EROSION AND THE SUMMIT LEVEL OF THE ALPS 15 



cycle by volcanic eruption or by changes of climate (other than the 

 normal decrease of precipitation and increase of temperature that 

 accompany the degradation of a highland) I have suggested should 

 be called accidents, in order to distinguish them from interruptions 

 due to crustal movements. 



The three terms, initial, sequential, and ultimate, referring to 

 the opening, the prosecution, and the ending of a complete cycle 

 of erosion, are due to Gulliver. The application of words taken 

 from the organic cycle of hfe to indicate stages in the inorganic 

 cycle of erosion was first made — not poetically, but physiographi- 

 cally — as far as I know, by Chamberhn and Salisbury in 1885, when 

 they described a district with narrow valleys as young, and one 

 with open valleys as old;^ I modified this suggestion by calling a 

 district with open valleys mature, and reserving old for a district 

 in which the inter- valley hills have been almost worn away; but 

 always with the understanding that these organic terms should 

 serve only to suggest, in the briefest possible manner, the general 

 surface features of a structural entity : they must be supplemented 

 by many details in a full description. Even when their application 

 is hmited to a structural entity, it is important to recognize that its 

 weaker rocks are more rapidly eroded than its stronger rocks, and 

 hence that belts of weak strata may already be old when belts of 

 resistant rocks are only mature, as is repeatedly the case in the 

 Pennsylvania AUeghenies. 



One of my later proposals was that an almost worn-down 

 surface should be called a peneplain; that new word being invented 

 in order to avoid the term, plain of erosion, which excited opposition 

 as demanding too long a stationary condition of the earth's crust 

 for its production. Conoplain has been proposed by Ogilvie to 

 name a surface of subaerial degradation that declines gently in all 

 directions from a central area.^ Monadnock, following the analogy 

 of meander — the name of a particular serpentine river now com- 

 monly used to indicate the curves that similar rivers follow — is 



^"The Driftless Area of the Upper Mississippi," Sixth Ann. Rept. U.S. Geol. 

 Surv., pp. 205-322. 



^ Amer. Geol, Vol. XXXVI (1905). 



