the period beginning with the formation of the earth and ending in the present day. It is often tabulated beginning with the Cambrian period, since the majority of fossil forms date from then. Precambrian time began with the formation of the earth about 4,600 million years ago. Dating of rocks is usually carried out by the measurement of radioactive decay. The older the rock the less radioactive it is. Organic remains are dated by CARBON DATING. See Appendix A.