142 NEW YORK STATE MUSEUM 



In comparing the sketch map, chart I with the diagram 

 [chart III] 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 Precambric areas, the Isle Wisconsin [D 2 ] and the 

 Isle Adirondack [E 2 ] in quite symmetric positions. Each has 

 its extension connecting it with the protaxis in symmetric posi- 

 tion, that of the Isle Wisconsin being directed northeast (partly 

 submerged by Lake Superior), that of the Isle Adirondack north- 

 west. 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 the Appalachian 

 folding and probably much obscured by extensive overthrusting 

 from the southeast along " Logan's line." The effect of Appa- 

 lachian folding by crushing in one side of the symmetric structure 

 here set forth, will be discussed more fully in another chapter 

 [see p. 145.] 



From each of these cornerstones there extends southward like 

 an arm, a broad belt of Precambric and early Paleozoic rocks, 

 nearly the full length of the continent. The western arm can be 

 traced by the great southward extension of the Precarbonic 

 rocks of Isle Wisconsin to near the neighborhood of Burlington, 

 the Siluro-Devonic inlier along the Mississippi above its junction 

 with the Missouri and the large Precambric-Cambro-Siluric 

 inlier or uplift of the Ozarks in Missouri and Arkansas 1 [D 3 ]. 

 Its " Leitlinie " is shown in red overprint in the line passing from 

 D 2 through D 3 . The eastern arm [E 2 -E 3 ] has been badly over- 

 ridden, broken up and forced inward by the tangential pressure 

 that has produced the Appalachian folds. It is, nevertheless, 

 still easily recognized in the belt of Precambric and Precarbonic 

 rocks, extending south and southwestward from New York as 

 far as Alabama. 



The two arms have later been somewhat disturbed and ob- 

 scured, especially the western one, by the breaking down of 

 certain portions south of Isle Wisconsin, where the Carbonic 



1 The Ouachita mountains in Arkansas probably represent, according 

 to Dr Ulrich's description Tin Preliminary List of Papers, Am. Geol. 

 Soc. 21 Meet. 1908, p. 21] and as already indicated by their strike, a 

 different element and will, for this reason, be left out of the discussion 

 for the present. 



