SIXTH REPORT OF THE DIRECTOR I909 149 



remained above the sea since the end of the Lower Siluric. In 

 Upper Siluric time the Cincinnati parma had become a promi- 

 nent feature, although the Cincinnati and Nashville parts of the 

 same were again separated repeatedly, as in Hamilton and Mis- 

 sissippian times, by the submergence of the middle part. In 

 Carbonic (Mississippian and Pennsylvanian) times all the sub- 

 divisions of the platform distinguished above, were fully developed. 

 Since then the platform has remained land, with the exception of 

 the Mesozoic Mississippi embayment. 



Summary 



The writer endeavors to point out : 



1 That the Paleozoic platform of North America extending 

 south from the Canadian shield, forms, together with the latter, 

 a structural element of the continent, that is similar in outline 

 to the latter. 



2 That the Paleozoic platform exhibits a symmetric arrange- 

 ment of its parts. This symmetric arrangement consists in the 

 presence of a median basin (Paleozoic eastern basin) that is 

 flanked on both sides by broad elevations, extending southward 

 from the Isles Wisconsin and Adirondack which possess symmetric 

 positions with reference to the Canadian shield. Ozarkia and Ap- 

 palachia, the two remaining portions of elevations, hold like sym- 

 metric positions. 



3 The axial line of the Paleozoic eastern basin is occupied by 

 the Cincinnati-Nashville parma and the Michigan subbasin. 

 The former divides the Paleozoic eastern basin into two similar 

 and symmetric basins, the Eastern Interior and East Central 

 basins. 



4 The disturbing factor has been the Atlantic pressure, which 

 pushed the eastern arm in and produced the Appalachian basin 

 folds, its effect reaching as far as the Nashville uplift. 



