606 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



sippi Valley, and proceeding with occasional warping in the former and 

 gradual resubmergence of more inland areas to the west and north; a 

 third stage characterized by profuse warping and sea shifting which 

 began with the narrow Atlantic seas of the Blonnt stage, attained maxi- 

 mum development in the Black Eiver and grew less during the Trenton 

 ages of wide interior seas ; and, finally, a fourth stage in which the move- 

 ments were broad in scope, without the irregular effects of regional warp- 

 ing, tending, on the whole, to mid-continental elevation, and in which 

 the submergences were extensive in the Appalachian and Allegheny 

 basins, but fell so far short of preceding stages in northerly and westerly 

 directions that Cincinnatian deposits seem entirely absent in the Missis- 

 sippi Valley north of Tennessee and west of Indiana. 



Except for modifications due to the intervening highly emergent phase 

 marking the transition from the Eopaleozoic to the Neopaleozoic era, a 

 similar series of stages is recognizable in the Silurian. In the latter 

 period the Kichmondian corresponds, in a general way, to the "Saint 

 Peter" series, the Clinton to the Stones Eiver, the later Niagaran or 

 Chicago ages to the Black Eiver and perhaps early Trenton, and the 

 Cayugan to the Cincinnatian. The average altitude of the continent 

 during the Silurian being greater than in the Ordovician, the areal 

 extent of most of the Silurian formations is inferior to that of the cor- 

 responding formations in the older period. Moreover, and for the same 

 reason, certain well-defined parts of the Ordovician submergent stages 

 seem wholly unrepresented in the accessible depositional sequence of the 

 Silurian in southeastern [N'orth America. Most notable of these is the 

 absence of any stage corresponding to the Ordovician Blount group. Nor 

 can I recognize beds whose diastrophic history is comparable to that of 

 the middle and late Trenton. 



Whether the early stages of a period effected submergence of median 

 areas of the continent — particularly the Mississippi Valley — depends on 

 the relative average altitude of the continent during the period. (See 

 page 342). If this average was low, as during the Ordovician, then the 

 continental seas were often broad, and the marine sedimentary record 

 in them is correspondingly full; if the average was high, as during the 

 Tennessean, Pennsylvanian, and Jura-Trias, then sea invasions during the 

 period were fewer and smaller and the accessible record correspondingly 

 incomplete. 



Applying this idea to other systems, as, for instance, the Waverlyan, 

 we may compare the evenly spread Chattanoogan with the Stones Eiver, 

 the exceedingly oscillating Kinderhookian with the Black Eiver, and 

 the Osagian with the Trenton. In each case the Waverlyan formations 



