391 



> FORMATIONS AND PHYSICAL HISTORY 



linn 'stone of this series (Fig. 76). North of Missouri, the formation 

 is not know.i far west of the Mississippi, but it extends into Mis- 

 souri, Arkansas, and perhaps even to the Arbuckle Mountains of 

 Oklahoma. It is found also in western Texas. The southern border 



the interior sea in which this formation was deposited is not 

 known, but it may have been separated from the Gulf of Mexico, 

 by a land barrier (Fig. 347). 



A significant feature of the distribution of the Niagara forma- 

 tion is its great development in northerly latitudes (p. 390). The 

 known patches in the north appear to be remnants of a once continu- 

 ous formation, and since the fossils are much like those of northern 

 Kurope, it is inferred that there was shallow-water connection be- 

 tween the Mississippi basin and 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- 

 bution of Niagaran strata is not known in detail. Their exact 

 (.(|uivalents have not been identified in the West. West of New 

 York the formation is mainly limestone. It is the oldest formation 

 in which well-developed coral reefs have been identified, though 

 coral-secreting polyps had lived 

 before (p. 385). Reefs are 

 known in eastern Wisconsin, 

 Indiana, and elsewhere. 



In the east where the Ni- 

 agara is known, it has a thick- 

 ness of but 100 to 300 feet, 

 while in Wisconsin it attains a 

 maximum of 800 feet (perhaps 



Fig. 348. The Wabash dome in the 

 Niagara limestone. (Kindle.) 



including some Clinton), all of which is limestone. While the 

 Niagaran beds of the interior are in general nearly horizontal, they 

 an domed in many places, giving the beds a high dip (Fig. 348), 

 as at various points about the south end of Lake Michigan. 



Cayugan series. The Salina formation, which overlies the Ni- 

 agara n srrirs in parts of New York, Pennsylvania, Ohio, Michigan, 

 and Ontario, is much less widespread, indicating the emergence of a 

 considerable area in the Mississippi basin at the close of the Niagaran 

 cpoi-h. The formation embraces all common sorts of sedimentary 

 roik, and in addition rock salt, and gypsum. Shale is the most 

 abundant sort of rock, and seems to have originated after the fashion 



