Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.
Prior to the late 17th century, geologic time was thought to be the same as historical time.
Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.
Thus, we can deduce that the mudstone and shale are older than the rhyolite dike.
But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.
Once these age relations were worked out, another principle fell into place - the principle of fossil succession.
We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.Thus, we know that the dike is younger than the mudstone, sandstone, and shale.Similarly, the rhyolite dike cuts only the mudstone and the sandstone, but does not cut across the shale.Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.Sediment deposited on steep slopes will be washed away before it is buried and lithified to become sedimentary rock, but sediment deposited in nearly horizontal layers can be buried and lithified.