404 jRveff(?)i.ann—Si/ni?ni'fric A nui nyement in the 



Fig. 1. 





P-C 



Al 



M-P 



C-0 



P-C, Preoambrian. Al, Algonkian. 

 C-0, Cambro-Ordovician. S, Silurian. 

 P, Pennsylvanian. 



S D M 



M-P, Metamorphosed Paleozoic. 

 D, Devonian. M, Mississippian. 



Fig. 1. Geological Sketch Map of the Eastern United States. 



MN, southern boundary of Canadian shield, A. D2 and Ej, the symmetric 

 Isles Wisconsin and Adirondack. Di and E,, the adjoining marginal depres- 

 sions of the shield. Ds-Da, guide line of western arm of platform ; E2-E3, 

 that of the eastern arm. Dj, Isle Ozark. Ea, Isle Appalachia. Bi-Bj, 

 guide line of axis of eastern basin. Bj, Cincinnati geanticline. Ba, Michi- 

 gan basin. Ba and B4, symmetric subbasins. 



