288 



G. II. CIIADWICK POST-ORDOVIC'IAA' DKKORMATION 



region 



Nearly circular domes, with quaquaversal dips, are marked fea- 

 tures of the good exposures along the Grass and Eaquette rivers, occa- 

 sioning several inliers, while bowl-shaped synclines containing outliers 

 also occur. Though the dips are all low, seldom over five degrees, they 

 reverse frequently or the strike veers rapidly in all the large outcrops. 

 The general resultant is the very zigzag trace for the formation bound- 

 aries, as shown on the accompanying map, figure 2.^ 



No such zigzags are sho^\ai on the geologic maps of the State, as may 

 be seen at once on comparison with the last State map of 1001. Their 



Figure 1. — Map showing Location of Canton Quadrangle and Belts of Formalions 

 adjacent thereto (after Logan and Merrill) 



recognition has become possible through the refinement of the strati- 

 graphic units in this region inaugurated by Gushing and Ulrich. In 

 place of the old divisions, "Potsdam" and "Calciferous," we must nuw 

 recognize the following formations in descending order : 



* See Gushing, op. cit., p. 113. 



" A word of explanation is in place here. The Ogdensburg quadrangle is being pre- 

 pared by Professor Gushing, wliose manuscript map has been liindly transmitted to the 

 writer ; but the generalized boundaries drawn in figure 2 had been previously woriced 

 out by the latter in a crude way. Since they form a part of the evidence material to 

 this paper, they are here used with apologies to Professor Cushing, \^■hose map shows 

 the same essential fact of the larger zigzags. 



A similar explanation and apology is due to Dr. J. G. Martiu. who has mapped the 

 pre-Gambrian rocks for the Ganton sheet. 



