NIAGARA PERIOD. 243 



Halysites catenulata, Spirifer crispus, Atrypa lamellate/,; and these are not all 

 beyond doubt. Moreover, three of them are cosmopolite species. The beds are, 

 therefore, strikingly different in life from the Niagara, and appear to represent 

 a later epoch. Among the species there are very large spiral chambered shells 

 of the new genus Trochoceras Hall, which are unknown in other formations. 



General Observations on the Niagara Period. 



Geography. — The facts upon which we have to rest our conclu- 

 sions with regard to the geography of the Niagara period are, — 



1st. The occurrence of the Oneida conglomerate over the region 

 from central New York southward through the length of the 

 Appalachians. 



2d. The Medina sandstone covering the same region, but spread- 

 ing a little farther westward on the north. 



3d. The Clinton group having the same range on the east and 

 extending over a considerable part of the interior basin to the 

 Mississippi ; shales characterizing the formation in the Appala- 

 chian region, shales and sandstones prevailing over limestone in 

 New York, and limestones, more or less argillaceous, mostly con- 

 stituting the beds in the West. 



4th. The Niagara rocks having nearly the same eastern limit in 

 New York (that is, absent almost entirely from the eastern third of 

 the State) ; spreading over the Appalachian region and also through 

 a large part of the interior basin ; consisting of shales with some lime- 

 stone in central New York, more limestone in the western part of the 

 State, shales almost solely in the Appalachians, limestones in theWest. 



5th. The formations six to eight times thicker in the Appala- 

 chians than in the West. 



The position of the coarse conglomerate rocks of the Oneida epoch, 

 spreading neither over eastern New York nor the interior basin 

 west of the State, apparently indicates that along its line was the 

 sea-coast of the time, and that the ocean reached it in full force. 

 Such coarse beds of marine formation are formed either in front 

 of the waves, or under the action of strong marine currents. The 

 latter never have sufficient force for the purpose, except in narrow 

 straits and over limited areas, and are not capable of making 

 accumulations of so great extent. It is stated on page 228 that 

 the Appalachian region in Vermont, if not also in southern New 

 England, must have been out of water : the absence of this forma- 

 tion and the others of the Niagara period from eastern New York 

 harmonizes with this view. 



The fine sandy and clayey character of the Medina beds shows 



