This is a problem that is commonly encountered in attempting to date rocks. The fossil-bearing rocks tell us the geological period. Yet most fossils are found in sedimentary rocks that cannot be isotopically dated directly. Igneous and metamorphic rocks are easier to directly date but they almost never contain fossils. Following is a chart of isotope ages calculated from the igneous rocks.
| Dike | 400 million years |
| Lava flow | 600 million years |
So how old is the Cambrian?