Page:Popular Science Monthly Volume 1.djvu/585

Rh ooze of an ancient sea-bottom, and the Conularia tells us that here the sea was open and deep.

A time came when mud-sediments were no longer brought down, and the bottom of an ocean, clear, warm, placid, over an area which extended from the Hudson far beyond the Mississippi, was a vast grove of coral. In sheltered nooks of the coral-grove were gardens of waving crinoids, and three-lobed, many-jointed, many-eyed trilobites were crawling over the coral sand, and mollusks in richly-sculptured shells were everywhere on sand and coral. The Niagara limestone is a monument of that ancient life. With the formation of this rock and its uplift from the sea, the geologic record here about Niagara closed, until the coming of the Ice.

We turn now to the geology of the lake-region. The area of the lakes is estimated at 90,000 square miles; and the area whose streams flow into the lakes, at 400,000 square miles. This immense area is one of the oldest on the globe. On the north shore of Lake Huron and Lake Superior we find the azoic rocks, and on the borders of Lake Erie and Lake Michigan we find no rock newer than the lowest members of the Devonian. The whole water-shed of the St. Lawrence was reclaimed from the ocean before the close of the Devonian epoch. If the drainage has always been through the gulf of St. Lawrence, the Niagara should be one of the oldest rivers on the globe. And yet, as we have seen, in the geological calendar it is very young. How shall we account for this gap between ocean-history and river-history? A little more of geology and something of topography will help us to understand why the Niagara has recorded such a small segment of the time which lies between us and the Devonian seas.



Borings made a few years ago at La Salle, on the Illinois, revealed the fact that the valley had been eroded forty feet below the present river-bed. Pot-holes and water-worn ledges at Athens mark the course of an ancient river. Other evidences of the ancient river are found in the valley of the Des Plaines and along the Calumet feeder of the Illinois Canal.

The topography of the lake-basins and the Niagara plateau will explain that old river-bed.