40 TRENTON GROUP. 



Phragmoceratidce, and Gyroceratidce here first developed their essential characters. 

 In the Birdseye limestone at Montmorency, Canada, petroleum exudes in drops 

 from fossil corals, supposed to have its origin either in the marine animals or fu- 

 coidal vegetation. 



CHAPTER IX. 



TRENTON QROUP. 



78. THE Trenton Group was named from Trenton, Oneida County, New 

 York. The limestone at the Falls, where it is more than 100 feet thick, was called 

 the Trenton limestone long prior to the use of the words in a geological sense. In 

 1838 Vauuxem referred to the Trenton limestone, but it was not until 1842 that 

 he and Prof. Emmons so described the Group as to establish it. At Trenton Falls 

 there are two kinds of stone one a dark, fine-grained limestone, in thin layers, 

 separated by black shale, and abounding in fossils ; the other a gray, coarse-grained 

 limestone, in thick layers, forming the top of the mass, and much less fossiliferous. 

 The Group has quite an extensive surface distribution in belts upon the margin of 

 the older rocks in New York, and varies somewhat in its characters, but seems at 

 all times to be a limestone, with the exception of shaly partings. It is 400 feet 

 thick at Chazy, the greatest exposed thickness, and from here it thins toward 

 the east. 



79. It enters Vermont from New York in three narrow outcrops, consisting 

 of black layers and seams of limestone and occasional argillaceous matter, with a 

 maximum thickness of about 400 feet. It enters New Jersey, and crosses the 

 counties of Warren and Sussex, with a maximum thickness of about 200 feet. It 

 is frequently exposed in the broken-up hills and mountains of Pennsylvania, show- 

 ing a thickness from 300 to 700 feet. The exposures continue to occur southerly 

 in the Appalachian Mountains in crossing Virginia, North Carolina, and Tennessee, 

 where, in the eastern part of the latter State, there is a thickness of 1,100 feet, and 

 in the middle part of about 500 feet. It is exposed by an ancient uplift in the 

 central part of Kentucky over several counties, forming a large part of what is 

 called the Blue-grass Region, and reaches as far north as the Ohio River. The thick- 

 ness is about 700 feet. 



80. It has an extensive geographical distribution in Canada. The Montreal 

 and Ottawa sections have each a thickness of 600 feet. The sections in Western 

 Canada, on the Trent River and at Collingwood, have a thickness of 750 feet, but 

 it thins westerly, and in following the outcrops around Lakes Huron and Michigan 

 the exposures rarely exceed 50 feet in thickness. In passing south of Lake Su- 

 perior it crosses Sulphur, St. Joseph's, and Great Encampment Islands, and thence 

 stretches west and south-west near Little Bay de Noquet and Green Bay, and en- 

 ters Wisconsin near the mouth of the Menominee River. From here the exposure 

 extends south-west across the State, displaying a large area in the south-western 

 part, and, entering the State of Illinois, occupies more or less of the surface in four 

 or five of the north-western counties. From here the exposures bear north-west and 

 north, occupying several counties in North-eastern Iowa, with a continuing belt across 



