 Another outlet of Lac des Deux-Montagnes, the Rivière des Prairies, flows along the north shore of the island, separating it from Île Bizard and Île Jésus, and flows into the St. Lawrence at the northeastern tip of the Island of Montreal, while the Rivière des Mille-Îles separates Île Jésus from the northern mainland.Most of the bedrock of the island consists of Chazy limestone, with some Utica shale.At the centre of the island stand the three peaks of Mount Royal, a volcanic intrusion (not an extinct volcano, contrary to popular belief).