INLAND EXTENSION OF THE CRETACEOUS. 559 



upoo the unconformable structures discovered below ; but in that case they 

 could not exhibit a distinct accordance with the structures. The inland ex- 

 tension of the Cretaceous ocean should therefore be determined by the area 

 over which the streams of to-day exhibit an indifference to the rock structure 

 over which they flow ; and further inland than this they should be led by 

 the soft rocks. 



McGee* has shown that this is the case in southeastern Pennsylvania, 

 where the Cretaceous sea crept inland a moderate distance over the Phila- 

 delphia belt of schists; but it is not necessary to suppose that all of even 

 this slight advance was gained by its own shore-cutting ; it may have crept 

 in over a nearly baseleveled and slightly submerged area. I have found 

 the same relation in northern New Jersey ,t where streams of both super- 

 imposed and adjusted courses may be recognized, the former over so narrow 

 a strip of the crystalline plateau that they may be neglected in speaking of 

 the Cretaceous peneplain as a whole. This great topographic area must 

 therefore be regarded as essentially a product of subaerial denudation, and 

 the date assigned to it in New Jersey may be accepted for the whole area, 

 until evidence to the contrary is presented. 



Extension of the Cretaceous Peneplain ivest and southwest of New Jersey. — 

 Having traced the peneplain from New England into New Jersey, having 

 there determined the date of its completion and the processes concerned in 

 its production, we may again turn to searching for its further extension. 



Going now to the northwestern side of the New Jersey highlands, and 

 looking across the Kittatinny or great Appalachian valley from such a com- 

 manding point of view as Jenny Jump mountain (see figure 3), the long, 

 even crest-line of Blue or Kittatinny mountain rises in the distance. It 

 looks like a dark blue wall rising to an even height against the light blue of 

 the sky. At the Delaware water gap the wall is cut through to the base ; 

 at the wind gap, a little farther westward, it is cut part Avay down ; else- 

 where, so far as the sight can follow it, it is surprisingly level. Thus it con- 

 tinues for hundreds of miles. Here and there it is trenched by a river that 

 comes from the back-country lowlands, or is half cut by a wind gap; occa- 

 sionally it twists or turns where a fold or wrinkle is passed ; at the synclinal 

 points of these turns a somewhat greater height is maintained than elsewhere, 

 while where the dip steepens the height falls away by a small amount; yet, as 

 a whole, the crest-line is extraordinarily level. The same may be said of the 

 other ridges of Medina sandstone, of which so many are to be found in the 

 Susquehanna drainage area of central Pennsylvania. Whatever their struc- 

 ture, however high their folds would carry them, they are all truncated in 

 the most obedient manner when they rise to the level of the Cretaceous pene- 



* Three formations of the Atlantic Slope: Amer. Tour. Sci., 3d. ser., vol. XXXV, 1888, pp. 133, 134. 

 t The Rivers of Northern New Jersey : Nat. Geogr. Mag., vol. II, 1890, p. 81. 



