TIMES OF rORMATION OP THE PENEPLAINS 581 



Some have gone so far as to work the rule the other way, and instead 

 of dating a peneplain by deposits of known age date a deposit by a pene- 

 plain on which it is believed to rest. Fuller and Clapp,^^ for example, 

 map and describe a formation as Eocene because they believe that the 

 Tertiary peneplain was completed at about that time and that the deposit 

 was laid down on the old plain. , 



The general implicit faith in the statements as to the age of the Ap- 

 palachian peneplain is further exemplified by the dating of peneplains 

 thousands of miles away by comparison with the Appalachian peneplains 

 of supposedly known age. In his Forty-ninth Parallel report Daly^^ says : 



"The evidences against the hypothesis of a mid-Tertiary peneplain on the 

 Front ranges seem to be powerful. First, the time allowed is not sufficient 

 for peneplanation or even past-mature development, followed by uplift and 

 mature dissection in a second cycle. All post-Cretaceous time has not been 

 enough to destroy the large monadnocks on the well established Cretaceous 

 peneplain of the Appalachians, though their rocks are not sensibly stronger 

 than those of the Front ranges of the Cordillera." 



Evidence as to Age of Peneplains 



general statement 



The best evidence as to the age of the Appalachian peneplains seems 

 to the writer to lie in the results of correlation with unconformities and 

 deposits in the Coastal Plain and in knowledge concerning the rate and 

 amount of erosion. The correlation rests on continuous tracing of pene- 

 plains and coincidence of their projected planes and on the principle that 

 deposits laid down near the close of an erosion cycle arc more scant and 

 finer in grain than those connected with an earlier part. The data as to 

 rate and amount of erosion consist of (1) the present rate under various 

 conditions and an estimate of the length of certain periods of geologic 

 time, (2) the amount of material which has certainly been removed from 

 some areas, and (3) the quantity of the deposits which have been derived 

 from the province in various periods and epochs. 



CORRELATION WITH BURIED PENEPLAINS 



Statements concerning the local altitude and slope of peneplains in 

 various parts of tlic Appalachian province arc much more, abundant than 

 those concerning the supi)osed l)uricd correlatives. It is often ]'cmarked 

 concerning a peneplain being described that it lias such and sucli alti- 



21 M. L. Fuller aud F. G. Clapi) : U. S. Geol. SuvvVy, Patoka Fofio, No. 105. 

 " U. A. Daly : Norlh Americau Cordillera at the Forty-ninth Parallel, pt. ii, Memoir 

 No. 38, Canada Department of Mines, 1912, p. G08. 



