ORIGIN AND NATURE OF THE MOON — UREY 259 



convinced that merely looking in this way will never provide very 

 detailed evidence. If only we could see the crystalline structure of the 

 lunar rocks! Lava, especially basaltic lava, is a dense liquid of con- 

 siderable fluidity which might push over and distort crater walls in 

 its path. I can see no evidence for this having happened in Mare Nu- 

 bium or Oceanus Procellarum. I have for the most part studied only 

 pictures, but a little observing convinces me that gross features of this 

 kind are not likely to be missed when good photographs are available. 

 But lava must flow downhill, whereas dust can fall anywhere. The 

 coloring in some photographs indicates a smooth hill over the eastern 

 wall of Prinz and over the craters Hippalus and Weiss : other exam- 

 ples are less clear. On the other hand, the crater Wargentin is full to 

 the brim with what appears to have been a liquid. Gilbert noted that 

 Julius Caesar is filled as high as a breach in the southern wall and that 

 Posidonius is filled to the height of a similar breach, which suggests 

 that the contents of both were once fluid. 



Mare Tranquillitatis has a very irregular shape and is very black. 

 One would expect that the settling of dust would give the moon a uni- 

 form color or that color variations would be gradual. The boundary 

 between the black of Mare Tranquillitatis and the gray of Mare Sere- 

 nitatis in the western part of the latter is very sharp, and there appears 

 to be no difference in level. In fact. Mare Tranquillitatis does look 

 like a lava flow. Some of its craters — for example, IMaclear — seem to 

 be distorted. The fragments of craters in the western part of the 

 mare look as though they may have been pushed about. 



Possibly both lava and dust were produced by the collisions, some- 

 times one and sometimes the other. Possibly a planetesimal fell ver- 

 tically and produced Mare Serenitatis and a pool of very dark lava 

 which flowed into Mare Tranquillitatis. It had only a small content 

 of volatile substances, and its material was distributed to only a lim- 

 ited extent over the moon. Then the Imbrian planetesimal, containing 

 an appreciable concentration of volatile substances, fell and distrib- 

 uted gray dust widely. Possibly it was some other series of events. 

 Gold's dust hypothesis has at least stimulated some serious thinking 

 about the long-accepted lava hypothesis. 



DURATION OF THE BOMBARDMENT 



It is notable that there are no large craters of later date than the 

 maria in the collision maria : in the case of Mare Imbrium this applies 

 only to the collision area within the broken circle of plate 2. The- 

 ophilus and Piccolomini, near the shore of Mare Nectaris, and another 

 on the Altai Mountains at the south are certainly postmare, as are 

 other craters near the shores of other maria. These circular collision 

 maria must have been fluid, that is, either true liquid or dust "fluid- 



