Why is the earth compared to a grindstone in A Tale of Two Cities?

This is an elegant metaphor, and everyone should appreciate just how good it is. The earth, and the grindstone, turn to produce. Just as the Earth simply moves, the grindstone simply moves, and all that is dull becomes sharp. It is the same with the earth, all that is weak becomes stronger with time. The metaphor can also be demonstrative of how everything fades in time, and that the earth will grind away all that seems important.