378 GEOLOGY. 



much of New York, Ontario, and Ohio, but also all or nearly all the 

 southern peninsula of Michigan, much of Indiana and Illinois, and 

 parts of Wisconsin and Iowa. North of Missouri, it is not known to 

 occur far west of the Mississippi. It was formerly supposed to be 

 present in the Black Hills of South Dakota, but this correlation seems 

 to be doubtful. 1 It may yet be found where it has not been recog- 

 nized, or where its identification has not been placed beyond question. 

 The formation probably never extended east over the Cincinnati arch 

 or the similar Nashville dome to the south, though it extended east 

 between them. To the westward it extended beyond the Mississippi 

 into eastern Missouri, Arkansas, and perhaps even to the Arbuckle 

 mountains in Indian Territory. It was in this epoch, as the relations 

 of the next succeeding formation show, that the submergence which had 

 been in progress since the beginning of the Silurian period, reached 

 its maximum in the upper Mississippi basin. In the southeastern 

 part of the same basin, the sea of the Niagara epoch was probably 

 more restricted than that of an earlier time. 



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

 not to have been connected with the ocean in this direction. The 

 land barrier which limited it on the south appears to have been in 

 the position of the Gulf States. These general facts are expressed 

 on the map, Fig. 174. 



Beds of sedimentary rock contemporaneous with the Niagara of 

 the interior may exist in the northern part of the Appalachian belt, 

 but their faunas are so unlike those of the Niagara of the Mississippi 

 basin as to indicate the continued separation of the Appalachian basin 

 from the interior. Because of these conditions, exact correlations 

 between the second group of the Middle Silurian series (p. 371) of the 

 interior, and the corresponding beds of the Appalachian trough, have 

 not been made. 2 The upper part of the Niagaran series (above the 

 Clinton) is believed to be absent from the southern Appalachians, the 

 region where most of the preceding formations of this period and of 

 the Ordovician are well represented. Their absence from this region 

 appears to mean that the shore of the interior sea was temporarily 

 shifted to the west, either because of crustal movements which caused 



1 Beecher, Am. Geol., Vol. XVIII, p. 31. 



2 Weller, Rept. on the Paleozoic Paleontology of New Jersey, Geol. Surv of N. J. 



1903. 



