558 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



ence drawn therefrom that the maximum thickness of each of the forma- 

 tions invading from the north is older or younger, as the case may be, 

 than the whole of its nearest southern contemporary. 



Demonstration by Niagaran overlaps. — The Niagaran oscillations cover 

 a longer time and the variously derived formations are but seldom 

 brought into direct superposition. Surely, too, the depositional record 

 of this stage in the continental basins is more than usually incomplete. 

 The problems, therefore, are correspondingly more complex and the 

 solutions in most instances too largely theoretical to be satisfactory. 

 However, the Clinton part of the story as now understood seems rea- 

 sonably convincing, while the sequence worked out for the upper half 

 of the series is perhaps sufficiently plausible to deserve trial. 



The oldest known Clinton is the Brassfield limestone or Ehinopora 

 verrucosa zone — in other words, the ^'Ohio Clinton." This easily identi- 

 fied zone, though somewhat irregular in outcrop, is yet widely recognized 

 in the Ohio Valley and western Tennessee. In Oklahoma it commonly 

 forms the basal part of the Hunton formation; in Arkansas, where it has 

 been seen at three localities in the Yellville quadrangle, it is provisionally 

 mapped with the Saint Clair limestone. Logan and Hall studied it at 

 Hamilton and other points in Ontario and placed it in the Clinton, but 

 at Niagara Falls, where the same horizon is more arenaceous, authors 

 have generally included it in the Medina. In the Appalachian Valley 

 it seems to be confined to the Tennessee basin. In this it was observed 

 in Elk Valley, White Oak Moimtain, La Follette, and Eockwood, in 

 Tennessee, and Lavender Mountain, Georgia. The fauna doubtless in- 

 vaded from the Gulf of Mexico. 



The Brassfield is succeeded — always abruptly and with evidence of 

 hiatus — by various formations at different localities. At Niagara Falls 

 and in the Tennessee basin later Clinton deposits rest on it, in Indiana 

 and western Tennessee the Osgood limestone (equals Eochester shale) 

 overlaps it, in Oklahoma any one of several much younger Silurian 

 horizons may be in contact with it. Taking the Indiana succession — 

 Brassfield and Osgood, both southern invasions — we would expect, as in 

 the preceding Pamelia-Lowville case, at least one intermediate Atlantic 

 invasion. On investigation this seems to be precisely what occurred, free 

 communication with the Atlantic while the middle Clinton deposits were 

 being laid down in New York and the western part of the Appalachian 

 Valley as far south as northern Tennessee being as firmly established as 

 fossil evidence can do it. These middle Clinton zones contain graptolites, 

 a distinctive coral, peculiar bryozoa and brachiopods and several ostracods, 

 all of types never seen in Silurian faunas that invaded from the Gulf of 



