In the "Round Earth" model gravity causes massive amounts of matter to pull toward each other which causes higher and higher pressure build ups in the center of a mass. As people dig holes in the ground, (simply put. huh?) molecules of every element are found more compact. With higher densities, the forces acting upon these elements must be stronger further down in the Earth's crust.
When we translate these findings to the FE theory, gravity is explained by the acceleration of the whole planet at the rate of gravity. If this is so, then why is more force exerted on rocks the further we dig? This also can relate to the issue that gravity or "gravitation" is lesser at high altitudes.