No. 3.] SPENCER — PAL^OZOIO GEOLOGY. 147 



YI. — NIAGARA FORMATION. 



l^opograpJii/ and Distribution. — Overlyinu- the Clinton form- 

 ation, the most important member of the series — the NiaErara 

 (proper) — is much more widely developed than tlie lower por- 

 tions of the trroup which are hirirely made up of mechanical 

 deposits. Owin^' to the hard limestones of the Niagara epoch 

 surmountinix several hundred feet of soft Medina and Clinton 

 shaly rocks, it forms a conspicuous feature in the country — the 

 summit of the Niairara escarpment — as along its northern and 

 north-eastern margins, the softer material forming the base of 

 ridge has been removed by erosion, leaving abrupt cliffs. 



The most eastern exposures of this formation in New York 

 are near the town of Catskill, on the Hudson River. From this 

 place it extends westward through the central and western parts 

 of the State, forming the bold slopes, u few miles south of, and 

 parallel to, Lake Ontario. Entering Canada at the Niagara 

 River, its direction is westward, nearly parallel with its strike, 

 as far as Dutidas, at the extreme western end of Lake Ontario. 

 Here the range of hills changes its course and extends to Cape 

 Hurd, and thence through Manitoulin and Cockbuni L«lands. 

 The range of hills south of the lake, as we have noticed, is about 

 400 feet hii:h and getierally has an abrupt face. However, from 

 Dundas to Georgian Bay. although the country is of a higher alti- 

 tude, the features are less broken on their eastern side, as they 

 recede from Lake Ontario. 



The southern portion of the basin of Lake Ontario is excavated 

 in Medina shales, while its northern side is scooped out of the 

 various rocks of the Hudson River, and the shales of the Utica 

 formation, which once formed the margin of the old sea in the 

 Niagara period. 



From the northern end of Lake Huron the Niagara forma- 

 tion extends into Drumujond Island, and thence along the whole 

 northern and western shores of Lake Michiuan, Again, the 

 margin of the seas in this period abutted against the Appalachian 

 chain as far south as Tennessee, as is shown by the remains of 

 their old deposits. The large island of the "Cincinnati Arch" 

 formed part of the barrier at the southern margin of the Medi- 

 terranean Sea, which extended over a region of thirteen degrees 

 of longitude and eight of latitude, in the Niagara period, or, we 

 may say, in the Silurian age. 



