Ruedemann — Paleozoic Platform of North America. 403 



Art. XLV. — On the Symmetric Arrangement in, the Ele- 

 ments of the Paleozoic Platform of North America ^" by 

 Rudolf Rdedemann. 



We wieli to present certain facts indicatina; that the struc- 

 tural development of eastern North America lias taken place 

 in sacli a fashion that a notable symmetric arrangement of its 

 elements has resulted. 



This arrangement becomes especially distinct when the large 

 area of Paleozoic rocks extending from the Canadian protaxis 

 southward is considered by itself. This area, which is roughly 

 bounded on the west by a line connecting the head of Lake 

 Superior with the Ozarks and on the east by a line inclosing 

 the Adirondacks and Appalachia, we may for convenience 

 term the Paleozoic platform of North America. It corre- 

 sponds in its relation to the Canadian shield with that of the 

 " Russian platform " of the European geologists to the Baltic 

 shield. A glance at the geologic map of North America will 

 show that this platform is a direct southward continuation of 

 the Canadian shield or protaxis and bounded by southward 

 converging lines that are direct continuations of the boundaries 

 of that shield, f as described by Suess and Willis. In the west 

 the platform, like the Canadian shield, is separated from the 

 Rocky mountain area by the north-south transcontinental 

 depression that extends from the Gulf of Mexico to the mouth 

 of the Mackenzie river and is occupied by Cretaceous and 

 Tertiary rocks. 



In comparing the sketch map (fig. 1) with the diagram (fig. 

 2) in which separate shading brings out the elevated and 

 depressed regions, it is seen that on either side of the Cana- 

 dian shield or protaxis [A], there stand out, like cornerstones, 

 two separate Precambrian areas, the Isle Wisconsin [DJ and 

 the Isle Adirondack [EJ in quite symmetric positions. Each 

 has its extension connecting it with the protaxis in symmetric 

 position, that of the Isle Wisconsin being directed northeast 

 (partly submerged by Lake Superior), that of the Isle Adiron- 

 dack northwest. From each of these extensions there runs 

 outward, along the margin of the shield, a deep depression, 

 the Lake Superior basin [DJ and the St. Lawrence basin [EJ. 

 The latter is less distinct through the disturbing influence of 



* Condensed from N. Y. State Museum Bulletin 140, pp. 141-149, 1910. 



•)• The Mesozoic and Cenozoic embayment of the Mississippi Valley is, in 

 this discussion, left out of consideration, because of younger age ; likewise 

 the belt of Carboniferous rocks to the west and southwest of the Ozarks, 

 that forms the outer slope of the western arm of the platform, roughly cor- 

 responding to the area of metamorphic rocks on the opposite slope of the 

 other arm, and properly belonging to the transcontinental depression. 



