Elements of the Paleozoic Platform of North America. 411 



There are facts available that indicate approximately the 

 time when the symmetric arrangement of the Paleozoic plat- 

 form took place. As we have noted before, Algonkian sedi- 

 mentation took place around Lake Superior (see fig. 1), but 

 aside from this somewhat independent depression, the whole 

 platform was, according to Walcott's investigation,* above sea 

 level until Upper Cambrian time, with the exception of the 

 Appalachian geosyncline. The relation of the Upper Cam- 

 brian deposits to the Isles Wisconsin and Adirondack would 

 indicate that in this period the separation of the Paleozoic 

 eastern basin and of the inclosing arms took place and proba- 

 bly also the beginning of the breaking up of the arms. The 

 Isles Wisconsin and Adirondack, Ozarkia and Appalachia have 

 remained above the sea since the end of the Lower Silurian. 

 In Upper Silurian time the Cincinnati parma had become a 

 prominent feature, although the Cincinnati and Nashville 

 parts of the same were again separated repeatedly, as in Ham- 

 ilton and Mississippian times, by the submergence of the mid- 

 dle part. In Carboniferous (Mississippian and Pennsylvanian) 

 times all the subdivisions of the platform distinguished above 

 were fiilly developed. Since then the platform has remained 

 land, with the exception of the Mesozoic Mississippi embay- 

 ment. 



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 lat- 

 ter, a structural element of the continent, that is similar in out- 

 line to the latter. 



2. That the Paleozoic ^^latform 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 south- 

 ward from the Isles Wisconsin and Adirondack which possess 

 symmetric positions with reference to the Canadian shield. 

 Ozarkia and Appalachia, the two remaining portions of eleva- 

 tions, hold like symmetric 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 simi- 

 lar and symmetric basins, the Eastern Interior and East Cen- 

 tral basins. 



4. The disturbing factor has been the Atlantic pressure, 

 which pushed the eastern arm in and produced the Appala- 

 chian basin folds, its effect reaching as far as the Nashville 

 uplift. 



*See Walcott, U. S. Geol. Survey Bull., Ixxxi, pi. 2, 3, 1891. 



