Therefore, since sedimentary rock is the only kind of rock that bears fossils, a relative date is estimated by the position of a sedimentary rock in relation to an igneous outflow.
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Most texts teach that it takes a long time for these sediments to build up, with older layers buried beneath younger layers.
Fossils found in lower layers are deemed to be older than those in the upper layers, older on the bottom younger on the top.
Similarly, some rocks return radiometric “ages” twice as old as the accepted age for earth.
Most rocks return conflicting radiometric “ages.” In these cases, researchers select results that match what they already believe about earth’s age (see the section Brand New Rocks Give Old “Ages” for details of this study and several others like it).
These assumptions are: (1) the radioactive element decays at a constant rate (2) the rock crystal being analyzed is not contaminated by infusion of excess end product (3) the rock crystal contained no end product when it was formed (4) leaching of the parent element out of the rock sample did not occur.