LOWER SILURIAN. 195 



the Archaean farther north. The shale is in some places three hundred 

 feet, or more, thick. It extends westward through Canada, and 

 beyond, probably into Wisconsin and Iowa, though a very thin de- 

 posit at the west ; and also southward along the Appalachians, being, 

 in Pennsylvania, from three hundred to seven hundred feet thick. 



The rock is a crumbling shale, mostly of a dark blue-black or 

 brownish-black color, and frequently bituminous or carbonaceous, — 

 so much so, in certain places, as to serve as a black pigment. It 

 sometimes contains thin coaly seams ; and much money has been 

 foolishly spent in searching for coal in this deposit. Thin layers of 

 limestone are occasionally interpolated, especially in the lower part. 



The rocks of the Cincinnati epoch (called formerly the Hudson 

 River), are shales in New York and Canada, but become calcareous to 

 the west, and consist of limestone, largely mingled with shale, about 

 Cincinnati, in Ohio, and farther west. The shales in New York 

 (called Hudson River and Lorraine shales) cover a narrow area 

 through the centre of the State, near the Mohawk, which widens to- 

 ward the Hudson. West of New York, the shales extend through 

 western Canada, and southward of the State, along the Appalachians. 

 The greatest thickness in New York is 1,000 feet. 



The Cincinnati limestone continues from Ohio westward, outcrop- 

 ping in several of the States of the Mississippi valley. There is 

 limestone of this epoch also in the Island of Anticosti, in the Gulf of 

 St. Lawrence, about 1,000 feet thick. 



In the Green Mountains, there are strata of mica schist, gneiss 

 ;and quartzyte, overlying the great Stockbridge limestone ; and, since 

 they are quite certainly Lower Silurian, and at the same time newer 

 than this limestone, they probably belong to the Cincinnati epoch. 

 Some of the higher summits in southern Vermont are reported to 

 consist mainly of this quartzyte ; the elevation of Bald Mountain, in 

 Bennington, for example, is 3,124 feet above the sea, and of Mount 

 Prospect, in Woodford, 2,690 feet. 



1. Trenton Epoch. _ (a.) New Yor-h and to the Eastward. —In New York, the 

 Trenton limestone is grayish-black to black. It is sometimes bituminous, especially in 

 its upper portions. Its layers are often thin, and beds of shale in many places inter- 

 vene. The black color is due to carbon or carbonaceous substances, as is shown by its 

 burning white. The crystalline points of the Birdseye are not always present, and occur 

 in other limestones. The color of this rock 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, N. Y., east of Lake Ontario. The color is generally dark, nearly black. 



In Canada, the Trenton outcrops over a large area about Ottawa, and "also over an- 

 other of less width along the north side of the St. Lawrence, from Montreal eastward 

 nearly to Quebec, and at intervals beyond to Murray Bay ; and a branch passes south- 

 ward from Montreal to Lake Champlain. Near Montreal, the whole thickness is 530 

 feet, and that of the lower part, including the Black River limestone and Birdseye 

 limestone, 38 feet (Logan). 



