Appalachian Geosyncline. 105 



resistant areas is the product in very large part of Cretaceous 

 erosion. The fair degree of preservation of that plain upon 

 the resistant rocks and the present broadly mountainous character 

 of the Appalachians are due largely to the comparative recency 

 of the last strong upward movements, which appear to date 

 from the close of the Miocene. The previous moderate uplifts 

 of the early Tertiary had not given the rivers sufficiently steep 

 grades to attack vigorously the regions of resistant rocks, 

 especially those situated near their headwaters. 



We look in vain in the Appalachians for the mountains 

 made by the original folding, ft was once thought that their 

 time-worn slopes rose still from the present valley floors, even 

 where the folding dated from the close of the Ordovician. 

 Then it was seen that most of the Appalachian mountains were 

 the remnants of a dissected plateau which had in the late 

 Mesozoicbeen a plain of erosion lying near the level of the sea. 

 The mountains existing from the Appalachian revolution were 

 then regarded as those higher masses rising in isolated groups 

 above the plateau. But with further searching the mountains 

 of even Permian folding are found to recede continually like 

 the proverbial foot of the rainbow. 



Absence of Structural Relations between Present and 

 Original Limits. 



As discussed under the subject of erosion cycles, the ten- 

 dency in the past has been to draw the original boundaries of 

 formations not far beyond the present limiting outcrops, even 

 where such a conclusion is not suggested by the nature of the 

 strata ; the mind is dominated by that which remains in evi- 

 dence ; that which is eroded is no longer visible to testify to 

 its former extent. In the case of folded structures now eroded 

 Willis, as an outgrowth of his work on initial dips and their 

 supposed relation to folds, has crystallized this relationship of 

 outcrop to original limits more definitely and strictly than have 

 other geologists. Under the subject of initial dips, he argues 

 that a line of downwarping during sedimentation determines 

 the axis of a syncline during future compression ; an upwarp- 

 ing, the axis of an anticline.* In applying this to the problem 

 of ancient shore lines he states : 



" The decided dip from the shore seaward is in a position to be 

 sharply upturned ; it may become vertical or overturned. In 

 this position it may become eroded to great depth without much 

 change in the position of its outcrop, which remains marking 

 approximately the ancient shore line." f 



*U. S. Geol. Surv., 13th Ann. Keport, Pt. II. pp. 253-263, 1893. 

 f Relations ot" synclines of deposition to ancient shore lines, Am. Geol., 

 xiii, p. 141, 1894. 



