340 CYCLES OF DENUDATION 



resist wear better than the anticlines, and standing up above the 

 level, form the synclinal mountains of many ancient ranges. 



From the geological point of view mountains must be regarded 

 as short-lived and ephemeral ; low-lying plains persist for a longer 

 time than do lofty ranges, as rivers may outlast many generations 

 of lakes. Consequently, among the mountain chains of the globe, 

 we everywhere find that the lofty ranges are those of compara- 

 tively recent date, while ancient mountains have been worn down 

 into mere stumps. The Appalachians have been reduced nearly 

 to base-level, and their present condition is that of a reelevated 

 and dissected peneplain, the ridges and valleys of which are deter- 

 mined by the position, attitude, and alternation of harder and 

 softer strata. In its pristine state this very ancient range may 

 well have been as lofty as the Alps or Andes. Of course, there 

 is no mathematical ratio between the youth of a range and its 

 height, for moderately folded strata of moderate thickness never 

 could have formed very high mountains, but in a general way it is 

 true, that very high ranges are youthful, and that very old ranges 

 are low. The process of degradation may go so far as to sweep 

 away a mountain range to its very roots, leaving only the intensely 

 plicated strata of the plain as evidence that mountains ever 

 existed there. Of such a nature is the upland of southern New 

 England and the great metamorphic area of Canada, both of 

 which probably once carried ranges of high mountains. 



Successive Cycles of Denudation. 



We have seen that any region, however lofty and rugged, must 

 eventually be worn down to base-level, provided only that the 

 country remain stationary with reference to the sea until the pro- 

 cess of degradation is complete. It is doubtful, however, whether 

 any extensive region of hard rocks has ever been absolutely reduced 

 to base-level : the usual result is the formation of a peneplain, a 

 low-lying, featureless surface of gentle slopes and with only occa- 

 sional eminences rising above the general level. Reelevation of 

 such a peneplain at once revivifies the streams and gives all the 



