PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 561 



or the greater part of each of the Wisconsin Niagaran formations was 

 deposited when the Gulf waters were excluded from areas in Tennessee 

 and elsewhere in which their Silurian deposits are now accessible; (3) 

 that the Lockport dolomite of New York is wholly of the age of the 

 Guelph and represents an eastward transgression of this Arctic sea from 

 Wisconsin comparable to the much earlier Prosser (early Trenton) 

 transgression described on page 369 ; (4) that the hiatus between the 

 Eochester and Lockport in New York represents the Waukesha and 

 Eacine formations, and probably also the Ma^^ville, in Wisconsin, and 

 the formations in Tennessee and Indiana beginning with the Laurel and 

 ending with the Beech Eiver; and (5) that with the possible exception 

 of the Mayville, which may be late Clinton, and the Iron Eidge ore bed, 

 which is probably of the age of the Brassfield, the Clinton group is not 

 represented in Wisconsin. Except those covered by these conclusions 

 and those placed in the Cayugan, the positions assigned in the following 

 table of Silurian formations in America generally referred to the Niaga- 

 ran are entirely hypothetical and provisional. 



It is to be noted that the formations making up the sequence are 

 arranged in three columns, the first giving the southern formations as 

 developed in western Tennessee, north Arkansas, and Oklahoma, the 

 second those in the Appalachian and Allegheny basins having Atlantic 

 connections and third, those in Wisconsin which are regarded as originat- 

 ing in the north. 



(18) Correlation based on inferred rhythmic shifting of the area of 

 maximum deposition in successive ages. — It is an interesting and sig- 

 nificant fact that the location of the area in which occurred the maxi- 

 mum deposition referred to a given period, epoch or age, or to which 

 the deposits of the several ages are confined, varied from time to time. 

 The shifting of the area, especially in the early periods, seems to be in 

 accordance with some definite plan of oscillation and structural defor- 

 mation. The reconstruction of this plan is of course no easy matter, 

 and no determined effort to do so will be made at the present time. Our 

 immediate concern relates to the fact that the presence or absence and 

 the areal extent of many and perhaps most of the stratigraphic units 

 recognized in the Appalachian Valley is largely determined by the alti- 

 tude or degree of development of certain permanent lines of weakness, 

 and of consequent positive tendencies, that run parallel with the general 

 strike of the valley and of others that cross the strike. The location of 

 these anticlinal axes along which stratigraphic overlaps commonly occur 

 is shown in a generalized and imperfect manner on figure 1, page 293. 



