Radiometric dating instruments
Fortunately, we are able to date older fossils using the radiometric breakdown of other elements (Potassium-Argon dating, Argon-Argon dating, and Rubidium dating [I'm writing this without any refs - so this last one might be wrong]).Usually the radioactive 'clocks' for these elements are started when the elements are deposited by a volcanic eruption (usually in the form of ash).The radioactive isotope Carbon 14 has a half-life of 5,730 years.This has made it useful for measuring prehistory and events occurring within the past 35 to 50 thousand years.Even in the case of very long half-lives, modern scientific instruments are now accurate enough to give very fine readings.We usually hear of Carbon 14 dating, which is very important in archaeology.
Also, it is of little use in anything more recent than 5,000 years ago.
During the 19th century, and even well into the twentieth, geological chronology was very crude.
Dates were estimated according to the supposed rate of deposition of rocks, and figures of several hundred million years were bandied out; usually arrived at through inspired guesswork rather than anything else.
However, although 5730 years is the correct half-life, it is not the one used for most C-14 dating, simply because the original half-life used to determine dates back in the 1950s was wrong, and to be consistent we still tend to use the wrong value (a bit like the direction of current flow in electronics, which is the opposite of that which the electrons take, but was the original and incorrect assumption).
The practical range for dating is in the order of a few hundred to about 40,000 years BP.