304 New York State Museum 



dant growth of corals and Stromatoporas which formed 

 the reefs or reeflike accumulations, so characteristic 

 of the Niagaran (Middle Silurian), which are best ex- 

 emplified in Wisconsin and the Manitoulin Islands and 

 also found in Michigan, Ohio and Indiana. Except in 

 maritime eastern Canada, the thickest deposits of Silu- 

 rian (mainly Middle Silurian) limestones occur in west 

 central Tennessee. The dolomitic Niagara series in 

 northeast Wisconsin and northern Michigan comes 

 next. Between the limestone area of the interior and 

 the region of sands and muds in the east was a transi- 

 tion zone in which sometimes sandstones and shales 

 were deposited, then again limestones. The lime- 

 stone deposits of the interior region belong largely to 

 the Middle Silurian. Nowhere is there a thickness 

 over 1000 feet, and usually it is much less, consider- 

 ably under 500 feet. 



At the base of the Middle Silurian (Niagaran) series 

 is one of the most widespread iron ore deposits known. 

 It was accumulated during the Clinton stage, and in 

 New York the formation is known as the Furnaceville 

 ore. Outcrops occur from New York, through Penn- 

 sylvania to southern Virginia, and others of corre- 

 sponding age occur also in Alabama. There are several 

 beds at different horizons in the formation, varying 

 from a fraction of an inch to about 40 feet in thick- 

 ness, though beds with a thickness of as much as ten 

 feet are the exception. This ore is believed to be a 

 chemical precipitate deposited in marshes and lagoons 

 along the shore, the iron having been leached by 

 streams from igneous rocks, over which they flowed, 

 and brought to the sea. Fossil fragments are common- 

 ly found in the ore with the shell substance replaced 



