Abstract
Crystalline rocks from Mare Tranquillitatis have yielded potassium-argon dates as old as 3.8 x 109 years. Crystalline rocks from Oceanus Procellarum give potassium-argon ages as old as 2.8 x 109 years. Evidently the maria are ancient features of the moon and were not formed contemporaneously, a conclusion that also has been verified by other methods. The potassium-argon ages of rocks from Oceanus Procellarum show much more loss of argon than the rocks from Mare Tranquillitatis, an indication that the rocks at Oceanus Procellarum have experienced more severe shock effects or longer cooling rates.
