198 National GeograjpTiic Magazine. 



also. Finally it may be fairly urged that it is more accordant 

 with what is known about old mountains in general to suppose 

 that their mass has stood at different attitudes with respect to 

 base level during their long period of denudation than to suppose 

 that they have held one attitude through all the time since their 

 deformation. 



It is natural enough that the former maintenance of some lower 

 altitude than the present should have expression in the form of 

 the country, if not now extinguished by subsequent erosion. It 

 is simply the revei'se of this statement that leads us to the above- 

 stated conclusion. We may be sure that the long maintained 

 period of relative quiet was of great importance in allowing time 

 for the mature adjustment of the rivers of the region, and hence 

 due account must be taken of it in a later section. I say relative 

 quiet, for there were certainly subordinate oscillations of greater 

 or less value ; McGee has detected records of one of these about 

 the beginning of Cretaceous time, but its effects are not now 

 known to be of geographic value ; that is, they do not now mani- 

 fest themselves in the form of the present surface of the land, 

 but only in the manner of deposition and ancient erosion of cer- 

 tain deposits.* Another subordinate oscillation in the sense of a 

 moderate depression seems to have extended through middle 

 and later Cretaceous time, resulting in an inland transgression of 

 the sea and the deposit of the Cretaceous formation unconform- 

 ably on the previous land surface for a considerable distance be- 

 yond the present margin of the formation. f This is important 

 as affecting our rivers. Although these oscillations were of con- 

 siderable geological value, I do not think that for the present 

 purposes they call for any primary division of the Jura-Creta- 

 ceous cycle ; for as the result of this long period of denudation 

 we find but a single record in the great lowland of erosion above 

 described, a record of prime importance in the geographic devel- 

 opment of our region, that will often be referred to. The surface 

 of faint relief then completed may be called the Cretaceous base- 

 level lowland. It may be pictured as a low, undulating plain of 

 wide extent, with a portion of its Atlantic margin submerged 

 and covered over with a relatively thin mai'ine deposit of sands, 

 marls and clays. 



*Anier. Jour. Science, xxxv, 1888, 367, 448. 



f This statement is based on a study of the geographic evolution of 

 nortliern New Jersey, in preparation for publication. 



