STRATIGEAPHICAL GEOLOGY. 5 



places on top of the 1st Magnesian, and are usually classed with it. The Group 

 has likewise been doubtfully identified in Wisconsin and other Northwestern 

 localities. 



6. The Birdseye Limestone, which is a well marked Group of rocks in New 

 York. The rocks break with a conchoidal fracture, and the surface presents 

 numerous crystalline spots due to calcspar in the tubes of Tetradium Jibratum, 

 which it contains in abundance, and from which the Group takes its name. It 

 is not, however, so distinct in its fossil remains as to be characterized and deter- 

 mined elsewhere, and therefore some geologists treat the name as a synonym for 

 the Black River Group. 



7. The Black River Group (including the Birdseye limestone or its equiva- 

 lent) which is found in New York, Pennsylvania, Canada and the Island of An- 

 ticosti, and is supposed to underlie Ohio, Indiana and Illinois, for it is again 

 found cropping out under the Trenton Group in Missouri and other places west 

 of the Mississippi. It has a very wide geographical range, and in Pennsylvania 

 attains the important thickness of 5,500 feet. 



The Orthooeratites, which commenced an existence in the Upper Potsdam, 

 reach their greatest development in this Group of rocks. Some shells are found 

 more than ten feet in length and exceeding a foot in diameter. Other Cephalo- 

 poda are found in abundance, and several new genera commence an existence. 



The synonyms for the Black River Group in the early geological reports 

 are Mohmek limestone, Base of the Trenton limestone, Blue limestone, Black 

 marble of Isle La Motte, Bald Mountain limestone, sparry limeroclcs, Transi- 

 tion chequered limestone, Seven foot Tier and Metalliferous limerock. 



8. The Trenton Group, which takes its name from Trenton, Oneida county. 

 New York, is found almost everywhere on the Continent, where the Lower Silu- 

 rian rocks are exposed. In New York, Pennsylvania, Kentucky, Illinois, Mis- 

 souri and Canada, its greatest thickness does not exceed 1,000 feet; while on the 

 Island of Anticosti it reaches 1,700, and in Tennessee 2,500 feet. The Galena 

 limestone of Illinois and Wisconsin, and the Cape Girardeau limestone of Mis- 

 souri, belong to it. 



The limestones of this Group are literallj^ a mass of fossils, and withal so 

 well preserved, that several hundred species and many genera have been defined. 

 No Group of paliBozoic strata has been studied with more interest, or has yielded 

 more facts beneficial to science. It is a magnificent museum of well preserved 

 shells, representing almost every character of the ancient j)opulation of the sea. 



9. The Utica Slate Group, which takes its name from Utica, New York, and 

 seems to be confined in its Geographical range to Pennsylvania, New York and 

 Canada, and reaches its greatest thickness in Pennsjdvania at 400 feet. Its syn- 

 onj'ms are Black slate, Pulaski shales, Frankfort slate and Lorraine shales. 



10. The Hudson River Group, which takes its name from the exposure on 

 Hudson River, New York, has an extensive geographical range, and reaches its 

 greatest thickness in Canada, at 2,000 feet. The fossils which characterize it 

 are nearly the same as those which characterize the Trenton Group. The inter- 

 vention of the Utica slate in New York and Canada furnishes the only excuse 

 for separating the two groups. In the Western and Southern States, wh'ere the 

 Utica slate is absent from the strata, the upper part of the Lower Silurian is 



