82 



Life finally disappeared almost completeK^ from this 

 region, and for a long period the waters were practically 

 uninhabited. The Upper Devonian conditions were ushered 

 in slowly, some of the Genesee species appearing early during 

 the Moscow age. This indicates that the change from the 

 upper Middle to the lower Upper Devonian was a gradual 

 one, and that abrupt changes did not occur, either in 

 physical conditions or in life. 



Meanwhile, during the whole progress of the Hamilton 

 period in the east, there were regions in the west and south- 

 west, where neither the limestone nor the shale-forming 

 conditions of this period existed. In these regions, bitumi- 

 nous shales were deposited from the close of the Lower 

 Devonian, far into the Upper Devonian. The changes in 

 phvsical geography which allowed the development of the 

 Hamilton beds in New York State after the deposition of 

 the Marcellus beds, did not occur in the south-west, and 

 with the continuance of the same physical conditions, the 

 material deposited as well as the life continued very uniform 

 throughout. The "black shale " of Ohio and other states 

 reprCvSents these deposits, ^vhich continued uniformU', during 

 all the time that the Hamilton beds were being laid down 

 over New York. This enables us to understand why the 

 species of fossils which flourished in the Marcellus epoch, 

 should return during the Genesee epoch, while during the 

 intervening Hamilton epoch they were absent. 



If the centre of distribution of these animals, which found 

 their natural habitat under conditions necessar\' for the 

 formation of the fine black shales, was in the southwest, 

 and there continued undisturbed during the whole of the 

 Middle and later Devonian periods, new emigrants could 

 travel eastward when the favorable conditions were, in part 

 at least, restored. In Central New York a limestone making 

 age, that of the Tully limestone, preceded the Genesee age, 

 and while it continued, deposition was almost at a stand- 

 still in the west. The Tully limestone at some places in 



