Principle of Original Continuity If layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock.
Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon.
The Grand Canyon is particularly good for this because different sedimentary rocks have different colors.
For example, the mudstone, sandstone and shale are cut by the basalt dike, so we know that the mudstone, sandstone, and shale had to be present before the intrusion of the basalt dike.
In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.
Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
Prior to the late 17th century, geologic time was thought to be the same as historical time.