Smithsonian Report, 1960 — Urey 



PioleiiKicus, showing the many small craters. The surface is not smooth, and the smooth 

 gray material covers some craters which antedated the formation of the material which 

 covers the floor. The walls have been scarred by projectiles which arrived from the 

 north-northeast and which were part of the debris from the Imbrian collision. 



