THE SILURIAN PERIOD 539 



Clinton beds have not been identified in the western half of the 

 continent. 



The Clinton formation was succeeded by the Niagara formation 

 (subdivided in New York into Rochester shale, Lockport limestone, 

 and Guelph dolomite, p. 537), which extends farther west than any 

 of the preceding Silurian formations, showing that the progressive 

 submergence of the earlier epochs still continued in the upper 

 Mississippi basin. The falls of Niagara River are over the limestone 

 of this series (Fig. 113). North of Missouri, the formation is not 

 known to occur far west of the Mississippi, but it extends into 

 Missouri, Arkansas, and perhaps even to the Arbuckle Mountains 

 of Oklahoma. It is also found in the trans- Pecos region of Texas. 

 It was in this epoch that the submergence of the upper Mississippi 

 basin reached its maximum, so far as this period is concerned. The 

 southern border of the interior sea is not known, but it appears to 

 have been separated from the ocean in this direction, by a land 

 barrier somewhere in the Gulf -State region (Fig. 397). The barrier 

 was probably south of Tennessee, as Niagaran limestone occurs in 

 the western part of that state. 



A significant feature of the distribution of the Niagara formation 

 is its great development in high latitudes. It occurs in patches 

 in Manitoba, west of Hudson Bay, and at numerous points farther 

 north, up to latitude 80. The patches appear to be remnants of 

 a once continuous formation, and since the fossils are much the 

 same throughout, and very like those of northern Europe, it is in- 

 ured that there was water connection between the Mississippi basin 

 id northern Europe by way of the Arctic islands, which permitted 

 the intermigration of the shallow-water sea-life of the two regions. 



East of the Appalachians and west of the Mississippi the distri- 

 mtion of Niagaran strata is not well known. They probably 

 ?cur in New Hampshire and Maine, and in the Provinces between 

 states and the St. Lawrence. In Nova Scotia, the Niagaran 

 iries is represented by shale, affording another illustration of the 

 ?t that different sorts of rock may be accumulating in different 



ions at the same time. The exact equivalent of the Niagara 

 Formation has not been identified with certainty in the West. 



West of New York the formation is mainly limestone. Perhaps 



