 He established that lead was the final decay product of uranium, noted that the lead-uranium ratio was greater in older rocks and, acting on a suggestion by Ernest Rutherford, was the first to measure the age of rocks by the decay of uranium to lead, in 1907.He got results of ages of 400 to 2200 million years, the first successful use of radioactive decay by Pb/U chemical dating (isotopes not discovered yet).More recently, older mineral deposits have been dated to about 4.4 billion years old, close to the best estimate of the age of earth.