Proceedings of tJie Ohio State' Academy of Science 25 



Davis who, more than any other one person, has given definite- 

 ness to this conception of the geographical cycle. 



It has been stated that the existing peneplains of eastern 

 United States are not intact, but have been dissected since their 

 formation. In central New England, for example, the even sky 

 line of the plateau, which stands two thousand feet above sea- 

 level, represents an old peneplain, which was later raised, and 

 into which streams have cut their vallevs to a deoth of more than 

 a thousand feet. In many cases broad lowlands, themselves pene- 

 plains of a later cycle, have been cut below earlier peneplains. 

 It is evident that the present position of such peneplains gives us 

 important information regarding the amount and character of the 

 successive uplifts of the region. For if such plains were made 

 near base-level, and are now found far above base-level, the 

 difference in height measures the amount of uplift. 



In the case of the Appalachians, the earliest peneplain has 

 been arched up to a maximum of over four thousand feet above 

 sea-level. But in other mountain areas old peneplains are recog- 

 nized by the evenness of sky-line, at much greater heights. Por- 

 tions of the Cascade range of central Washington are cut from 

 an eight thousand foot plateau which was a peneplain formed 

 in Pliocene time.° In other mountain ranges similar con- 

 ditions have been found. The bearing of these facts on mountain 

 origin is evident. These mountains have the folded and dis- 

 ordered structure typical of mountain areas, but the original high 

 relief which this disordered structure suggests had been reduced 

 to a peneplain before the present relief was produced. In the 

 case of both the Appalachians and the Sierra Nevada, the range 

 of today is the second which has stood on that site, and we have 

 evidence, in the peneplain from which the later range is cut, of 

 complete destruction by erosion of the earlier* range. 



Our knowledge of peneplain development in the past brings 

 out with great clearness another great fact of geology, the im- 

 portance of which is only recently fully recognized, namely the 

 alternation of periods of crustal disturbance with long periods of 



''Smith and Willis: Contributions to the Geology of Washington, 

 Professional Paper No. 19, U. S. Geol. Survey, plates 8 and 19. 



