194 PALEOZOIC TIME. 



very few fossils. It is possible that these limestones were largely 

 made from the minute shells of Rhizopods ; and, if so, they may have 

 been accumulated in deep water. 



3. TRENTON PERIOD (4). 



1. American. 



Epochs. — 1. Trenton epoch (4 a), or that of the Black River 

 and Trenton limestones. 2. Utica (4 b), or that of the Utica shale. 

 3. Cincinnati (4 c), or that of the Hudson River group and the 

 Cincinnati limestones and shales. 



I. Rocks : kinds and distribution. 



The rocks of the Trenton series over the United States are almost 

 solely limestones. They occur extensively along the range of the 

 Appalachians, in New York and Canada ; probably in western New 

 England, as part of the Stockbridge or great Green Mountain lime- 

 stone ; in Ohio and Indiana, in Illinois and other States over the 

 wide Mississippi basin (where it includes the " Galena limestone " of 

 Wisconsin and the adjoining States) ; and in a broad band stretching 

 northwest from Lake Superior, by Winnipeg Lake, west of the 

 Archaean. 



In the State of New York, the formation is exposed to view on the 

 eastern border of the Chazy area, not far from the border of the 

 Archaean peninsula, and also south of the Archaean, resting on the 

 Calciferous. It constitutes the high bluffs of the gorge at Trenton Falls, 

 on West Canada Creek, and thence derived its name. It occurs in 

 the Ottawa region in Canada, and extends northeastward to Quebec. 



The formation in New York is divided into the Black River and 

 Trenton limestones, the latter being the upper; and these divisions are 

 recognized in Canada and some parts of the States west of New 

 York. The lower part of the Black River limestone is distinguished 

 by the New York geologists as the Birdseye limestone, from crystal- 

 line points scattered through the rock. 



The thickness of the series in northern New York and Canada, 

 where probably lay the ocean's border, is generally from 100 to 300 

 feet ; yet, in the region of Ottawa — a great St. Lawrence Bay in the 

 earlier Silurian era (see map, p. 165) — it is about 800 feet. West of 

 the Appalachians, the thickness averages about 300 feet. Along the 

 Appalachian region in Pennsylvania, it is made 2,000 feet by Rogers. 



The Utica shale, or the rock of the Utica epoch, is the surface-rock 

 along a narrow region in the Mohawk valley, New York (see 4 b on 

 map, p. 165), following a course nearly parallel with the outline of 



