304 E. 0. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Mississippi Valley. At the same time it advanced westward, covering 

 areas previously land in Oklahoma and central Texas. 



Extent of Paleozoic Continental Seas 

 general discussion of the subject 



The relief of Paleozoic land areas — a subject to be discussed in detail 

 under the head of Displacements of the Strandline — was generally very 

 low. At certain times, however, notably the early Cambrian, early Silu- 

 rian, and early Pennsylvanian, the highly clastic character of the sedi- 

 ments justifies the belief that the submarginal lands — ^that is, the areas 

 outside of the great interior flat of the continent — were high enough to 

 encourage abundant and rapid erosion. Comparatively high lands, 

 though more limited in extent, are indicated also in the early and late 

 parts of the other periods (see pages 467 to 477), but following their 

 reduction each of the periods entered a long enduring stage of low lands 

 and shallow epicontinental seas. The evidence of still other crustal 

 movements during the course of each period is preserved in the strati- 

 graphic and organic records. These last movements (see pages 405 to 

 430), though less violent than those marking the beginning and close of 

 the periods, nevertheless effected often greater geographic changes. Under 

 nearly baseleveled conditions it is readily seen that a slight tilt of the 

 continental platform, or only a small subsidence of the same, might have 

 produced great extensions of tlie seas, and when the movement was re- 

 versed, equally great withdrawals. iVdmitting an average low relief, it 

 follows that erosion during a long part of each period must have been 

 very slight. Following this, it is plain why even very long intervals of 

 emergence may be so obscurely recorded in the stratigraphic succession 

 that the fact of emergence may be very difficult to establish by physical 

 evidence alone. 



Numerous instances of discontinuity of sedimentation, and presumably 

 emergence, have been observed, the time values of which, if suspected at 

 all, could not possibly be determined without the evidence of fossils. 

 Assuming the competency of their evidence, the facts concerning a few 

 of the more convincing, in part unpublished, instances will be cited in 

 the hope that they may be accepted as establishing the following propo- 

 sitions: (1) prevalency of conditions approximating baseleveling over 

 large areas of the Paleozoic continent; (2) the present distribution of 

 Paleozoic epicontinental formations is fairly indicative of the extent of 

 their respective seas; (3) the Paleozoic continental seas were never deep, 

 and most of them very shallow, and (4) the surface of the continents is 

 unstable and in Paleozoic ages was subjected to frequent more or less 



