602 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Hence, it may be further suggested, that the full time value of a geo- 

 logical period — assuming that they are approximately equal in value — 

 corresponds to something near that of 7,000 to 10,000 feet of limestone. 

 In view of the known exceedingly uneven distribution of marine deposits 

 in the continental basins, and knowing further that the sedimentary 

 record in the accessible parts of these basins, however thick it may be. 

 ever exhibits evidence of interruption and incompleteness, this suggestion 

 certainly is not improbable. We know, for instance, that a clearly broken 

 sequence of over 4,200 feet of Canadian limestone was laid down in cen- 

 tral Pennsylvania, and that no rocks of this age are to be found in certain 

 parts of the Appalachian Valley in Tennessee and Alabama. Here, on 

 the contrary, we find a similarly broken sequence comprising thousands 

 of feet of Ozarkian dolomites that are wholly unrepresented by deposits 

 in those parts of central Pennsylvania in which the Canadian is best de- 

 veloped. And each of the breaks in the two sequences ma}^, as was shown 

 by the Stones Eiver-Lowville contact, represent deposition of thick beds 

 elsewhere. After such proof of oscillation and long continuance of emer- 

 gent conditions in first one and then in another part of the same geosyn- 

 cline, there surely is no just cause for surprise in the suggestion that 

 uninterrupted deposition in each of these periods may somewhere have, 

 reached a total of 8,000 to 10,000 feet. The idea is not of immediate 

 consequence, but seemed worthy of mention to show the possibilities in 

 the way of future expansion of the geological column. 



Ehythm in the order of occurrence, location, and extent of oscillations, 

 causing submergences and emergences of continental areas, doubtless will 

 later on be determinable. At present, however, little of this is shown by 

 published paleogeographic maps. Probably their unpromising exhibit is 

 due to the often highly composite, not to say heterogeneous, nature of 

 most of the maps. 



RECURRENCE OF GEOGRAPHIC PATTERNS 



As will appear in succeeding descriptions, the Paleozoic systems are 

 distinguished by movements similar in intensity and effectiveness to those 

 described on page 582 in showing why the Waverlyan and the Tennessean 

 are systems and not merely series. The results, so far as the shifting of 

 the strandline is concerned, vary from time to time ; but strikingly simi- 

 lar recurrences in geographic patterns are common. These are to be noted 

 more especially on comparing invasions from the same oceanic basin in 

 the same period and these again with stages in the second succeeding or 

 preceding period. For instance, the Ozarkian geographic facies are to a 

 large extent like those in the Ordovician and these again like certain 



