Elements of the Paleozoic Platform of JSovtli America. 407 



warping of the axis of the basin, two separate symmetric 

 basins have been formed,* one, the Eastern Interior, and the 

 other, the East Central basin. On account of the approach of 

 the Precambrian arms in the south, these basins do not extend 

 north and soutli, but extend symmetrically to northwest and 

 northeast. The Ohio river from the Pennsylvania to the West 

 Virginia line flows along the axis of the eastern basin. 



The northern portion of the Paleozoic platform that lies to 

 the north of the Cincinnati geanticline assumed the aspect of 

 a separate subcircular basin, typically indicated by the Michi- 

 gan coal field and the locations of Lake Michigan and Lake 

 Huron. It also lies symmetric to the whole arrangement and 

 with the Cincinnati uplift it is on the line of symmetry. It is 

 possible that this Michigan basin, instead of being an inde- 

 pendent depression, originated from the same warping force 

 as the Cincinnati uplift, being the result of a longitudinal 

 oscillation of the axis of the same geanticline, comparable to 

 those more intensive longitudinal oscillations of the axes, 

 which have been observed in some of the Alpine folds (see 

 Haug, Traite, p. 211). The Canadian geologists, however, 

 have claimed to find the influence of the Cincinnati uplift 

 extending from the west end of Lake Erie farther north to 

 Lake Huron. In this case it would seem that the Atlantic 

 pressure had affected the entire extent of the uplift [see p. 408] 

 giving it a direction subparallel to the Appalachian folds, and. 

 the Michigan basin would have to be considered as inde- 

 pendent of the Cincinnati uplift, a view distinctly not sup- 

 ported by the general distribution of the formations around the 

 basin. 



The development of these symmetric structures may have 

 taken place as shown in figures 1 and 2. In figure 1 the 

 Canadian shield A and its Paleozoic platform are separated by 

 the line M-N. First, then, an extensive depression affected 

 the middle portion of the platform producing the Paleozoic 

 eastern basin, fig. 2, B„, B^, and leaving two long embracing 

 arms standing, the western one D,.3 and the eastern one Ej.3. 



The following changes took place in the two arms and in 

 the inclosed basin. The arms were broken up, with the result 

 that on either side two principal isles, Isle Wisconsin [D,] and 

 Isle Ozark [Dgj, Isle Adirondack [EJ and Isle Appalachia [E,] 

 were formed. These isles are distinctly paired. On either 

 side between the Canadian shield and the first isle a depression 

 formed, the Lake Superior basin [D,] and the St. Lawrence 



* Dana [Areas of Continental Progress in North America, etc. Bull. Geol. 

 Soc. Am., i, 41, 1890J, recognizing the importance of the regions of shallow 

 seas represented by the Cincinnati uplift and the Precambrian region of 

 Missouri as regards rock- making, has distinguished these basins by the 

 terms here used. 



