He was employed at Caltech's Division of Geological & Planetary Sciences at the time of writing the first edition.
Assuming a strictly literal interpretation of the week of creation, even if some of the generations were left out of the genealogies, the Earth would be less than ten thousand years old.
But because God has also called us to wisdom, this issue is worthy of study.
Rocks are made up of many individual crystals, and each crystal is usually made up of at least several different chemical elements such as iron, magnesium, silicon, etc.
When the glass is turned over, sand runs from the top to the bottom.
Radioactive atoms are like individual grains of sand--radioactive decays are like the falling of grains from the top to the bottom of the glass.
Radiometric dating techniques indicate that the Earth is thousands of times older than that--approximately four and a half billion years old.