DEVONIAN AGE. 275 



Coccosteus, Ctenacanthvs, and Lvptacanihus'" (Dawson). The gigantic Dinichikys of 

 Newberry, from the Black Shale of Ohio, occurs in great concretions, some of them 

 twelve feet in diameter. Remains of a species of Palceoniscus have been found in the 

 top of the same shale, near Danville, Ky. (F. H. Bradley.) 



General Observations. 



Geography. — The positions and nature of the Hamilton beds in- 

 dicate similar geographical conditions to those of many earlier periods, 

 — that a shallow sea covered New York and spread widely to the 

 west, and that many changes were experienced in the water-level. 

 The beds are to a great extent mud-beds, whence we learn that they 

 were deposited in quiet waters : the fossils are marine, proving marine 

 waters. The beds in New York are thickest about its central parts, 

 and yet spread to its eastern and western limits, excepting the upper- 

 most, the Genesee shale, which is not known in the eastern part ; 

 they are partly calcareous in the lower part of the Marcellus beds, 

 proving that the change from the condition of the limestone-making 

 Corniferous period was gradual ; limestone layers occur higher up, at 

 intervals, indicating changes of level, which favored at times the 

 growth of Crinoids and Corals ; ripple-marked flags make up some 

 layers, proving, by their evenness and extent, and the regularity of 

 the lamination, that the sea, at the time of their formation, swept over 

 extensive sand-flats, coming in over the present region of the Hudson 

 River or of New York Bay. The existence of a barrier of sand along 

 the ocean, such as is thrown up and at intervals removed again by 

 the waves, would account for the varying conditions and also for 

 changes in the living species by extermination. 



Moreover, while these mud- accumulations were here in progress, 

 there were Hamilton limestones forming in some of the Western 

 States, indicating again the existence of the Interior or Mississippi 

 sea, — a feature in a large part of both Silurian and Devonian geog- 

 raphy ; and then the wide-spread, but thin, Hamilton black shale, 

 almost destitute of fossils, and very bituminous, indicating probably 

 quite shallow waters over the Interior basin, — yet not so shallow but 

 that fishes could live abundantly through them. 



The Appalachian region was still an area of vastly the thickest 

 deposits, and hence of the greatest change of level by subsidence ; and 

 the great thickness of the formation (1,000 feet) in central New York 

 makes it another example of the prolongation of the subsiding Appa- 

 lachian region northward over southern New York. This fact and the 

 thinning of the beds toward the Hudson River indicate that the Green 

 Mountain region was above the sea, so that the great New York bay, 

 alluded to in the observations on the Oriskany beds, was still outlined 

 on the east, although communicating westward more or less perfectly 

 with the Interior basin. 



