CLASSIFICATION GRADATIONAL AND LITHOLOGIC CRITERIA 473 



Clinton, Tuscarora, Medina, Juniata, and Oswego formations. These 

 lands I regard as having been nearly baseleveled at the close of the Clin- 

 ton, and that following the Clinton they merely shared in a general ele- 

 vation that embraced the whole Appalachian region. This conclusion is 

 based (1) on the fact that the last deposits of this group or formation 

 locally consist chiefly of limestone, and (2) on the similar fact that the 

 Cayugan and Helderbergian deposits which succeed them in the middle 

 and northern parts of the valley consist almost entirely of calcareous mud 

 rocks and pure limestones. 



Warping strong enough to effect the resubmergence of a large part of 

 the Appalachian Valley (Tennessee to Xew York) and to produce lagoons 

 and more or less effectively inclosed basins farther inland was deferred 

 to the Cayugan, during which time, apparently, the latter condition pre- 

 vailed in a notable degree also in the British Isles. But there is no sedi- 

 mentary evidence of folding at this time in any region near enough, or so 

 situated that it could contribute distinctly clastic material to deposits 

 now accessible. The hiatus that separates the "transition beds" beneath 

 the Coeymans from the Manlius, at Schoharie, New York, and the older 

 Helderbergian (Keyser) rocks from the Cayugan in the middle and 

 northern Appalachian Valley, together with the total absence of lower De- 

 vonian beds in all of the median regions of the North American continent 

 north of Saint Louis, suggests sufficient actual and relative elevation to 

 insure total withdrawal of continental seas at the close of the Silurian. 

 Between the operation of forces resulting in suboceanic spreading on the 

 one side and continental creep on the other, the Appalachian Valley geo- 

 syncline accommodated itself to the strain by subsidence to stages per- 

 mitting earlier return of marine waters here than elsewhere. The Hel- 

 derbergian limestones laid down at this time show that no folding of 

 consequence occurred in adjacent areas. Such folding is indicated a 

 little later by the locally highly clastic deposits of the Oriskany. Some 

 elevation and warping of Ozarkia also is indicated at this time by con- 

 siderable deposits of quartz sand containing Oriskany fossils, which occur 

 locally in southern Illinois and possibly extend beneath younger forma- 

 tions into north-central Tennessee. Subsequently, Ozarkia contributed 

 similar amounts of quartz sand to the late Devonian limestones on its 

 north flank and again when the Chattanooga shale, with its introductory 

 Sylamore sandstone, overlapped its southern side. Appalachia, Taconia, 

 and the Adirondack uplift, however, continued to supply finer or coarser 

 elastics to the close of the Waverlyan. Evidently the belt of gentle fold- 

 ing had moved farther inland in the Neopaleozoic than it was in the 

 Eopaleozoic. (See figure 16, page 440.) 



XXXII— Bull. Geol. Soc. Am., Vol. 22, 1910. 



