272 N. S. SHALER SPACING OF RIVERS 



For this iiKiuiry the most itn[)ortant contrast in the fields above men- 

 tioned is in the measure of their degradation, their hight above the sea, 

 and the form of their reliefs. Considered in a general way, it is evident 

 that the Green Mountain district, embodying the Berkshire hills and the 

 AUeghanies, may be, as regards their altitude, not unreasonably grouped 

 together, their bights being in the whole similar, the peaks which sur- 

 pass a;n altitude of about 2,000 feet being rather exceptional, and perhaps 

 explainable on the theory that they are due to local resistance to down- 

 wearing action. So, too, the Piedmont area, where the slope rises from 

 tidewater westward to the level of about 1,000 feet, exhibits much uni- 

 formity in the hight of the crests at equal distances from the central 

 Appalachians, but with many " monadnocks " or steep, isolated peaks 

 rising far above the general plane of crest altitude. In the central sec- 

 tion there is, in my opinion, no such accordance in the bights that point 

 to baseleveling followed by elevation, the range in elevation being from 

 about 1,000 feet in New Jersey to about 7,000 feet in North Carolina- 

 There are what might be taken as signs of a progressive rise of these 

 elevations to the southward were it not that from the Smoky mountains 

 of North Carolina there is a steep decline toward the gulf of Mexico, as 

 well as a rise from New Jersey northward in ridges of related age, cul- 

 minating in the White mountains of New Hampshire. 



Taking first the Alleghanian field we find that the measure of accord in 

 its summit levels is not what it seems at first sight to be. Looking from 

 any summit which commands awide prospect, the observerisalwaj^s struck 

 with the apparent regularity of the skyline of the distant mountains. On 

 close study of this impression, however, he finds reasons to doubt its va- 

 lidity. It is easy to see that at a distance of, say 40 miles, differences of 

 hight of 500 or even of 1,000 feet are not conspicuous unless the additional 

 elevation have a peaked shape. If the rise is gentle at either end it is classed 

 with the prevailing skyline formed by the other ridges. It is only by the 

 careful inspection of good maps which by contours indicate the bights 

 that an adequate conception of the altitudes of the reliefs can be gained. 

 Until within a few j^ears the higbts of the AUeghanies were not enough 

 known to afford a basis for this accurate inquiry. At present, however, 

 the topographical maps of the area made b}'- the United States Geological 

 Survey are sufficiently accurate for the })urpose. From them we learn 

 that the range in the bights of the crests is considerable, amounting to 

 as much as one-third of the maximum altitude above the plane of the 

 sea, and to as much as one-half the hight to which tlie crests rise above 

 the plane to which the greater rivers have cut down their channels. It 

 these differences were presented to the eye in a group of peaks brought 



