3b PROCEEDINGS OF THE AXX ARBOR MEETING 



RESTORATIOX OF THE FORMER LIMITS OF GREATLY ERODED FORMATIOXS 



There has been a tendency among paleogeographers to draw the orig- 

 inal boiindaries of geologic formations not far from the present limiting- 

 outcrops. Because of the existence of detached outliers and because of 

 the evidence of the superposition of drainage lines, as Davis showed to 

 be the case in Xew Jersey, the original boundaries of some formations 

 have been indicated as lying some distance from the present outcrops of 

 the main bodies of the formations. Barrell in his paper, ''The Upper 

 Devonian delta of the Appalachian geosyncline/*' presented additional 

 criteria for ascertaining the approximate limits of formations that have 

 undergone great erosion. One section of this article is devoted to show- 

 ing that, except on the borders of fault troughs, there is no necessary 

 structural relation between the present and the original limits of forma- 

 tions, therein taking issue with Willis's opinion on initial dips and the 

 original limits of formations. The additional criteria adduced by Barrell 

 are derived from the character of the sediments of the marginal outcrops 

 and the rate of thinning at the present margins. If continental clastic 

 sediments show by the rounding of pebbles and by gradual reduction in 

 size of pebbles away from the source of the constituents, thereby indicat- 

 ing transportation for long distances, it is obvious that the original mar- 

 gin of the formation lay nearer the source of supply of the sediments than 

 does that part of it still preserved. Deductions from rate of thinning of 

 sediments as to the position of the original boundary rest upon the as- 

 sumption that "the thinning shown near the i)resent margin continued 

 uniformly to original limits — an assumption which certainly is not ex- 

 actly true, but which on the other hand serves as an approximation." 



THE SIGXIFICAyCE OF SOME SPEC Li L FEATfRES OF SEDIMEXTS 



Barrell repeatedly emphasized that really sound conclusions on the 

 origin and conditions of deposition of sediments shouhl result not from 

 a single line of evidence, but from the convergence of several independent 

 lines. However, a few special features of importance in interpreting 

 continental deposits will be mentioned. 



The most favorable conditions for the formation and preservation of 

 mud-cracks are undoubtedly floodplains that are subjected to periodic 

 overflow and on which are made deposits of non-calcareous clav alternat- 

 ing with somewhat coarser material. Although Barrell gave his first 

 comprehensive discussion of the significance of mud-cracks in 1806,^* 

 the subject is brought up again and again in his later papers. He says: 



" Relative geological importance of continental, littoral, and marine sedimentation. 

 Jour. Geol., vol. 14. 1906. pp. 524-568. 



