626 



E. O. ULRICIl REVISION OF THE PALEOZOIC SYSTEMS 



of the Paleozoic periods. The "birth of the Appalachian protaxis" 

 surely did not occur at the close of the lower Cambrian, for it was in 

 evidence long before, having formed the eastern shore of the lower Cam- 

 brian trough, at least to the south of New York. It may be true that 

 the Chilhowee-Green Mountain barrier of Ulrich and Schuchert is first 

 clearly indicated at this time; but this barrier can not possibly be the 

 Appalachian protaxis. It lies to the west of that axis, and the date of. 

 its^ origin is still undetermined. In my opinion, the Chilhowee axis 

 existed as a low ridge in pre-Cambrian times and that it formed the 

 western border of the lower Cambrian trough just as the Eome barrier 

 at times, especially in the Ordovician, limited the westward expansion of 

 Atlantic waters in east Tennessee. (See table, page 544.) Further, I be- 

 lieve that the Eome barrier, and certain other axes to the west of it^ were 

 in rudimentary existence, if not in pre-Cambrian times, then at least 

 during the middle Cambrian. However, our inquiry in the case of these 

 old warps concerns itself less with the date of their origin than with 

 the determination of the age when they first gave lodgment to a Paleo- 

 zoic sea. 



As I see it, the principal geographic difference between the lower and 

 middle Cambrian in the middle and southern thirds of the Appalachian 

 Valley region is that, whereas the older continental seas were largely 

 confined to a trough lying to the east of an old axis over which remnants 

 of the lower Cambrian Chilhowee rocks are now piled by westward thrust- 

 ing, the middle Cambrian seas were mostly — perhaps entirely — developed 

 in the narrow crumpled area between the Chilhowee barrier on the east 

 and the Powell barrier (between the Newman and Clinton troughs) on 

 the west. Except that other relatively subordinate parallel axes were 

 developed in the meantime, the Cambrian shifting of the seas in the 

 Appalachian region was essentially similar to the east-west shifting 

 that took place, as described in Part 11, so frequently in the same region 

 during the Ordovician. 



If the dift'erences between the lower and middle Cambrian seas were 

 of sufficient importance to justify the systemic separation of the two 

 sei'ies, then the upper Cambrian would be entitled to similar distinction. 

 Then, too, the Ozarkian would embrace two, or perhaps three, systems, 

 the Canadian two, the Ordovician at least four, and the Silurian three 

 or four. The free entrance of Atlantic waters into the Appalachian 

 troughs and their exclusion during some preceding or succeeding stage 

 or epoch is a very unsafe criterion on which to base systemic distinctions. 

 It is well worth consideration in discriminating groups and series or 

 stages and epochs, but a geologic period spans a longer time interval in 



