442 



HISTORICAL GEOLOGY 



Niagaran, received its name. The limestone of this stage is thickest 

 in the Mississippi Valley, where it probably had been accumulating 

 for a longer time than in New York. Such a great thickness of 

 limestone indicates a long period during which few oscillations in 

 level occurred, and when the lands were so low that little sediment 



was carried to the 

 sea. 



The thickness of 

 the Silurian in Mary- 

 land is about 3000 

 feet ; in western Ten- 

 nessee, about 1500 

 feet; in Alabama, 

 about 500 feet ; in 

 central Tennessee, 

 300 feet ; in Wiscon- 

 sin, about 800 feet ; 

 and in Nevada, about 

 1000 feet. 



Clinton Iron Ore. 

 — One of the most 

 widespread iron ore 

 deposits known was 

 accumulated during 

 the Clinton stage of 

 the Silurian Period. 

 It outcrops in one or 

 more broken belts 

 from New York, 

 through Pennsyl- 

 vania to Alabama, 

 and occurs in beds 

 at different horizons in the formation, sometimes as many as four beds 

 being present in one locality. The thickness of the ore beds varies 

 from 40 feet to a fraction of an inch, but a bed 10 feet thick is unusual. 

 I he ore is called " fossil " and " pea " ore because fossil fragments are 

 commonly found in it with the shell substance entirely replaced with 

 hematite, while some beds are made up of rounded grains of a concre- 

 tionary character. The ore was deposited close to shore, probably 

 in lagoons and marshes, and was probably a chemical precipitate, 



Fig. 412. — Map showing the probable outline of 

 North America during a portion of the Upper Silurian. 

 (Modified after Schuchert.) 



