74 



constituted a large proportion of the accumulation formed. 

 These conditions, however, did not continue very long, for 

 this fauna was soon driven out and replaced by the first of 

 the trtie Hamilton faunas, which began with the develop- 

 ment of corals, brachiopods, and large Nautili. It thus 

 appears that the change from the Marcellus to the Hamilton 

 conditions was a slow and gradual one, consisting in the 

 deepening and purification of the water. Hamilton species 

 gradually appeared, and although the change from the 

 Strophalosia bed to the Nautilus bed is an abrupt one, the 

 way for this change was being prepared during the long- 

 preceding ages. Not so, however, at the beginning of the 

 Mesodevonian period, for the change from the Corniferous 

 coral reef to the Marcellus mud-flats was sudden, resulting 

 in the extinction of numerous forms of life, many of which 

 disappeared forever. What became of the fauna of the 

 Strophalosia bed at the opening of the Hamilton (Spiri- 

 feroides) epoch, is not known. The survivors probabl}' 

 migrated eastward, where the conditions continued more 

 favorable. 



At the commencement of the Aliddle Devonian period, the 

 character of the sea-bottom, and the relative depth of the 

 ocean, became more varied. In the west, owing probably to 

 the absence of coincident subsidence and deposition, the 

 conditions continued to remain uniform into the Upper 

 Devonian. In the extreme east, ho\vever, subsidence went 

 on at a uniform and continuous rate. This accounts for the 

 fact that the beds are much thicker in the eastern portion of 

 the State of New York than they are on Lake Erie or west- 

 ward. For thick beds of fragmental material can not 

 accumulate imless there is coincident subsidence, the sea floor 

 sinking at a uniform rate, and thus making room for the 

 material constantly brought in b}' the streams. While, 

 however, the subsidence was gi'cater in its totality and 

 more continuous in the east, than in the wCvSt, it was more 

 sudden in the Erie County region, so that, at the opening of 



