Elements of the Paleozoic Platform of North America. 405 



the Appalachian folding and probably much obscured by 

 extensive overthrusting from the southeast along " Logan's 

 line." The eifect of Appalachian folding by crushing in one 

 side of the symmetric structure here set forth, will be discussed 

 more fully in another chapter (see p. 408). 



From each of these cornerstones there extends southward, 

 like an arm, a broad belt of Frecambrian and early Paleozoic 

 rocks, nearly the full length of the continent. The western 

 arm can be traced by the great southward extension of the Pre- 

 carboniferous rocks of Isle Wisconsin to near the neighborhood 

 of Burlington, the Siluro-Devonian inlier along the Mississippi 

 above its junction with the Missouri and the large Precam- 

 brian-Cambro-Silurian inlier or uplift of the Ozarks in Mis- 

 souri and Arkansas* [DJ. Its "Leitlinie" is shown in the 

 thick broken line marked D^ through D^. The eastern arm 

 [E^-EJ has been badly overridden, broken up and forced 

 inward by the tangential pressure that has produced the Appa- 

 lachian folds. It is, nevertheless, still easily recognized in the 

 belt of Precambrian and Precarboniferous rocks, extending 

 south and southwestward from New York as far as Alabama. 



The two arms have later been somewhat disturbed and 

 obscured, especially the western one, by the breaking down of 

 certain portions south of Isle Wisconsin, where the Carbonifer- 

 ous has transgressed it, and the eastern one by the submergence 

 of portions southeast of the Adirondacks and by extensive 

 folding. In their original position the two arms may be con- 

 ceived as approaching each other somewhat in the south, 

 although not nearly so much as they do now, in consequence 

 of the forcing inward of the eastern arm, for if the consider- 

 able shortening of the eastern basin indicated by the Appa- 

 lachian folds is taken into account and the basin spread out to 

 its original width, the eastern arm would probably take a posi- 

 tion fully corresponding to that of the western. 



These two arms bound a large basin, the " Paleozoic 

 eastern basin," now occupied by the basin of Ohio and the 

 Great Lakes. In the middle of this an elongated low eleva- 

 tion formed, now indicated by the Cincinnati and Nashville 

 "uplift." 



The axial position of this uplift (see line B^-Bj, fig. 1) sug- 

 gests that it may partake of the nature of the " geanticlinal 

 median "f that according to Haugij: forms along the median 



* The Ouachita mountains in Arkansas probably represent, according to 

 Dr. Ulrich's description [in Preliminary List of Papers, Geol. Sec. Am., 

 21 Meet. 1908, p. 21] and as already indicated by their strike, a different 

 element and will, for this reason, be left out of the discussion for the present. 



t Dana clearly recognized this uplift as a geanticline. 



X Haug, ^mile. Bull. Soc. Geol. Fr. (3), xxviii, 617, 1900 ; and Traite de 

 G4ologie I, 164, 1907. 



