218 



PALEOZOIC TIME. 



B. UPPER SILURIAN. 



Marine life, large oceans, small lands, and uniform climates — the 

 features of the Lower Silurian — continued to characterize the open- 

 ing period of the Upper Silurian. 



The periods and epochs indicated in the New York rocks have been 

 mentioned on p. 164. The periods are — the Niagara (5), the Sa- 

 lina (6), and the Lower Helderberg (7). 



I. NORTH AMERICAN. 



1. NIAGARA PERIOD (5). 

 Epochs. — 1. Medina epoch, or that of the Oneida conglomerate 

 and Medina sandstone (5 a). 2. Clinton epoch, or that of the Clin- 

 ton group (5 b). 3. Niagara epoch, or that of the Niagara shale 

 and limestone (5 c). 



I. Rocks : kinds and distribution. 



The rocks of the Medina epoch in New York are mainly sandstones 

 and conglomerates ; and much of the sandstone is argillaceous. It is 

 not known west of the State of New York, except in Upper Canada 

 and northern Michigan. The lower member is a pebbly sandstone or 

 grit, called the Oneida conglomerate, being so named from its occur- 

 rence in Oneida County, N. Y. The upper is called distinctively the 

 Medina sandstone, and is usually a red or mottled argillaceous sand, 

 stone. Both are thin to the north, the former 100 to 120 feet in 

 Oneida County, and the latter 300 to 400 feet along the Niagara 

 River. The conglomerate is 500 feet thick in the Shawangunk Moun- 

 tains, where it is called the Shawangunk grit, and 700 feet in some 

 parts of Pennsylvania and Tennessee. The Medina beds are 1,800 

 feet thick in Pennsylvania and 500 feet in Tennessee. 



In the Eastern-border region, at Anticosti, several hundred feet of 

 limestone represent this epoch. 



The rocks of the Clinton and Niagara epochs have a much wider 

 range ; and both formations thin out toward the Hudson River. The 

 Clinton beds occur near Canajoharie, in New York, and stretch on 

 west through Canada to Michigan, and along the north side of Lake 

 Huron ; and also appear in Ohio, Indiana, and Wisconsin ; also south, 

 in Pennsylvania, Virginia, and Tennessee. The rocks in New York 

 and along the northern border of the United States are shaly sand- 

 stones, shales, and limestone. 



In the formation, there are one or more thin beds of red argillaceous 

 iron ore, made up mostly of small flattened grains ; these outcrop in 



