The slope of the line determines the date, and the closeness of fit is a measure of the statistical reliability of the resulting date.
Here is one example of an isochron, based on measurements of basaltic meteorites (in this case the resulting date is 4.4 billion years) [Basaltic1981, pg. Skeptics of old-earth geology make great hay of these examples.
For example, creationist writer Henry Morris [Morris2000, pg.
This will be covered in the chapters, Primitive Environment (chapter 9), and DNA (chapter 10).
Third, evolution of living organisms into more advanced life forms by natural selection or mutations.
Using the potassium-argon method, Fitch and Miller were the first to measure the age of the tuff.
Their result of 212–230 million years did not agree with the age of the fossils (elephant, pig, ape and tools) so they rejected the date.They said the sample was contaminated with excess argon.Using new samples of feldspar and pumice they ‘reliably dated’ the tuff at 2.61 million years, which agreed nicely.Later, this date was confirmed by two other dating methods (paleomagnetism and fission tracks), and was widely accepted.Then Richard Leakey found a skull (called KNM-ER 1470) the KBS tuff, a skull that looked far too modern to be 3 million years old.Tests by other scientists using paleomagnetism and fission tracks confirmed the lower date.