THE PAST AND FUTURE OF NIAGARA. 



569 



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 ex- 

 tended from the Hudson- far beyond the Mississippi, was a vast grove 

 of corah In sheltered nooks of the coral-grove were gardens of waving 

 crinoids, and three-lobed, many-jointed, many-eyed trilobites were 

 crawlino- 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. 



Fig. 3. 



Hj-pothenuse of the Triangle, 1,600 Miles long. 



Ideal Section of toe St. Lawrence and its Lakes. 



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 ex- 

 plain that old river-bed. 



