Obviously just the opposite holds when the ratio of lead-206 to lead-207 is too small.
The result of this enormous array of tests is a consensus. The process rate must always have been the same" (Morris 1974a, 138).
The second worry is that extra amounts of the daughter element may enter the system after the original formation of the rock, thus giving the impression that more of the parent element has undergone radioactive decay than has actually been the case.
In both the examples I have described, there are ways of checking that such intrusions have not occurred.
Given this value of Dit is then possible to use either decay process to calculate the age of the rock.
If the results agree, they are said to be concordant, and geologists are usually confident that concordant ages are the true ages of the rocks under consideration.