330 E. O. ULRICII REVISION OF THE PALEOZOIC SYSTEMS 



In the course of my field studies I have perhaps paid more attention to 

 the investigation of overlap phenomena than to any other feature of 

 etratigraphic geology. Nearly every formation, and in many cases the 

 well marked members of formations, found outcropping on the flanks of 

 the old gentle uplifts of the interior flat of the North American conti- 

 nent, exhibit unquestionable evidence of overlap structure. A similar 

 structure pertains also to the formations in the folded Appalachian region, 

 only here the sedimentary record is more complete and the easily recog- 

 nizable gaps are usually separated by thicker beds and have a correspond- 

 ingly inferior time value. However, those that have been observed in the 

 Appalachian region, although commonly representing a much shorter 

 period of time, are no less distinctly marked than are the corresponding 

 hiatuses in the thinner sections of the interior areas. Detailed correla- 

 tions between the thick Appalachian sections and the thinner, less com- 

 plete records exposed on interior uplifts like the Cincinnati dome, enable 

 us to reach a fair conception of the relative importance of the numerous 

 gaps in the latter. Obviously, the hiatuses which are as sharply defined 

 in the relatively complete Appalachian Valley sections as those seen in 

 more interior areas like the Cincinnati uplift must be of much greater 

 taxonomic value than those which are entirely unrecognizable, or but ob- 

 scurely indicated, in the former. 



Progress of overlaps interrupted and modified by emergent phases. — 

 Sea transgressions over continental areas probably never progressed unin- 

 terruptedly through the whole of a period and certainly never continued 

 into and through the succeeding period. There is therefore no basis in 

 fact for the rather commonly accepted view of a continuous, gradually 

 increasing submergence of the American continent, beginning with re- 

 stricted early Cambrian troughs and ending in a great lime depositing 

 Ordovician sea, that is supposed to have covered the greater part of the 

 continent. Neither is there any ground for the proposition not long ago 

 submitted by Grabau^^ of great "progressive overlaps," the first of which 

 began with the Cambrian and, spreading in various directions in different 

 parts of the continent, continued to advance to Ordovician time, while an- 

 other began with the Devonian "Black Shale" and continued to the Ten- 

 nessean. As is rather fully shown in discussing the effect of currents in 

 continental seas (pages 362-374), the continents were never subjected 

 to such long continued and gradual encroachments of the sea nor to the 

 great and deep submergences believed in certain quarters. In fact the 



"Types of sedimentary overlap. Bull. Geological Society of America, vol. 17, 1906, 

 pp. 567-636. 



i 



