464 



A. W. GRABAU THE SHEEBUIINE SANDSTONE 



derived from the Naples and none from the lowan fauna. The central 

 region is again essentially intermediate. 



^ B 





k.:'i 



Paleogeograppiic Conclusions drawn from 

 THE Faunal Distribution 



^^^e may now attempt to outline the sequence 

 of events in N"orth Anjerica during Portage 

 time as indicated by the faunas and sediments 

 (see section, figure 2). The Hamilton period 

 came to a close with the contraction of the 

 Xorth American epeiric sea and its division 

 into three distinct basins, separated in part at 

 least by land barriers. Before the separation 

 was complete, however, members of the lowan 

 Traverse fauna were temporarily enabled to 

 enter jSTew York district. This gave us the lead- 

 ing elements of the Tully fauna, such as Hypo- 

 tliyris cuhoides, Pugnax pugnus, Schizophoria 

 tulliensis, etcetera, which continued to flourish 

 during the period of deposition of the Tully 

 limestone. That isolation of the central New 

 York area was elfected soon after the incursion 

 of the western fauna is shown by the fact that 

 the residual Hamilton fauna began to undergo 

 the modification which eventually led to the 

 Ithaca fauna. This separation was effected on 

 the east by the Sherburne bar, which extended 

 from the old land of Atlantica, in the north, 

 southAvard through Pennsylvania, possibly as 

 far as the Harpers Ferry region, cutting off an 

 eastern arm of the Atlantic from the central 

 l)asin. In this eastern arm the Hamilton fauna 

 continued to flourish in its purity, though much 

 restricted as to numbers of species on account 

 of the less favorable facies of the sea bottom. 

 That the fauna nevertheless remained true to 

 type can only be explained by the assumption 

 of continued connection with the center of dis- 

 tribution of the Hamilton fauna. This center 



