Principles of relative dating of geologic events dating shows yahoo answers
We could assume that this igneous intrusion must have happened after the formation of the strata.
Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.
It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.