the rapidity as well as the amount of the temperature changes. 

 This form of insolative disintegration, dependent upon solar heat, has 

 been operative with nearly equable effect throughout the whole his- 

 tory of the moon and has been most intense in its equatorial latitudes. 

 It may be further aided by the passage of lunar heat and gases at 

 times of exceptional volcanic activity, but the evenness of the dis- 

 integrative etTect suggests a dominance of external temperature 

 changes as the cause. Craters which show no appreciable insolative 

 disintegration are therefore relatively young. From these sharp- 

 edged craters we may trace back successive generations in time until, 

 as in the region of Tycho, may be faintly seen the almost obliterated 

 ruins of great crater rings that appear to be twenty or even one hun- 

 dred times older. Measured by the scale of terrestrial chronology, 

 the great craters Tycho and Copernicus, each 65 miles in diameter, 

 may be as recent at least as the younger Tertiary. 



Judged by these means, how old are the maria? Here we must 

 decide between the craters that are within or adjacent to the maria, 

 but are older than the}', and the craters that are younger. The outer 

 l^ortions of the Mare Nectaris^^ show an ancient topography, but 

 the bounding scarp of the Altai Mountains at the edge of this mare is 

 very high and steep. The floor of Nectaris appears, consequently, to 

 have been formed by a downsettiing accompanied by a melting in, 

 but the latter process has not extended to the walls. The great crater 

 Theophilus situated in this mare is, however, younger than the mare, 

 as shown by the depression of its floor, but it is without the ray sys- 

 tem, which appears to mark the very youngest of the craters. Thus 

 we reach the striking conclusion that the maria are among the 

 younger features of the moon, though not the youngest. 



" The smaU mare in the southwest quadrant nearest to the center. In pi. 1 the 

 features spoken of do not show. 



