j 50 PALAEONTOLOGY OF NEW YORK. 



therefore, to trace the extension of this group westward, along a line north 

 of any important influence arising from the Cincinnati axis, and to show its 

 gradual lv increasing calcareous character with constantly diminishing thick- 

 ness. The same law has undoubtedly prevailed over the entire bed of the Ham- 

 ilton ocean ; and there is no reason to suppose that the same law and conditions 

 which operated upon the sediments which we trace with slight interruptions 

 from the Hudson valley to Wisconsin, should not have prevailed also in a 

 southwesterly direction, giving at one time an expansion of these calcareous 

 and semi-calcareous beds, of which the continuity in that direction has been 

 broken by the Cincinnati axis. 



Moreover, in considering this condition, it should not be forgotten that the 

 eastern outcrop of these sedimentary formations, extending from the Held- 

 erberg mountains in New York, southwesterly through Pennsylvania, Mary- 

 land and Virginia, in an unbroken line of more than six hundred miles of the 

 coarser sediments, have, in their western extension, followed the same law 

 which prevails in the three hundred miles of continuous outcrop within the 

 State of New York. The operation of the same law has given us the Hamilton 

 group at the Falls of the Ohio, represented by about thirty feet of impure and 

 magnesian limestones. 



We now come to the consideration of the age and relations of the Black 

 slate which lies upon the Hamilton limestones in the neighborhood of the 

 Falls of the Ohio and at numerous localities in Indiana. To do this appre- 

 ciatively, we must consider the relations of the Hamilton group to the 

 succeeding rocks in the State of New York and other eastern localities. These 

 relations have been heretofore fully described, and we need only a cursory 

 review of the facts. 



In central and western New York, the fossiliferous, semi-arenaceous and 

 semi-calcareous shales of the Hamilton group are succeeded by a black, fissile 

 slate, known as the Genesee slate ; and this Black slate is succeeded by a 

 green or olive shite or shale, followed by successive alternations of black. and 



