692 ABSTRACTS OF PAPERS 



still show traces of marine cliffs. So far as the speaker has observed, this is 

 not the case. Water- worn pebbles ou the upland surface may be of stream 

 origin as well as of marine origin, and hence special care is necessary to elimi- 

 nate the possibility of stream origin before such gravels are accepted as a 

 confirmation of the theory of marine planation. The speaker would also like 

 to know whether Professor Barrell found the terrace levels to be horizontal 

 over wide areas. 



Prof. W. M. Davis : The introduction of a new view by a critical investigator, 

 who thus traverses an earlier view that had gained general acceptance, is 

 likely to lead us nearer to the correct interpretation of natural phenomena, 

 particularly if the investigator himself had previously accepted the earlier 

 view which he is now obliged to give up ; and, as in the present case, the new 

 view is based on facts not previously recognized and, indeed, in the earlier 

 lack of topographic maps hardly recognizable. Hence on general grounds Pro- 

 fessor Barren's new interpretation of the uplands of southern New England 

 seems more likely to be correct than my own earlier one. 



As an additional test for the occurrence of a series of ancient sea cliffs in 

 these uplands, let me suggest a search for "cliffs of decreasing height," par- 

 ticularly along the latest-made (lowest-standing) shorelines; for if a sea cliff 

 is cut back over a distance several times greater than the diameter of the hills 

 into which the background is dissected, and if the shoreline is many times 

 longer than a hill diameter, a certain number of the hills must be transected 

 by the retreating shoreline on the inland side of their summits, so that the 

 cliff height must decrease with further recession. The absence of such cliffs — 

 indeed, the prevailing absence of any cliffs — along the inner hilly border of the 

 even uplands of Devonshire-Cornwall and of Brittany has led me to regard 

 these uplands as uplifted peneplains of normal erosion, in spite of their near- 

 ness to the sea, and in spite of the occurrence on them of gravel patches with 

 marine fossils. These are explainable by a brief submergence of the peneplain 

 after it was degraded and before it was uplifted and dissected. On the other 

 hand, the occurrence of cliffs, and especially of cliffs of decreasing height, along 

 the inner border of an even upland would go far toward proving its marine 

 origin. However, if the hills of the background were elongated ridges, cliffs 

 of decreasing height would be rare or absent. The long and narrow ridges, 

 which the inclined trap sheets of the Triassic area would soon form in an 

 uplifted platform of abrasion, would be reduced to a lower level by sea action 

 south of a shoreline, while retaining their former level little reduced north of 

 the shoreline. As there are many shorelines and many trap ridges, it would 

 seem probable that some of the latter should be transected by some of the 

 former, and at the point of transection a sudden increase of height should 

 occur independent of the fault-lines which traverse the trap ridges. 



As to the inland reach of a primitive Chesapeake Bay far northward in 

 Pennsylvania, the transverse course of the Susquehanna, where it escapes from 

 the syncline of the Wyoming Valley, ought not, in my opinion, to be regarded 

 as an example of superposition, for a river must somehow escape from a syn- 

 clinal valley by a transverse course, whether it is superposed or not. Like- 

 wise, the longitudinal course of a stream, pointed out by Professor Barrell 

 along the northern side of the west end of the Wyoming syncline. is as easily 

 explained by adjustment to structure as by indirect effect of marine action. 



