MEDIAL OR NUCLEAR AREA OF NORTH AMERICA 165 



it is also known as the neMtral area. Kober has recently called these parts 

 •of the continents "Kratogens" (from Jcratos, strength, and genos, produc- 

 ing). Suess has called these neutral areas forelands, and it is against 

 them that the geosynclines have been pushed and folded. 



~ The Canadian shield occupies the greater northern part of the neutral 

 region, while in the United States the Paleozoic seas more often covered 

 its southern extension west of the Cincinnati geanticline and east of 

 .Siouia, which was less often warped beneath the ocean level (see figure 3). 

 In the late Paleozoic, neutral Siouia, however, became mobile again and 

 the site of the Ancestral Eocky Mountains geanticline (see page 186). 

 The ancient land Columbia, or greater Mexico, may have been another 

 ishield, and during most of the Paleozoic it appears to have been a low- 

 land, furnishing but little sediment to the adjacent seas. 



It is most often around, and rarely across, the nuclear region that the 

 inland or epeiric seas have flowed. Because this region has not under- 

 gone orogeny since the Proterozoic, its Paleozoic and later strata remain 

 nearly horizontal, though in places blocks have been faulted or warped 

 into the old masses. Accordingly, all of the flat-lying sedimentary for- 

 :mations around and upon the nucleus are included under the term "neu- 

 tral portion of the continent" — neutral in relation to the average levels 

 ■of the oceans because unaffected by mountain-making forces. 



SWELLS 



Undoubtedly there are many domed areas within the neutral or nuclear 

 ;areas of JSTorth America, and especially within the eastern United States. 

 In Middle Ordovician time the Cincinnati geanticline began in two 

 .swells, the Nashville and Cincinnati domes. In early Middle Silurian 

 time these became confluent into a single arch having the general trend 

 of the Appalachians. The Ozark dome is one of the greatest and most 

 persistently rising of these swells, and northern Wisconsin is another but 

 less actively rising one. The Mississippian seas have repeatedly flowed 

 between these swells and at times have completely gone over them. On 

 the other hand, the Sioux Falls region of southeastern South Dakota and 

 the Baraboo range of Wisconsin are persistent Huronian quartzite ridges, 

 remnants of the Killarney Mountains that rose in late Proterozoic time. 

 The buried granite ridge (Nemaha Mountains) also appears to be of tliis 

 orogenic entity. 



