590 E. O. ULRICH KEYISION OF THE PALEOZOIC SYSTEMS 



nized. It is at least possible, if not probable, that this represents the 

 Cypress sandstone of the Kentucky-Illinois finorite district. 



On the basis, then, of (1) the widely recognizable stratigraphic break 

 between the Saint Louis and the Sainte Genevieve, (2) the intervening 

 diastrophic movements which occasioned very considerable shifting of 

 seas, and (3) the introduction of many species which are otherwise un- 

 known beneath the Cypress, Tribune, or Birdsville, the Sainte Genevieve 

 seems more naturally classified as basal Chesterian than as uppermost 

 Meramecian. 



The principle of introductory fades of faunas and deposits — Discussion 

 and illustrations. — The relations of the Sainte Genevieve to the succeed- 

 ing beds of the Chester series are essentially the same as those of the 

 Warsaw to the Spergen. They are also like those of the Eichmondian to 

 the Niagaran and of the Helderbergian to the middle Devonian. More 

 exactly, though, they are comparable with the relations of the Lowville to 

 the middle and upper Black Eiver Mohawkian formations and of the Key- 

 ser limestone to the succeeding members of the Helderbergian. In all of 

 these cases the first named stratigraphic unit — at least its lower part- 

 commonly resembles the next underlying deposits more than it does the 

 overling beds. Yet, if our classification is to be chronoligic and based on 

 diastrophic principles in which neither the purely lithologic nor the pale- 

 ontologic criteria are of paramount importance, these units should be re- 

 ferred to the younger and not the older group, series, or system, as the 

 case may be. In each of the mentioned cases I have endeavored to asso- 

 ciate and classify the formations in accord with the principle that, quoting 

 from Weller (op. cit., page 290), "the time boundary between two geologi- 

 cal ages (periods) should be marked by the time of maximum withdrawal 

 of the sea or the subsequent readvance during which new sets of conditions 

 were introduced.^^ From this quotation, which most geologists will in- 

 dorse, it is seen that we agree in principle and theory. Differences arise 

 chiefiy in practice, when principles are frequently forgotten and mere like- 

 ness of deposits and absence of conspicuous evidence of diastrophic breaks 

 in sedimentation is likely to dominate conclusions. In the case of the 

 Lowville-Stones Eiver contact its importance and really great significance 

 has been shown in a preceding part (see pages 553 to 557). The Cayu- 

 gan-Helderbergian contact also is now conceded to be unconformable in 

 all known exposures and the usual hiatus between the two is admitted to 

 be of greater time value than had been supposed. The same, again, is to 

 be said of the Maysville-Eichmond contact. The structural evidence of 

 the stratigraphic hiatus in all of these instances is surely quite as clear 



