BIGSBY PALAEOZOIC ROCKS OF NEW YORK. 381 



sufficient only to carry on the coarser particles to a certain distance, 

 where they were deposited. The finely levigated mud was carried 

 beyond this point, being floated by less force than the sand. 



" Some portion of the clay was deposited with the sand toward the 

 central part of the State ; and but little of the latter extended beyond 

 this point. Finally the current became more gentle, and the clay 

 was deposited to a certain extent, beyond which the power of the 

 current was insufficient to carry even this material ; and the deposit 

 consequently thinned out in that direction." 



Transition. — This is very gradual, and from the Marcellus Shale. 

 In fact, Prof. H. D. Rogers, with the sanction of all American geo- 

 logists, has included within one and the same series Marcellus Shale, 

 the Hamilton group, and Genesee Slate, and has named them the 

 " Cadent series." 



Place. — It is seen to rest on Marcellus Shale in numberless places, 

 among which are Varick and Fayette townships in Seneca County, 

 and on Flint Creek, Ontario County. 



If we leave the territory of New York, the extent of the Hamilton 

 group is enormous, and far beyond any profitable details. 



Taking our departure from Coeymans, a village on the Hudson 

 River, we trace its eastern outcrop for 244 miles southwards to Or- 

 wigsburg. 



From Coeymans, again, following the northern outcrop westwards, 

 it stretches across New York, and by Buffalo to South Harbour on 

 Lake Michigan, a distance of 913 miles, — while it forms a belt of 1000 

 miles in length around the Illinois Coal-field. But this is not all ; 

 it takes its proper place among Devonian groups in vast prolonged 

 stripes throughout Ohio, Kentucky, and Tennessee. 



In Eastern New York the Hamilton group passes through the 

 counties of Sullivan, Ulster, Greene, Albany, Schoharie, and Otsego. 

 In Central New York it is entirely confined to the counties of Otsego, 

 Madison, Onondaga, Cayuga, Herkimer, Oneida, Courtland, Che- 

 nango, and Tompkins. In the first four of these last it covers much 

 surface — one-half of Madison ; in the last five, very little. In Western 

 New York it traverses the counties of Ontario, Livingstone, Genesee, 

 and Erie. 



Position. — It is conformable with its associate strata ; but the hard 

 blue shale of this group about Kidder's Ferry is distinctly seen to 

 rise and fall in undulations (Hall). 



Thickness. — In the east of New York it is 1000 feet thick (De 

 Verneuil). In the central and western parts of the State it varies 

 from 700 to 300 feet (Hall and Vanux.) ; but it always thins west- 

 wards, and especially from the River Genesee. On the shore of Lake 

 Erie it is not half as thick as at Lake Seneca (Hall, Report, p. 187). 

 Its greatest thickness in Pennsylvania is 600 feet, according to 

 Rogers. This group has suffered much denudation. The valleys of 

 Seneca and Cayuga Lakes are palaeozoic excavations in these shales 

 for more than half their length. 



Fossils. — The fossils of the Hamilton group are remarkable for 

 their great numbers and exquisite perfection. We seem to be 



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