238 



PALEOZOIC TIME — UPPER SILURIAN. 



water species, as Corals and Crinoids. (3.) The Niagara epoch was 

 eminently one of the limestone epochs of the American Silurian. 



The limestones are usually made up of fossils, and are often mag- 

 nesian. In western New York, near Niagara Falls, there are 165 

 feet of limestone (directly at the Falls, 85 feet) overlying 80 of 

 shale (fig. 357 A) ; and the undermining of the limestone by the 

 falling waters, which readily remove the shale, is the occasion of 

 the slow retrocession of the Falls. Along the Appalachian region 

 in Pennsylvania, the beds have a thickness exceeding 1500 feet, — . 

 thus, like the earlier formations, surpassing many times in extent 

 the beds of the interior basin. 



a. Interior Continental basin. — At Rochester, N.Y., there are about 80 feet of 

 limestone, overlying 80 of shale. Farther eastward, in Wayne co., the lime- 

 stone is 30 or 40 feet thick, and in Cayuga co. still less. The formation thins 

 out altogether in Herkimer co. 



In the States west of New York the limestone lies directly upon the Clinton 

 limestone. In the peninsula of Michigan the thickness is about 100 feet 

 (Winchell). In Ohio, Indiana, and Illinois, this rock and the Corniferous over- 

 lying it have been called the Cliff-limestone, because it often stands in bold 

 bluffs along the river-valleys. Such bluffs are a common feature in all lime- 

 stone regions where the strata are nearly horizontal and in heavy beds. 



In the Helderberg Mountains there is a layer of limestone 25 feet thick, called 

 Coralline limestone, which has been referred to the Niagara epoch. It has been 

 suggested that it should be united rather with the overlying formation. 



b. Appalachian region. — In Pennsylvania the formation consists of two dis- 

 tinct deposits of marl or fragile shale. The lower is about 450 feet thick where 

 most developed, near the middle belt of the Appalachian zone, and decreases 

 both to the southeast and northwest. The upper deposit is 1200 feet thick 

 in the northwest belt, and declines to the southwest (H. D. Rogers). These 

 strata may include, besides the true Niagara, strata of the Salina or Salt- 

 group period. 



c. Eastern border region. — The Niagara limestone is supposed to occur in 

 eastern Canada some distance south of the St. Lawrence, in the course of a 

 limestone belt running between northern Vermont and Gaspe on the gulf; but 

 it has not yet been identified with certainty. 



Near New Canaan, in Nova Scotia, there are clay slates of the Niagara 

 epoch. " 



d. Arctic regions. — In the Arctic, the Niagara limestone has been observed 

 between the parallels of 72° and 76° on the shores of Wellington and Barrow 

 Straits, and on King William's Island. The common Chain-coral Halysites 

 (Catenipora) catenulata has been found at several localities, along with other 

 Upper Silurian species. (See, further, p. 242.) 



The color of the Niagara limestone is commonly dark bluish-gray to drab. 

 It is sometimes quite impure, and good for hydraulic purposes. A specimen 

 from Makoqueta, Jackson co., Iowa, afforded J. D. Whitney — carbonate of lime, 

 52.18, carbonate of magnesia, 42.64, — with 0.35 of carbonate of soda, a trace of 



