Final answer:
The fundamental geologic principle that allows geologists to use the remains of organisms when correlating rock units is fossil or faunal succession.
Step-by-step explanation:
The fundamental geologic principle that allows geologists to use the remains of organisms when correlating rock units is fossil or faunal succession. Fossils are useful for dating and correlating rocks because certain fossils only exist within specific time periods. By examining the fossils found in different rock layers, geologists can determine the relative ages of the rocks and how they correlate with each other.