In the same way, by identifying fossils, he may have related Sedimentary Rocks B with some other rocks.
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For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.
Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.
Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fall and winter months.
When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings.
Simply counting the number of rings will give one a fairly good idea of the age of the tree.
Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes.
From his research, our evolutionary geologist may have discovered that other geologists believe that Sedimentary Rocks A are 200 million years old and Sedimentary Rocks B are 30 million years old.
Thus, he already ‘knows’ that the igneous dyke must be younger than 200 million years and older than 30 million years.
Thus, although "extinct", these nuclides are present in meteorites, but produced by a more recent process.
"The idea that Rb-Sr is the most used chronometer for meteorites is largely based on work done 10-30 years ago.
Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.