Description of absolute dating and numerical age
In the diagram to the right,the fault cuts the limestone and the sandstone, but does not cut the basalt.
Thus we know that the fault is younger than the limestone and shale, but older than the basalt above.
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.
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.
This principle is often stated as "the present is the key to the past".
By carefully digging, we have found that each trash pit shows a sequence of layers.
Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits.
Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.
If the enclosing rock is an igneous rock, the inclusions are called xenoliths.