55G W. M. DAVIS DATES OF TOrOGRAPHIC FORMS. 



land thiui that of to-clay. It is suggested that the evenness of the crest may 

 be the result of balanced forces of attack and resistance, and that after such 

 a balance has been attained further degradation would not produce irreg- 

 ularity, but would maintain evenness of outline. I,t appears to me that the 

 general principle here indicated is correct. After a certain amount of degra- 

 dation of any structure, variety of relief weakens and disappears, leaving 

 simplicity in its place ; but I do UQt think that this stage can be reached in 

 ridges that stand so high above the present baselevel as do those of the 

 Watchung lava sheets. However even their structure, however regular 

 the constructional topography following on their regular uplift, however 

 systematic the attack made upon them by the forces of denudation, their 

 topographic form must increase in variety for a certain share of the time 

 necessary for their complete baseleveling, and thus a maximum variety of 

 relief would be attained. After this, further denudation would reduce the 

 variety of relief until it disappeared in the featureless surface of the base- 

 leveled peneplain. But the most careful analysis that I can make of this 

 process leads me to think that ridges like those of the Triassic lava sheets 

 cannot be produced while their crest-lines are five or six hundred feet above 

 baselevel ; they will be lower than they are when they reach the stage of 

 even crest-lines in their present attitude with respect to baselevel. 



Evidence from the Palisades of the Hudson. — There is, however, in the 

 Palisades an example that does not leave this conclusion entirely to deduct- 

 ive argument. The Palisades are formed on the edge of an intrusive sheet 

 of lava, which may be traced from Haverstraw, on the Hudson, down the 

 river past Hoboken and Jersey City, opposite New York, and a little further 

 till it descends under the Cretaceous beds in the neighborhood of Amboy. 

 Some twenty miles farther southwestward, and apparently in continuation 

 of the Palisade curve, a perfectly similar intrusive lava sheet rises from 

 below the Cretaceous cover and forms the ridge of Rocky hill, back of 

 Princeton. The variation iu the height of the crest-line of this long, partly 

 buried ridge is considerable ; near its northern end it rises 700 or 900 feet 

 above tide; near New Brunswick or Amboy, where the Raritan crosses the 

 buried portion of its course, it is probably 100 feet below tide-water ; back 

 of Princeton it reaches an elevation of 400 feet. Now, if we are to suppose 

 that the present form of northern New Jersey has been produced by erosion 

 of a formerly greater mass, while it stood at its present attitude with respect 

 to baselevel, how can the variation iu the level of the Palisade-Rocky hill 

 ridge be accounted for? So far as its crest is visible, it is relatively even ; 

 but at what altitude above baselevel was this evenness attained? No suffi- 

 cient explanation for its form can be found in denudation without subse- 

 quent distortion. In the Amboy district, where the Palisade ridge sinks 

 below sea-level, we find the Cretaceous clays lying on the denuded, l)ase- 



