Scientists combine several well-tested techniques to find out the ages of fossils. The most important are relative dating, in which fossils and layers of rocks are placed in order from older to younger and radiometric dating which allows the actual ages of certain types of rocks to be calculated. In the 17th century, progress was made. Nicolas realized that the layers of rock closest to the surface generally were the younger than those underneath. William Smith concluded that the layers of rock containing the same fossils were the same age.
"... the deeper the layer, the older the rocks"
"... regardless of their compositions or locations, could be correlated with each other by when they were made"
Fossil Life ( student reading) https://staff.concord.org/~btinker/GL/web/geology/english/dating_rocks.htm
"... regardless of their compositions or locations, could be correlated with each other by when they were made"
Fossil Life ( student reading) https://staff.concord.org/~btinker/GL/web/geology/english/dating_rocks.htm