340 E. O. ULRICH REVJSIOK OF THE PALEOZOIC SYSTEMS 



intersystemic interval the general elevation of the interior area of the 

 continent, which had been the primary cause of the draining of the seas 

 of the preceding period, was in course of gradual reduction by continental 

 creep, and this seems to have proceeded without much local warping 

 through the early stages of the next period. At the same time the 

 emerged surface suffered slight degradation. Finally, when the general 

 subsidence of the interior ])lain reached the stage of submergence by 

 marine waters, the new continental seas naturally simulated those of the 

 preceding period in pattern and size. As stated on page 446, "continental 

 creep" is ever opposed by "suboceanic spreading." In the course of the 

 conflict between these and other factors the surface of the continent pul- 

 sates up and down, falling very gradually when the former is dominant 

 and rising, particularly in decidedly positive areas, more impulsively 

 when the latter prevails. In the case of the widely submergent introduc- 

 tory phase of the Silurian, as established by the extraordinary distribu- 

 tion of Eichmondian deposits, the downward trend of the land was pres- 

 ently interrupted by an emergence. The last movement evidently began 

 a series of warpings that, as "creep" once more prevailed, were sufficiently 

 accentuated to cause wide differences in the extent and general pattern 

 of ensuing Niagaran continental seas. It appears then that the early 

 submergences of these periods simulated preceding submergences chiefly 

 for the reason that warping of the interior areas was largely delayed to 

 later stages of the new period. 



Regarding the periods in which the accessible early depositional stages 

 are neither larger nor conspicuously different from subsequent stages, it 

 is thought that these indicate preceding intersystemic intervals of non- 

 accessible deposition and continental emergence of longer duration than 

 usual. In other words, that the epoch corresponding, for instance, to the 

 introductory stage of the Silurian was in these cases included in the 

 intersystemic interval whose record is inaccessibly buried under the 

 marginal shelf of the continent. Or it may be merely a peculiar phase of 

 the rhythm, something like this being suggested by the fact that the 

 periods whose first series of accessible deposits is of relatively small ex- 

 tent are those (Ordovician, Tennessean, and Cretaceous) which immedi- 

 ately precede the great revolutions. In favor of the former interpreta- 

 tion there is to be urged the fact that the physical as well as the faunal 

 break between the Waverlyan and the Tennessean, and perhaps also be- 

 tween the Comanchean and the (upper) Cretaceous, is greater than 

 between the other systems of their respective eras. Indeed, we might 

 say that the accessible sedimentary record of these systems was delayed 

 to the second stage — that is, deposition in the continental basins began 



