EROSIONAL HISTORY OF DPJFTLESS AREA 31 



a second cycle to degrade some of the land further and leave 

 the previous surface represented by the flat summit areas. 

 It seems that accordant summit levels would be as strong- 

 evidence of more than one cycle in regions of massive igne- 

 ous rocks as in regions of folded strata. 



Extrusive lava sheets and intruded sills might form flat- 

 topped hills, when eroded, without offering more than a 

 bare suggestion of more than one cycle of erosion. 



In any case there is an unanswerable question as to how 

 flat upland surfaces must be and how nearly to a common 

 level their remnants must come before weight is given to 

 them as evidences of more than one cycle of erosion. Even 

 in the regions which have suflfered more than one cycle 

 there are several causes of irregularity in the topography 

 of the upland plain. In the first place most peneplains at 

 the close of the first cycles are not flat. A total relief of 

 several hundred feet would not be incompatible with the 

 term peneplain, provided large portions of the surface had 

 been brought to grade. Secondly, the surface might be 

 warped, folded, tilted, or faulted as it is uplifted. Finally, 

 erosion might roughen the upland surface after rejuvena- 

 tion of the streams, without entirely destroying its former 

 characters. The larger and flatter summit areas are, and 

 the more nearly accordant they are, the more definitely can 

 the term "even-crested summit areas" be applied to them, 

 and the more certainly can the erosional histories of regions 

 be read from them. The failure of accordance in the up- 

 lands of a region could under no circumstances be taken as 

 proof that the region had not been eroded in more than one 



cycle. 



Even-crested summit areas should be used as evidence of 

 more than one cycle of erosion only after complete and care- 

 ful study. 



Intermediate Plains 

 The term intermediate plain has not been used before as 

 an evidence of more than one cycle, but the principle in- 

 volved has been used extensively and to good advantage, 

 without previously having been named. In the present con- 



