Why Can't Dinosaur Fossils Be Dated With Either C 14 Or U 238?

Uranium 238 can only be found in rocks that are either igneous or volcanic in origin. Therefore, it is not possible to date fossils directly using U 238. Since the half lifetimes of carbon 14 and uranium 238 are so drastically different from one another, it is impossible to employ them together. The radioactive isotope carbon 14 can only accurately date fossils that are quite recent in age.