The geologic law scientists use to date crust by studying layers of rocks is called the Law of Superposition.

Here's why:

* Superposition: This law states that in undisturbed rock sequences, the oldest layers are found at the bottom, and the youngest layers are at the top.

* Dating: By understanding the order of rock layers, geologists can determine the relative ages of different rock formations. They can then use techniques like radiometric dating to assign absolute ages to these layers.

Important Note: The Law of Superposition is a fundamental principle in geology. It helps geologists understand the history of the Earth and its formations.