The great curiosity of St. John is the "reversible cataract" in the river, caused in the gorge just west of the city by the enormous tides of the Bay of Fundy. The great river above the city is a wide estuary, but before entering the harbor it is compressed into a short, deep and narrow gorge, barely one hundred and fifty yards wide in some places, and obstructed by several rocky islets. As this is the best crossing-place, two bridges are thrown side by side over the chasm, one for a railway and the other for a street, resting upon the limestone cliffs a hundred feet above the water. As the tide ebbs and flows, the rushing river currents make the reversible cataract, almost under the bridges, with the water pouring down both ways at different tidal stages. Through this contracted pass the entire current of the vast St. John valley finds its outlet to the sea. When the ebb tide quickly empties the harbor below, the accumulated river waters cannot get into the gorge fast enough to reduce as rapidly the level of the broad basin above, and they consequently rush down, a cataract, swelling sometimes to ten or twelve feet at the upper entrance to the gorge, and make whirling, seething rapids below. When the tide turns, this outflow is gradually checked by the rise in the harbor, but soon the tremendous incoming flood from the Bay of Fundy overpowers the river current, fills up the gorge, and rapidly rising in the gorge rushes inward to the broad basin, thus making the cataract fall the other way. Twice every day this ever-changing contest is fought, and were it not for the obstruction made by this narrow, rocky gateway, these enormous tides would rush along in full force and overflow a large surface of the very low-lying interior of New Brunswick. The river makes a sharp bend just at the outlet of the gorge, turning from south to northeast around a rocky cape protruding far into the stream; then it broadens out into a rounded bay, and a short distance beyond sharply bends again into the harbor of St. John. Vessels are taken through the gorge at proper tidal stages, guided by tugs and floating at high speed with the rushing current. This is one of the most remarkable exhibitions made of the curious influence of these enormous Bay of Fundy tides.