THE SILURIAN PERIOD. 381 



of exposed Niagara are separated because the formation is concealed 

 at intervening points by younger beds. 



The distribution of the Niagara formation in the interior, taken 

 with the distribution of the earlier Silurian formations, would lead 

 to the conclusion that the Niagara should rest on the Hudson 

 River shales unconformably over a large area in the Mississippi basin. 

 Such is doubtless the fact, yet the degree of unconformity is so slight as 

 not to be readily detected. This might mean either (1) that the older 

 formation was elevated so little at the close of the Ordovician, and 

 stood so low during the Oneida, Medina, and Clinton epochs, as to have 

 suffered little erosion, or (2) that it was almost perfectly base-leveled 

 before the Niagara epoch. In both cases it would be necessary to 

 suppose that the strata were little deformed, since the bedding of 

 the two formations is essentially harmonious. Between these two 

 alternatives, the former is perhaps the more probable. 



In New York the beds which were formerly grouped under the 

 name Niagara, and now under the series name, Niagaran, consist of 

 shale (Rochester, p. 370) below, and limestone (Lockport and Guelph) 

 above. It is the more resistant beds of the latter overlying the more 

 yielding layers of the former (Fig. 176) which have given rise to the 

 Falls of the Niagara, the locality whence the formation takes its name, 

 and determined the position of the escarpment between Lake Ontario 

 and Lake Erie (Fig. 177) from which the Falls (at F) has retreated. 

 The same limestone has determined other falls, such as the upper- 

 most falls of the Genesee river, shown in Fig. 178. West of New York 

 the formation is mainly limestone. Except the Trenton, it is more 

 wide-spreacl than any preceding formation of limestone, and tells of 

 an expanse of clear water covering much of the great Mississippi basin. 



The Niagara formation is the oldest in which well-developed coral 

 reefs have been identified. While coral-secreting polyps had lived 

 before, as shown by their abundant fossils in older formations, and 

 while their secretions had helped to make the Trenton and other lime- 

 stones, the reef-building species seem to have first become abundant in 

 mid-Silurian time. The reefs are known in eastern Wisconsin and at 

 other points farther -south and east. Coral reefs and the adjacent 

 deposits illustrate as clearly as anything else among the ancient forma- 

 tions the methods by which limestone deposits were formed. The 

 reefs themselves are composed of the commingled relics of the life that 



