Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.
But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.
Look at the many photographs of the Grand Canyon in your textbook.
Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon.
A more modern way of stating the same principle is that the laws of nature (as outlined by the laws of chemistry and physics) have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.
Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.