SUBAERIAL ORIGIN OF CRETACEOUS PENP:PLA1N. 557 



leveled surface of the soft Triassic shales ; that is, in the very district where 

 the supposed Cretaceous baseleveled peneplain, as recorded on weak rocks, 

 is at sea-level, the hard rocks are also at sea-level. This, as it appears to 

 rae, gives good reason for believing that the even crest-line of this lava sheet 

 is the product of essential baseleveling, and not of a balance of resistances 

 and attacking forces at some considerable altitude above baselevel, as above 

 suggested. 



In addition to this argument, the general accordance of altitudes among 

 the diverse members of the peneplain may be quoted. In Massachusetts the 

 elevation of the upland on the crystalline portion of the old peneplain cor- 

 responds in a striking way with the height of the trap-ridges that rise above 

 the Triassic lowland ; it can hardly be thought that the altitude of balance 

 between wasting and resistance should be the same in great crystalline masses 

 and in tilted lava sheets. Moreover, the altitude of the peneplain, as indi- 

 cated either by the crystalline uplands or by the trap-ridges, increases north- 

 ward from Long Island sound. Such correspondence as this is not to be ex- 

 plained except by supposing that the whole region was once a lowland, and 

 that its present diversity of altitude is due to subsequent unequal elevation, 

 just as its present variety of form is due to still later inequality of erosion 

 on the elevated mass. 



Siiba'erial, not marine, Denudation produced the Peneplain. — The process by 

 which the Jurassic constructional topography was baseleveled may be next 

 considered. A few^ decades ago the answer to this question would pretty 

 certainly have invoked marine action, and the peneplain would have been 

 called a plain of marine denudation. Our government surveys in the west, 

 more than anything else, have called attention to the greater importance of 

 subaerial erosion, when large areas are concerned, and the pendulum of 

 opinion now swings to that side of the explanation. Something more judi- 

 cial than the general turn of opinion is needed in deciding such a problem 

 as this. Evidence of a more critical quality may be found in the character 

 of the Cretaceous deposits where they overlie the Triassic shales at Amboy 

 and elsewhere in New Jersey, and in the arrangement of certain river-courses 

 further inland. 



The lowest beds of the Cretaceous formation are so nearly free from ma- 

 terial ascribable to the underlying red shales of the Triassic formation that 

 Professor Cook was some years ago constrained to refer them to a hypothet- 

 ical crystalline area now submerged in the Atlantic. This explanation does 

 not accord well with the general origin of the Cretaceous formation else- 

 where along the Atlantic border, where it is best referred to the belt of crys- 

 talline rocks farther inland. There is no reason to think that New Jersey 

 was an exception to this rule. Hence it must be supposed that the Triassic 

 area of the peneplain was baseleveled by subaerial erosion before that sub- 



l.XXXII-BuLL. Geol. Soc. Am., Vol. 2, 1890. 



