Passage 8.15.2
ἄγοντες δὲ παρὰ ἔτος ἥντινα τελετὴν μείζονα ὀνομάζουσι, τοὺς λίθους τούτους τηνικαῦτα ἀνοίγουσι· λαβόντες δὲ γράμματα ἐξ αὐτῶν ἔχοντα ἐς τὴν τελετὴν καὶ ἀναγνόντες ἐς ἐπήκοον τῶν μυστῶν, κατέθεντο ἐν νυκτὶ αὖθις τῇ αὐτῇ. Φενεατῶν δὲ οἶδα τοὺς πολλοὺς καὶ ὀμνύντας ὑπὲρ μεγίστων τῷ Πετρώματι.
Each year, when performing what they call the Greater Rite, they then open these stones. Taking out from within them writings intended for the ceremony, they read them aloud in the hearing of the initiates, and then place them back in the same night. I also know that most of the Pheneatians swear by the Petroma in their most solemn oaths.