Geological time and radiometric dating
Laws of Stratigraphy Original Horizontality - sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
There are different ways that scientists can measure geological time.
The igneous activity that produced such intrusions...
...calculation was based on the assumption that the substance of the Earth is inert and thus incapable of producing new heat.
These radioactive isotopes are parent isotopes, which decay slowly to daughter isotopes, changing the rock’s isotopic character.
The rate at which the isotopes decay is in effect our "geological clock".