The portion, referred to as an eon, of geological time that extends for several hundred million years from the end of the accretion of the Earth to the formation of the oldest recognized rocks. According to current models, the inner planets formed by the accretion of planetesimals in an environment where gas and volatiles had been swept away by early intense solar activity. The accretion of the Earth appears to have been completed between 50 and 100 million years (m.y.) after the beginning of the solar system (T₀) as recorded in the oldest refractory inclusions in the Allende Meteorite, whose age of 4566 ± 2 m.y., ascertained by lead isotope dating, is taken as T₀. Core formation on the Earth appears to have been coeval with accretion and so preceded the Hadean. Any primitive atmosphere was removed by early collisional events, and the present atmosphere has arisen by a combination of degassing and additions from comets.