THE SILURIAN PERIOD. 377 



One of the notable features of the Clinton formation is the presence 

 in it at many points of iron ore. Chemically, the ore is generally 

 in the form of hematite. Physically, it is often made up of small 

 concretions which so resemble flaxseed in size and shape as to have 

 suggested the name " flaxseed ''ore. Locally it is known also as " fossil" 

 ore from the abundance of fossils which it contains. The ore is known 

 at many points between New York and Alabama, 1 as far west as Wis- 

 consin 2 (where it is the only representative of the formation which 

 has been commonly recognized), and as far northeast as Nova Scotia. 

 The ore beds are interstratified with other beds of the formation, and 

 are usually believed to have been accumulated by chemical precipi- 

 tation in marshy flats. Since the iron compounds in solution in the 

 water were derived from rock formations in process of decay, the 

 extraction of these compounds being itself one of the elements of decay, 

 the iron ore, as well as the other beds of sedimentary rock, had its 

 origin in the decay of the older formations. The iron was probably 

 deposited in the form of limonite (2Fe 2 3 , 3H 2 0) or of iron carbonate 

 (FeCOs), and subsequently changed into its present form (Fe 2 3 ). 

 This is the fourth series in America which contains a large amount 

 of iron ore, the first, second, and third being in the Archeozoic, the 

 Huronian, and Animikean respectively. Like the Clinton iron ore 

 of Wisconsin, the manganese ore deposits 3 of Arkansas lie at the 

 base of the Silurian system in that region, and the two classes of ore 

 may have had a somewhat similar origin. 



The Clinton formation has a thickness of 900 or 1000 feet in Penn- 

 sylvania, of 800 in Virginia, and of 200 or 300 feet in eastern Tennessee 

 and Kentucky. In the eastern interior, the formation is much thinner. 



Like the Oneida and Medina formations which preceded, the Clin- 

 ton beds have not been identified in the western half of the continent. 



The Niagara formation. — The Clinton formation of the interior 

 was succeeded by the Niagara (Rochester shale, Lockport limestone, 

 and Guelph dolomite of New York; see p. 370), which extends some- 

 what farther west than any of the preceding Silurian formations, show- 

 ing that the submergence of the earlier epochs still continued in the 

 Upper Mississippi basin. The Niagara formation covers not only 



1 See geological reports of the several States; also Jour. Geol., Vol. IX, p. 412. 



2 Geol. of Wisconsin, Vol. I, 1881. 



3 Penrose, Geol. Surv. of Arkansas, Vol. I of the Ann. Rept. for 1890. 



