282 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Paleozoic cycles 315 



Present and Paleozoic conditions briefly compared 317 



Oscillatory character of continental seas 320 



General discussion 320 



Middle Ordovician oscillation indicated by the Chambersburg lime- 

 stone in southern Pennsylvania 321 



Chambersburg limestone in the Chambersburg-Massanutten basin. 321 



Character and stratigraphic relations 321 



Basal member of the Martinsburg formation 323 



Chambersburg limestone in the Mercersburg, Pennsylvania, bands. 325 



Character and stratigraphic relations 325 



Blue Spring section 320 



The Dickeys, Pennsylvania, section 327 



Chambersburg limestone section near Saint Thomas, Penna . . 328 

 Resume of differences between the Chambersburg and Mercers- 

 burg belts 328 



Displacements of the strandline 329 



Stratigraphic overlaps 329 



General discussion 329 



Progress of overlaps interrupted and modified by emergent 



phases 330 



Minor interruptions indicating relatively local movements . . . 334 

 Frequency of alternation of submergent and emergent condi- 

 tions indicative of smallness of Paleozoic movements of 



land areas with respect to sealevel 335 



Bearing on paleogeography 336 



Initial submergences often larger than succeeding invasions 



of same stage or period 338 



The vertical displacement of the strandline and the relief of con- 

 tinental land-masses 341 



General discussion 341 



Discussion of illustrative diagrams 342 



Marine invasions from the four sides of the North American 



continent 344 



Intersystemic intervals. 345 



Comparison with Schuchert's submergence and emergence 



curves 348 



Time values of systems 349 



Relatively hopeless imperfections of the sedimentary record 350 



Climatic variations 352 



Frequent alternations of warm and relatively cool climates 352 



The hypothesis of reversed oceanic circulation 353 



Significance of black shale deposition 356 



Distribution and characteristics of the shale. 356 



Objections to prevailing interpretations 356 



Black shale faunas 358 



Cool climates a possible cause 359 



Shallowness of Paleozoic continental seas and absence of strong, trans- 

 continental currents 361 



Continental seas generally very shallow 361 



Effect of currents on deposition 362 



