Passage 10.5.6
εἶναι δὲ αὐτὴν τῶν περὶ τὸ ὄρος νυμφῶν. ἔστι δὲ ἐν Ἕλλησι ποίησις, ὄνομα μὲν τοῖς ἔπεσίν ἐστιν Εὐμολπία, Μουσαίῳ δὲ τῷ Ἀντιοφήμου προσποιοῦσι τὰ ἔπη· πεποιημένον οὖν ἐστιν ἐν τούτοις Ποσειδῶνος ἐν κοινῷ καὶ Γῆς εἶναι τὸ μαντεῖον, καὶ τὴν μὲν χρᾶν αὐτήν, Ποσειδῶνι δὲ ὑπηρέτην ἐς τὰ μαντεύματα εἶναι Πύρκωνα. καὶ οὕτως ἔχει τὰ ἔπη· αὐτίκα δὲ Χθονίης φωνὴ πινυτὸν φάτο μῦθον, σὺν δὲ τε Πύρκων ἀμφίπολος κλυτοῦ Ἐννοσιγαίου. Musaeus , Eumolpia χρόνῳ δὲ ὕστερον, ὅσον τῇ Γῇ μετῆν, δοθῆναι Θέμιδι ὑπʼ αὐτῆς λέγουσιν, Ἀπόλλωνα δὲ παρὰ Θέμιδος λαβεῖν δωρεάν· Ποσειδῶνι δὲ ἀντὶ τοῦ μαντείου Καλαύρειαν ἀντιδοῦναί φασιν αὐτὸν τὴν πρὸ Τροιζῆνος.
They say she is one of the nymphs who dwell around the mountain. Among the Greeks there exists a poem, known as the Eumolpia. The verses of this poem are attributed to Musaeus, son of Antiophemus. In this composition, it is related that the oracle originally belonged jointly to Poseidon and to Earth. The Earth herself uttered the oracles, while Pyrcon served Poseidon as his attendant in matters of prophecy. The verses run thus: "And straightway the voice of Chthonia spoke an insightful word, And Pyrcon, attendant to glorious Earth-shaking Poseidon..." According to the story in Musaeus' Eumolpia, somewhat later in time, Earth handed it over to Themis, and Apollo received it as a gift from Themis. In compensation for the oracle, Apollo gave Poseidon Calaureia before Troezen.