West of Lake Ontario, the chasm at the Falls of Niagara shows the strata of the country to be limestone, next slate, and lowest sandstone. Limestone and sandstone compose the secondary formations of a large portion of Canada, and of nearly all that vast extent of country in the United States drained by the Mississippi. At Niagara the interposing structure of slate is nearly forty feet thick, and fragile, like shale crumbling away from under the limestone, thus strengthening the opinion that there has been for many ages a continual retrocession of the Great Falls. Around Lake St. Clair, masses of granite, mica slate, and quartz are found in abundance. The level shores of Lake Huron offer little geological variety; secondary limestone, filled with the usual reliquiae, is the general structure of the coast, but detached blocks of granite and other primitive rocks are occasionally found: this district appears poor in minerals. The waters of Lakes Huron, Michigan, and Superior have evidently, at some remote period, formed one vast sheet, which probably burst its bounds by a sudden action of nature, and subsided into the present divisions, all lower than the former general level: the separating ridges of these waters are but slightly elevated; great masses of rock and huge bowlders of granite are found rolled at least 100 miles from their original situations, and immense alluvial beds of fresh-water shells, apparently formed since the deluge, but when the waters were still of a vast depth and extent, are found in the east of Lake Huron.