408 JRuedetnan?) — Symvieti'ic An^angement in the 



basin [E,], and other depressions between the first and second 

 isles. 



In tlie Paleozoic eastern basin a broad low anticline, the 

 Cincinnati-Nashville panna [B,], arose*; exactly in the axial 

 line of the depression and in coiitiiniation of this line of eleva- 

 tion the northern part of the l)asin sank down into an axial 

 basin, the ]\Iichi<j;an snbbasin [BJ. On either side of the 

 panna, between the latter and the Precambrian arms, two 

 basins, the East Central [B,] and the Eastern Interior basin 

 [BJ, were formed, in such an arrangement that they converge 

 southward and are exactly symmetrical to the axial line of the 

 eastern basin. 



The stress from the sub- Atlantic pressure, the cause of the 

 Appalachian folding and overthrusting crushed or crumbled 

 this symmetric structure from the southeast, its influence being 

 felt in the whole eastern portion of the area. Its effect is 

 seen by a comparison of figures 1 and 2. Following Claypole's 

 earlier estimates, Willis (Bull. Geol. Soc. Am., xviii, 404, 1907) 

 remarks that " it is a moderate statement to say that during 

 the Appalachian revolution that portion of the continent 

 southeast of the Cumberland Plateau rim moved northwest- 

 ward at least 50 miles." 



On account of its oblique direction to the north-south axis 

 of the Paleozoic eastern basin, the stress reaches deepest into 

 the latter in the south, where it has clearly turned the Nash- 

 ville portion of the Cincinnati-Nashville parma aside. It 

 further lengthened the Eastern Interior basin, and extended it 

 into southeastern New York, or considerably farther north 

 than the opposite East Central basin. Moreover, it may have 

 produced secondary depressions east of the Michigan basin, 

 which have finally found expression in Lakes Erie and 

 Ontario. The principal facts suggesting the latter view are 

 the general parallelism of these lake basins with that portion 

 of the Appalachian folds southeast of themf whence the push 

 came. It should, however, in this connection be taken into 

 account that it can have been but the last stages of Appa- 

 lachian folding that produced the gentle down-warping of 

 these basins, since the outwardly convex strike of the earlier 

 Paleozoic formations (best seen at the west end of Lake 

 Ontario) shows that this was an elevated region until at least 

 Devonian time. It is, therefore, quite possible that these 

 depressions are the counterparts of the late (early Tertiary) 



* Suess has termed stieli broad warpings " parmas." 



\ By drawing a straight line connecting the folds from the Tennessee- 

 Virginia line to the Pennsylvania- New Jersey line, one obtains a line that 

 indicates the general direction of this portion of the folds, and that line is 

 parallel to the two lake basins. 



