Passage 8.24.12
Ψωφιδίοις δὲ καὶ παρὰ τῷ Ἐρυμάνθῳ ναός ἐστιν Ἐρυμάνθου καὶ ἄγαλμα. ποιεῖται δὲ πλὴν τοῦ Αἰγυπτίου Νείλου ποταμοῖς τοῖς ἄλλοις λίθου λευκοῦ τὰ ἀγάλματα· τῷ Νείλῳ δέ, ἅτε διὰ τῆς Αἰθιόπων κατιόντι ἐς θάλασσαν, μέλανος λίθου τὰ ἀγάλματα ἐργάζεσθαι νομίζουσιν.
The people of Psophis have beside the Erymanthus river a temple and statue of Erymanthus. Statues of rivers are made from white marble for all rivers except the Egyptian Nile; the Nile alone, because it flows through Ethiopia on its way down to the sea, is believed to have its statues fashioned of black stone.