XIII. ] GEOGNOSY OF THE APPALACHIANS. 263 



than one horizon over the continental area, which, as the result 

 of widely spread movements, might be supposed to be repre- 

 sented in the Appalachian region. In the latter Logan has 

 already observed that ' the absence of all but the highest beds 

 of the Levis along the eastern limit of the Potsdam, near 

 Swanton, Vermont (while the whole thickness of them ap- 

 pears a little farther westward), makes it probable that there 

 is a want of conformity between the two ; and I have in 

 this connection insisted upon the entire absence, in this 

 locality, of the Calciferous, which is met with a little farther 

 south in the section just mentioned, as another evidence of 

 the same unconformity.* There are also, I think, reasons 

 for suspecting another stratigraphical break at the summit 

 of the Quebec group,t in which case many problems in 

 the geological structure of this region will be much sim- 

 plified. 



It should be remembered that the conditions of deposition 

 in some areas have been such that accumulations of strata, cor- 

 responding to long geologic periods, and elsewhere marked by 

 stratigraphical breaks, are arranged in conformable superposi- 

 tion ; and moreover that movements of elevation and depres- 

 sion have even caused great paleontological breaks, which over 

 considerable areas are not marked by any apparent discordance. 

 Thus the remarkable break in the fauna between the Calcifer- 

 ous and the Chazy is not accompanied by any noticeable dis- 

 cordance in the Ottawa basin ; and in Nebraska, according to 

 Hayden, the Potsdam, Carboniferous, Jurassic, and Cretaceous 

 formations are all represented in about 1,200 feet of conforma- 

 ble strata. J In Sweden the whole series from the base of the 

 Cambrian to the summit of the Silurian appears as a conform- 

 able sequence, while in North Wales, although there is no ap- 

 parent discordance from the base of the Cambrian to the sum- 

 mit of the Lingula flags, stratigraphical breaks, according to 

 Ramsay, probably occur both at the base and the summit 



* American Journal of Science (2), XL VI. 225. 



t See, for the evidence of this, Essay XV., Part Third. 



J American Journal of Science (2), XXV. 440. 



