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chondritic meteorites, but icy planetismals may have contributed components of the atmosphere andhydrosphere. By analogy with the evolution of the moon, the growth of the terrestrial mass and volumeresulting from meteorite impact and accretion occurred exponentially over a period of about 50 millionyears (Ouyang ZY et al., 2002). After 4.0 Ga the number of meteorite impacts decreased very rapidly,and the augmentation of the Earth's mass since that time, caused by the impacts, has been only 1025grams, 1/600 of its total mass. This implies that accretion of the Earth effectively ceased at about 4.0 Ga,and the mass and volume of the Earth has since remained essentially unchanged.