106 J. Barrell — Upjyer Devonian Delta of the 



In accordance with this view he draws the Upper Devonian 

 shore line through Pennsylvania within from one to five miles 

 of the present limiting outcrops,* following them so closely 

 that a peninsula of land five miles wide is shown extending 

 into his Upper Devonian sea in that region now highly folded 

 and lying between the Susquehanna and Juniata Rivers. Yet 

 the outcrops running so closely parallel and adjacent through 

 Pennsylvania to these postulated shore lines are from 6000 to 

 9000 feet in thickness. 



The truth or error of this principle proposed by "Willis for 

 the determination of ancient shore lines, — or better stated, for 

 the determination of the original margins of sedimentary for- 

 mations, since they need not be necessarily shores, — is of crit- 

 ical importance in paleogeography in general and in the 

 interpretation of the Upper Devonian of this region in partic- 

 ular. It therefore requires detailed discussion. 



First must be considered, the validity of the supposed rela- 

 tion of initial dips to the resulting folded structures. On the 

 map (fig. 1, p. 89) it is seen that the anthracite synclinorium 

 corresponds fairly well to the region of greatest thickness of 

 the Upper Devonian. It may be granted that such great 

 lenses of sediment, by producing a downwarp in a deeper com- 

 petent stratum, predispose the structure to form a synclino- 

 rium, and the formations below and above emphasize that 

 tendency. The north-south anticlinorium between Long. 77° 

 and 78° does not show, however, such a predetermination, and 

 in regard to the individual folds it would seem that the com- 

 petence of the beds and the necessities of mutual adjustment 

 between folds, not to speak of possible deeper forces acting 

 from below, must completely overshadow slight local variations 

 in initial dip. This skepticism regarding such supposed detailed 

 relations is not controverted by any data thus far published. 



If an examination of Willis' original article be made, it will 

 be seen that he repeatedly notes a somewhat indeterminate 

 nature of the field data. Furthermore, his discussion starts 

 with the assumption that the shore of Appalachia was near at 

 hand.+ In regard to both Massanntten and Bays Mountains all 

 that the data show is a thinning of the strata to the west, not 

 a maximum thickness under the synclines. But postulating a 

 shore a few miles east gives, of course, the necessary initial dip. 

 This reasoning in a circle was not obvious to such a keen thinker 

 as Willis because everyone held that the shore was nearby, and 

 that which is the general dictum tends to be viewed in time as 

 a proven fact. As to the data used in Pennsylvania, it does 

 not seem that here either is demonstrated a real relationship 



* Paleozoic Appalachia, Maryland Geol. Surv., vol. iv, PI. VII, 1900. 

 f Loc. cit.. p. 255. 



