270 N. S. SHALER — SPACING OF RIVERS 



jointing of tlie rocks and other details of structure and attitude which 

 do not liave controlling influence on surfixce rivers. 



Tiie foregoing considerations concerning the spacing of rivers, though 

 in themselves matters of interest, as they ])oint the way to a clear under- 

 standing of important geological prohlems, have a much wider applica- 

 tion than appears at first sight. I shall now endeavor to show tliat they 

 ma}' hear on the question as to the origin of the coincidences in moun- 

 tain crests, which is so generally held to indicate the existence of ancient 

 baselevels of erosion which have been lifted to a hight above the level of 

 the sea and then dissected by rivers. To perceive the value of these in- 

 dications we must again revert to the facts shown in the miniature valley. 

 It is noticeable that the divides between these streams are often main- 

 tained in about the same plane to whatever depth the general surface 

 may be lowered. I have seen instances of it where the valleys had cut 

 down to a depth greater than their width. This element of regularity is 

 quite as evident as the uniformity of spacing between the channels. The 

 crests, of course, rise toward the headwaters of the basins which they 

 separate, but where a considerable field is incised in this manner the 

 nodal points of the drainage remain approximately at the same hight 

 above the baselevel. 



Bearing of Evidence on Baseleveling 



relation of down-wearing to uniformity of level 



At first sight it may seem that this identity of attitude of the remain- 

 ing highlands in the system of miniature valleys is due to the original 

 equality of the surface on which the work of erosion began. Observa- 

 tions show, however, that even where there is a great variety' in elevation 

 the process of down-wearing tends to bring the varied levels into uni- 

 formity. In the small scale, rapidly forming drainage this action is more 

 easily recognized than it is in the case of true river basins. The process 

 may indeed be seen in actual operation. Where there are considerable 

 elevations their slopes are steeper and erosive work goes on upon them 

 much more speedily than elsewhere, until their declivities are brought 

 down to the uniform grade of the slopes which the material takes with 

 a given measure of rainfall. The important point is to discern that an 

 approach to uniformity of interval and lateral slopes of valleys effect- 

 ively tends to bring about a likeness in the hight of the divides even 

 where the original surface was of varied elevation. 



BASELEVELS OF THE APPALACHIANS 



To appl}' this principle to the matter of ancient baselevels, let us take 

 what is a most characteristic instance, that of the Appalachians. As re- 



