In the late 1700s scientists proposed that the layers in the rocks of mountains and canyons represented a record of the passage of time, with the younger layers lying on top of the older layers. what could they determine about the fossils they were finding in the rocks based on this observation?

That the younger fossils are present at the upper layer in comparison to the fossils in the lower layers.  

Explanation:

The scientists have illustrated that the layers of the rocks of the canyons and the mountains are aligned with period and that they are the demonstration of the geological past. As the scientists postulated this, it also signifies that the layers of the rocks have not got distracted by the geological procedure. Thus, they are aligned by age, that is, the youngest at the top, and the oldest being present at the bottom.  

In the case of fossils, this signifies that the scientists can now predict the oldness of the fossils, on the basis of the layer in which the fossils are present. Thus, the nearer they are witnessed at the top, the newer they are, and the deeper they are found, the mature they are.