EXTENT OF PALEOZOIC CONTINENTAL SEAS 317 



with the limestone at the top of the Clinton), but showing almost noth- 

 ing of the kind in its middle and later deposits. The Devonian differs 

 in that it begins with a limestone series, has a fair "cycle" in the middle, 

 and closes with shales and so much sandstone that Newberry, the father 

 of cycles of deposition, thought it advisable "to consider the Portage 

 sandstones ... as the true base of the Carboniferous system." The 

 Waverlyan often begins with a sandstone (Hardin in Tennessee, Syla- 

 more in Arkansas), followed in turn by shale (Chattanooga) and lime- 

 stone (Louisiana to Chouteau), but the upper, Osagian, series is litho- 

 logically too uniform to suggest a typical cycle. In the Tennessean also 

 cycles corresponding to its series divisions are very poorly indicated, and 

 the same is to be said of the Pennsylvanian. For these post-Ozarkian 

 systems therefore the ordinary criteria of cycles of sedimentation (de- 

 trital sediments passing into limestone) fail rather generally. Probably 

 the coarse elastics of the typical cycle may, in the case of these failures, 

 find a satisfactory substitute in the fine and relatively sparse detrital 

 material included in calcareous shale and argillaceous limestone. Or, 

 when the series consists almost entirely of mechanical sediments, cycles 

 may be established according to the grade of fineness to which they have 

 been reduced. But the prospect seems on the whole rather discouraging. 

 Considering that there are few and locally no limestones to complete 

 the cycles of the sedimentary record in the Appalachian and Atlantic 

 troughs east of the great valley, and often no sandstones with which to 

 begin them in the broader basins of the more inland areas, it is clear that 

 the local manifestation of cycles of erosion and deposition is dependent 

 on conditions having no readily determinable bearing on stratigraphic 

 taxonomy. If generally and regularly developed, or even if each im- 

 portant cycle had been adequately expressed in some, however local, but 

 accessible situation, they might constitute an excellent basis of classifi- 

 cation. But as it is, their possible use for this purpose is largely defeated, 

 or at least rendered uncertain by the frequent oscillations and emergen- 

 cies to which the areas of accessible record of marine deposition have been 

 subjected from the beginning of geologic time. So far as they can be 

 clearly determined they should be employed, but at their best the func- 

 tion of cycles of deposition in stratigraphic taxonomy is corroborative 

 and not initiative. 



PRESENT AND PALEOZOIC CONDITIONS BRIEFLY COMPARED 



The gradational processes and agents at work today, also the condi- 

 tions under which they operate, are essentially the same as in many of 

 the preceding geological ages. As today, so in past ages there must have 



