206 PALAEOZOIC TIME — LOWER SILURIAN. 



overlies the Calciferous, and underlies the Trenton : its relation to the Ohazy, 

 beyond this of position, has not yet been determined. 



The Chazy limestone has been stated to occur in Northern America, in the 

 Winnipeg region, west of the Azoic. 



(b.) Appalachian region. — The Chazy has not been recognized along any 

 portion of the Green Mountains. It is supposed to be represented in the upper 

 part of the Quebec group. In Pennsylvania, there is a magnesian limestone, 

 5000 to 6000 feet thick in some places, corresponding to the Trenton period, 

 according to H. D. Rogers; but what part is Chazy is not yet ascertained. 



(c.) Eastern border. — A limestone of the Chazy epoch occurs at the Mingan 

 Islands, in the Gulf of St. Lawrence. 



(d.) Arctic region. — Limestone strata, containing Chazy fossils, have been ob- 

 served in the Arctic,- on King William's Island, North Devon, and at Depot 

 Bay in Bellot's Strait (lat. 72°, long. 94°). The species Orthoceras moniliforme 

 Hall, and a Maclurea (31. Arctica Haughton) near M. magna, have been observed. 

 The limestone is in part a cream-colored dolomite. 



2. Trenton epoch. — (a). Interior Continental basin.— In New York and 

 Canada, the Trenton limestone directly overlies the Chazy. The lower part in 

 New York is made up of the Birdseye and Black River limestones (the latter 

 the upper) ; and these same subdivisions have been distinguished in much of the 

 Mississippi basin. 



The Birdseye limestone is so called from whitish crystalline points or spots 

 distributed through it. This peculiarity, however, is not always present, and 

 occurs in other limestones. The color is drab or dove-colored and brownish, 

 and not so dark as that of the overlying beds. 



The Black River limestone is named from Black River, east of Lake Ontario, 

 in New York, along which there are the best exposures of it. The color is 

 generally dark, nearly black. 



The Trenton limestone in New York is grayish-black to black. It is some- 

 times bituminous, especially in its upper portions. Its layers are often thin, and 

 frequently argillaceous, and beds of shale in many places intervene. The black 

 color is due to carbon or bitumen, as is shown by its burning white. 



The Trenton limestone has been recognized in the Winnipeg region in British 

 America, as well as over much of the Mississippi basin. The Galena or lead- 

 bearing limestone of Wisconsin and the adjoining States in the West constitutes 

 the upper portion of the Trenton series, and often alternates with layers of the 

 Trenton limestone. 



The rock, though generally common limestone, sometimes includes layers of 

 magnesian limestone ; and the Galena beds are generally magnesian. (See ana- 

 lysis on p. 84.) 



The thickness in New York seldom exceeds 300 feet. At Montreal it is 600 

 feet (Logan) ; on the Manitoulin Islands, in the St. Lawrence, not over 300 feet; 

 in the Michigan peninsula, about 32 feet (Winchell) ; in the region more to the 

 west, usually about 300 feet ; in middle Tennessee, where the beds are called 

 the Stones River group by Safford, 200 to 300 feet ; in Missouri, 400 to 500 feet 

 (Swallow). In Iowa the Galena limestone is 250 feet near Dubuque, and the 

 Trenton 20 to 100 feet (Hall). 



(b.) Appalachian region. — The limestones of the Trenton epoch have great 



