βάλλετο κρηπῖδα: P. 7.3: κρηπῖδ᾽ ἀοιδᾶν βαλέσθαι. The metaphor shifts rapidly, but the notion of drink - offering is not foreign to that of laying the foundation.
Παῖ Π.: Stately genealogical address, with effective position of vocative.
Πετραίου: Poseidon was worshipped in Thessaly as the Cleaver of the Rock, because he had opened a way through the rock for the Peneios. On the π's, see v. 150.