The main focus of this book is on how objects can be dated using measurements of radioactive isotopes and their products, that is the elements and isotopes that form after radioactive decay.
The author begins with a brief discussion of ideas about the earth's duration before the advent of dating techniques using radioactive isotopes. Here he discusses the duration of the earth as inferred from the Bible, the influence of James Hutton in moving scientific opinion towards a longer time scale, William Smith's use of fossils to come to a relative (that is, the order in which rocks were formed, but not when they were formed) dating of sedimentary rocks, and the conflict in the later half of the 19th century between geologists' belief in a long earth history and the physicist Lord Kelvin's model of a relatively short (20 million years in some versions of the model) duration for the earth.
With the discovery of radioactivity, in the early 20th century it became apparent that radioactive decay could be used a sort of clock. The physicist Ernest Rutherford was one of the first to attempt to estimate geological time scales using radioactive decay. The British geologist Arthur Holmes in his early work was one of the first geologist's to use the decay of uranium to lead to estimate geological time scales. These early efforts were hampered by the lack of understanding that different isotopes of the same element exist, and that there can be more than one radioactive isotope of an element.
As understanding of the complexity of the problem increased, more accurate methods resulted. Claire Patterson, at the University of Chicago and later at Caltech, came up with the roughly 4.55 billion year estimate of the duration of the earth's existence in the 1950s using the uranium to lead decay series, after much difficulty in eliminating laboratory contamination of lead from leaded gasoline. Starting in the 1940s at the University of Chicago, Libby and his graduate students developed carbon 14 dating, which is suitable for dating objects that contain carbon from roughly the last 50,000 years and is therefore useful for archaeologists, and for geologists who study ice ages. One thing I was interested to learn is that the carbon 14 method is the only one that involves the actual counting of radioactive decay; the other methods, such as uranium to lead or potasssium argon, actually require the measurement of the "parent" element and isotope (such as uranium) and the "daughter" element and isotope (such as lead) with a mass spectrometer, because radioactive decay is too slow for practical counting from small samples of these isotopes.
Each radioactive method is suitable for different time spans, The uranium lead method is suitable for very long (billions, hundreds of millios of years) time spans, the potsssium argon method for intermediate (in a geological sense!) time spans, and carbon 14 for the last 50,000 years or so. Because carbon 14 is produced at varying rates over time in the upper atmosphere (from the interaction of cosmic radiation with molecules in the air), to improve its accuracy it is calibrated with (mainly) tree ring data. The calibration at the moment goes back about 26,000 years.
Recent developments have allowed for collecting information from smaller samples, such as individual crystals of zircon,
I found the book easy to read. The author includes two appendices with some discussion of the mathematics of radioactive decay, a chart of the geological time scale, and the periodic table of the chemical elements.