Lectures on the Sūtra of the Collection of the Father and Son (Pitāputrasamāgamanasūtra) (20)

When the external earth element gradually took shape, it took twenty small kalpas—336 million years—to be established from empty space where nothing existed, gradually forming the inanimate universe of the three thousand great thousand worlds. After the three thousand great thousand worlds are destroyed, there is again nothing at all; everything subject to arising and ceasing is empty, without a single dharma.

The seeds of the earth element exist within the tathāgatagarbha, and these seeds are also formless and markless—they are empty. The external mountains, rivers, and great earth that are formed are empty in their self-nature as well; they have no self-existing essence and cannot determine their own existence, being created and fashioned by the tathāgatagarbha. The hardness of material objects is also empty, formless and markless, arising and ceasing along with the objects themselves.