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

 Cincinnati limestones and shales. 



I. Rocks: kinds and distribution. 



The earlier part of the Trenton period was eminently a limestone- 

 making era. A broad limestone belt of this era stretches across Cen- 

 tral New York, north of the Mohawk (4 a, on map, p. 165), and 

 extends westward through Canada to Wisconsin (where it includes the 

 "Galena limestone"), and into Minnesota; it occurs also in Missouri 

 and in central Tennessee. It stretches northwest of Lake Superior 

 in a broad band west of the Archaean, passing Winnipeg Lake. It 

 makes part of the crystalline limestone of the Green Mountains, as 

 identified by Trenton fossils in Vermont, and in Columbia and Dutchess 

 Counties, N. Y., where Trenton fossils occur ; and is also extensively 

 developed in the Eastern Appalachians, south of New York. 



In the State of New York 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 



