FORMATIONS AND PHYSICAL HISTORY 389 



Karh ft" tin- three series is made up of several formations, but the 

 ^divisions of one place do not fit another. A brief sketch of the 

 iat lire and distribution of the principal subdivisions of the system 

 Fords an outline of the history of the continent during the period. 



Silurian of the East 



Oswegan series. This series is known chiefly in New York, 1 

 >th the Oncida and the Medina formations appearing at the surface 

 >uth of Lake Ontario. Their equivalents may recur in the western 

 irt of the Appalachians farther south. The Oneida consists of 



conglomerate and sandstone, and the Medina of sandstone and shale. 



The sediments of these formations appear to have been deposited in 

 shallow interior sea, as shown by fossils, and by the cross-bedding, 



ripple-marks, etc., which affect them. Both formations are prob- 



ibly continuous beneath younger strata over considerable areas 

 mth of Lake Ontario, and west of the Appalachians, 2 and the 

 [edina is more wide-spread than the Oneida. 



Niagaran series. The Clinton formation overlies the Medina 

 mformably, but has a wider distribution, extending westward to 

 .ake Huron and Indiana, and perhaps to the Mississippi, and south- 

 ward, in the Appalachians, to Alabama and Georgia. Beds of 

 Clinton age have been recognized in Nova Scotia and at a few other 

 laces northeast of the United States, where marine sedimentation 

 /as probably continuous through the Ordovician and Silurian 

 iriods. In the Appalachian Mountains, the formation is largely 



)f sandstone and shale. In western New York and farther west, 

 mch of it is limestone. 



One of the features of the formation is its iron ore, generally in 

 ic form of hematite (Fe2Os). The ore is known at many points 

 cen New York and Alabama, as far west as Wisconsin, and in 

 fova Scotia. It is interstratified with other beds of the formation, 

 id is usually believed to have been accumulated by chemical 

 >redpitation in lagoons or marshy flats. 



The Niagara formation (subdivided in New York, p. 388), 

 itends farther west than any of the preceding Silurian formations, 



showing that the submergence begun earlier still continued in the 



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



1 The formations of eastern New York and New Jersey formerly classed as 

 O.ii-i'l;i. are of Salina age. Hartnagle, Bull. 107, N. Y. State Mus. 

 1 Perhaps the Richmond beds, the Maquoketa shales, etc. 



