186 WALKS AND TALKS. 



formation the Niagara Limestone. We find it at the Niag- 

 arajiyer, which jjives its name. It is seen along the gorge 

 from Lewiston to Hie Falls. It is the top rock of this gorge, 

 and over its brink at the Falls, the vast body of water is pre- 



Icipitated. SCljereaction of the water against the underlying 

 shale wears it away^ The limestone is undermined, and huge 

 pieces break off from time to time. So the Falls recede ; so 

 )the gorge is continued backward ; so the seven-mile gorge was 

 formed; and we have recently ascertained that during thirty- 

 three years the recession has been three feet a year. 



From the Falls eastward, this limestone continues its out- 

 crop to Rochester anoT^eyond. Westward and northwestward 

 it trends toward Cape Hurd, a promonotory separating Geor- 

 gian Bay from Lake Huron. Continuing under the north- 

 ern part of Lake Huron, it forms the southern portions of 

 the Manitoulin Islands ; it borders the northern shore of Lake 

 Michigan; separates Bay de Noc and Green Bay from Lake 

 Michigan, and borders the western side of the lake tqJJhicago, 

 extending beyond and into north-western Indiana. From 

 north-western Illinois, a belt stretches north-westward diagon- 

 ally across Iowa. At Sandusky, Ohio, an area expands like 

 a great spatula over parts of Ohio, Indiana, and Kentucky, / 

 stretching to the southern part of Kentucky. But through 

 the broadest part of this spatula is a great oval perforation, 

 within which are embraced Cincinnati, Richmond (Ind.), 

 Madison, Frankfort, and Lexington (Ky). On the great 

 Silurian mass _of limestone are situated Rochester, Niagara 

 Falls, Milwaukee, Chicago, Joliet, Huntington (Ind.), San- 

 dusky. Next below theP Niagara Limestone^ lies the Niagara 

 Shale, and then the Clinton formation ; but both of these be- 

 come limestones at the west, and unite with the Niagara lime- 

 stone to augment the central mass. Next above the Niagara 

 limestone comes the Salina formation, of shales, clays, and 

 marly limestones a formation which, as we stated in Talk 

 XXIII, yields the country a vast amount of salt and gypsum. 

 At the bottom of the Silurian are found two" fnurnu-ntal for- 

 mations, the Oneida Conglomerate and the Medina 



