422 E. 0. tlLRlCH — REVISION OF THE PALEOZOIC SYSTEMS 



long enduring and definitely established areas of uplift, are mostly of 

 Ordovician age, will probably be ascribed to the fact that the rocks of 

 this period have received a larger share of my stratigraphic and paleon- 

 tologic studies than others. But this is not the true reason, for much 

 evidence has been gathered to prove that essentially similar oscillatory 

 movements occurred in preceding and succeeding Paleozoic periods. 

 The true reason is that minor local tilting of the kind described is more 

 readily determined and seems, indeed, to have been more prevalent during 

 the Ordovician than in other periods. The evidence in hand leads to the 

 inference that in the other periods the movements were broader in scope. 

 Largely on this account their effects on the distribution of deposits and 

 faunas is less easily ascertained. However, the lack of clearly defined and 

 abundant faunal associations in the rocks of certain ages doubtless con- 

 tributes not a little to the difficulty of acquiring detailed knowledge con- 

 cerning local migrations of the strandline. 



Early Silurian oscillations in North America. — Movements of a dif- 

 ferential vertical character, simulating those of the Ordovician, except 

 that as a rule they affected relatively wider areas, are clearly indicated in 

 the early and middle Silurian. Post-Ordovician sedimentation in 

 America is believed to have begun with the Dubuque limestone, an ir- 

 regularly distributed and sparingly fossiliferous thin formation in the 

 upper Mississippi Valley. The age of this bed has not been positively 

 determined, but for the present it seems preferable to class it with the 

 Maquoketa in the Eichmond than with the underlying Ordovician de- 

 posits. It is limited both above and beneath by unconformities and, so 

 far as known, confined to the north. The Arnheim shale, which is pro- 

 visionally assumed to succeed the Dubuque, is known only on the flanks 

 of the Cincinnati dome and locally in west central Tennessee. This dis- 

 tribution, together with the fact that its fauna consists chiefly of un- 

 questionable though strongly modified derivatives of the Maysville fauna 

 of the same areas, indicates its southern origin. In Tennessee it is suc- 

 ceeded and overlapped by the Fernvale, a well marked and widely recog- 

 nizable zone. That differential movements intervened is proved by the 

 fact that whereas the Arnheim is well developed in the Cincinnati region 

 and wholly unknown in Arkansas, Missouri, and Illinois, the reverse is 

 true of the Fernvale. Both formations contain large faunas, but very few 

 of the species are common to the two. In view of this faunal difference, 

 it seems improbable that both invaded from the Gulf of Mexico. On the 

 other hand it seems unlikely that the characteristic and abundant Fern- 



