UPPER SILURIAN. 221 



augment in thickness to the northwest; (6) upper shale, 120 to 250 feet, which thickens 

 to the northwest; and (7) red shale or marl, 975 feet thick, at the Lehigh Water-Gap. 

 The formation spreads across the State, "from the northwest flank of the Kittatinny 

 Mountain to the similar slope of the last main ridge of the foot of the Alleghany 

 Mountains." (H. D. Rogers.) In East Tennessee, the rocks are 200 to 300 feet thick, 

 and include one or two beds of argillaceous lenticular iron ore. 



c. Eastern-border region. — The relations of the limestones of Anticosti to this epoch 

 have been mentioned on p. 206. 



In Nova Scotia, at Arisaig, where the rocks are shales and limestone, and have a thick- 

 ness of about 500 feet, fossils occur throughout the formation, and are very abundant in 

 the upper or more calcareous part. These rocks may be partly Lower Helderberg, accord- 

 ing to Dawson. At the East River of Pictou, there are also slates and calcareous bands, 

 probably of the same age. They include a deposit of oolitic iron-ore, like that of the 

 Clinton rocks of central New York, which in some places has a thickness of 40 feet. 

 Shales and sandstone occur also in New Brunswick, northeast and southeast of Passama- 

 quoddy Bay. 



3. Niagara Epoch. 



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

 limestone, overlying 80 of shale. Farther eastward, in Wayne County, the limestone 

 is 30 or 40 feet thick, and in Cayuga County still less. The formation appears to 

 thin out in Herkimer County. It is, however, represented in the Helderberg Moun- 

 tains, south and west of Albany, by a bed of limestone about 25 feet thick, called 

 the Coralline limestone. From New York, the formation extends westward into Canada, 

 and then northward around the north side of Lake Huron, the north and west sides of 

 Lake Michigan, and thence westward through northern Illinois into Iowa. In Ohio, 

 it outcrops, like the Clinton, around the area of Cincinnati limestone. Throughout 

 these regions, the rock is almost wholly limestone. In the peninsula of Michigan, the 

 thickness is about 100 feet; in Ohio, the lower part of the Cliff limestone, 80 feet. 



In West Tennessee, the Meniscus limestone, 150 to 200 feet thick, noted for its fossil 

 sponges, of which one is meniscus-shaped, is probably the equivalent of the Niagara 

 limestone. 



The Gait or Guelph limestone, well seen at Gait and Guelph in Western Canada, 

 and farther west, which was formerly supposed to be of the age of the Salina beds, 

 is now regarded as the upper part of the Niagara limestone. The Leclaire limestone 

 of Iowa has the same position. 



b. Appalachian region. — In Pennsylvania, the formation consists of two distinct 

 deposits of marl or fragile shale. The lower is about 450 feet thick, where most de- 

 veloped, near the middle belt of the Appalachian zone, and decreases both to the 

 southeast and northwest. The upper deposit, including some thin limestone layers, is 

 1,200 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. It is part, according to Logan, of 

 an extensive formation, which stretches from northern Vermont, eastward over a part 

 of northern New Hampshire and northern Maine, to Cape Gaspe on St. Lawrence Bay, 

 being, in this part, limestone with some massive and shaly sandstone. The formation 

 embraces also the strata of the Lower Helderberg, and possibly part of those of the 

 Lower Devonian. Niagara fossils occur near Lake Memphremagog and in the lower 

 part of the Gaspe limestone, as well as at some intermediate points. 



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 



