54 [Assembly 



series of layers of limestone forming the brink of the rocky wall : 

 these are the Niagara shale and the Niagara limestone. The 

 great cataract pours over their edges ; and its peculiar form, so 

 like a huge dam, is owing to the fact that the soft shale below 

 wears away more rapidly than the hard limestone which forms 

 the top of the fall, thus maintaining a hollow space behind the 

 descending sheet. 



These rocks are perfectly exhibited in the ravine of the 

 Niagara, especially along the Niagara and Lewiston railroad. 

 The shale decomposes rapidly where exposed to the air, until it 



' resembles a bank of grey clay. -It contains thin layers of lime- 

 stone in many places, the surfaces of which are often covered 

 with beautiful small corals of several species, and the shale itself 



- contains them in great numbers. The best locality for fossils 

 has been at Lockport, where they are more numerous than on 

 the Niagara, and where, during the construction of the double 

 locks, great quantities of the shale were excavated. The " deep 

 cut " of the canal above Lockport is through the Niagara lime- 

 stone, some layers of which there form a massive and beautiful 

 building stone. The same limestone and shale form the upper 

 falls of the G-enesee at Rochester ; and in the precipices below, 

 fossils are common. 



The limestone is at Niagara about 160. feet thick (of which 

 only the lower part is seen just at the Falls); at Rochester, about 

 70 feet only. It forms the summit terrace of that remarkable 

 range of hill known as the Mountain Ridge or Queenston Heights, 

 extending from New- York through Canada West : it can be seen 

 in the Manitoulin islands on Lake Huron ; ^Iso south of Green 

 Bay, and across Wisconsin and Iowa. (On the Upper Missis- 

 sippi, the Medina sandstone does not exist, and the Hudson-river 

 group is thin and so inconspicuous as to have been overlooked 

 by the earlier observers. They thus found the Galena or Upper 

 Trenton limestone apparently joining the Niagara limestone, and 

 both were described as one rock under the name of the Upper 

 Magnesian limestone, an error which has been corrected by 

 more careful examination.) It exists also in Ohio and Kentucky, 

 where it forms the lower part of what is there called the Cliff 

 LIMESTONE. Eastward from Rochester, the Niagara group gradu- 

 ally thins out, its limestone being traceable to the Mohawk valley, 

 and a few of its layers even farther : it is not distinctly seen in 

 Pennsylvania. 



