452 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



North American continent, as noted and explained on pages 407 and 409, 

 effected in perhaps every case extraordinarily rapid extensions of sedi- 

 mentary planes. Overlap strnctiire in these cases is demonstrable only 

 on the flanks of island domes and peninsular projections included in the 

 general area submerged at such times by northern waters. 



General prevalence of overlap structure. — With relatively few excep- 

 tions, lithologic or stratigraphic units, whether constituting the whole 

 or merely subordinate divisions of formational units, are bounded below 

 by progressive overlap structure. The exceptions consist almost solely 

 of instances of limestone or shale formations which grade laterally into 

 more clastic, near-shore, delta or fluviatile, deposits. N'otable instances 

 of such transitions have been shown to occur in the Devonian of New 

 York by Clarke, Williams, and others who have contributed to our knowl- 

 edge of the stratigraphy in this State. These cases are typical of a class 

 in which land debris is accumulated in a bay or along a shore, finer 

 sediments being laid down farther out. The Selma chalk band in the 

 southern States which, according to unpublished investigations by L. W. 

 Stephenson, passes at each end into less calcareous deposits, seems to 

 illustrate a different class, namely, deposition around a low blunt penin- 

 sula influenced by lateral drainage, so that relatively clastic deposits 

 formed on its sides and calcareous sediments off the rounded extremity. 



Though lateral transitions in lithic character have hitherto been sup- 

 posed to occur very commonly, a critical review of the cases has shown 

 that true transitions of this kind are by no means common. As a rule 

 the presumed lithically changijig stratigraphic zones proved to comprise 

 deposits that were not only laid down in physically distinct troughs or 

 basins, but also that their respective beds are not contemporaneous. 

 Faunal studies first suggested and have finally established the proposition 

 of small, disconnected, oscillating continental seas ; but, after all, the most 

 convincing and therefore necessary evidence was furnished by strati- 

 graphic overlaps. In the Appalachian Valley, for instance, most of the 

 Eopaleozoic formations were found to be confined to a single or to two 

 adjacent troughs. Other formations spread more widely and are found 

 in three or more basins. In the broader interior basins the stratigraphic 

 record as exposed on the flanks of low uplifts is punctuated at frequent 

 intervals by evidence proving oft-repeated advance and retreat of marine 

 waters. 



Owing to overthrusting of the middle and eastern troughs in the Appa- 

 lachian Valley, the overthrusting being incidental to the excessive folding 

 of the valley rocks (see figure 16, section 6, page 440, and figure 9, 



