For some length below Niagara the waters are violently agitated; however, at the distance of half a mile, a ferry plies across in safety. The high banks on both sides of the river extend to Queenston and Lewiston, eight miles lower, confining the waters to a channel of no more than a quarter of a mile in breadth, between steep and lofty cliffs; midway is the whirlpool,[135] where the current rushes furiously round within encircling heights. Below Queenston the river again rolls along a smooth stream, between level and cultivated banks, till it pours its waters into Lake Ontario. Ontario is the last[136] and the most easterly of the chain of lakes.[137] The greatest length is 172 miles; at the widest it measures 59 miles across; the circumference is 467 miles, and the surface is 334 feet below the level of Lake Erie. The depth of Ontario varies very much along the coast, being seldom more than from three to 50 fathoms; and in the center, a plummet, with 300 fathoms of line, has been tried in vain for soundings. A sort of gravel, small pieces of limestone, worn round and smooth by the action of water, covers the shores, lying in long ridges sometimes miles in extent. The waters, like those of the other great lakes, are very pure and beautiful, except where the shallows along the margin are stirred up by violent winds: for a few days in June a yellow, unwholesome scum covers the surface at the edge every year. There is a strange phenomenon connected with Ontario, unaccounted for by scientific speculation; each seventh year, from some inscrutable cause, the waters reach to an unusual height, and again subside, mysteriously as they arose. The beautiful illusion of the mirage spreads its dreamy enchantment over the surface of Ontario in the summer calms, mixing islands, clouds, and waters in strange confusion.[138] The outline of the shores is much diversified: to the northeast lie low lands and swampy marshes; to the north and northeast extends a bold range of elevated grounds; southward the coast becomes again flat for some distance inland, till it rises into the ridge of heights that marks the position of Niagara. The country bordering the lake is generally rich and productive, and was originally covered with forest. A ridge of lofty land runs from the beautiful Bay of Quinte, on the northwest of the lake, westward along the shore, at a distance of nine or more miles: from these heights innumerable streams flow into Ontario on one side, and into the lakes and rivers of the back country on the other. At Toronto the ridge recedes to the distance of twenty-four miles northeast from the lake, separating the tributary waters of Lakes Huron and Ontario; thence merging in the Burlington Heights, it continues along the southwest side from four to eight miles distant from the shore to the high grounds about Niagara.