Smith ] 



Some Aspects of Erosion. 



161 



LIMITATION OF THE TERM. 



If the necessary conditions of peneplanation, then, are so difficult 

 of fulfilment, it is certainly reasonable to suppose that actual and 

 extensive peneplains could have been developed only in rare 

 instances, where there was an unusual combination of the most 

 favorable circumstances. But it is by no means necessary, on this 

 account, to discard altogether the idea of the actual occurrence of 

 peneplains. 



It is possible that a given region may remain relatively quiet 

 through a considerable geologic time. Such relative quiescence 

 would allow of a continuous slow elevation, providing it was so slow 

 that erosion could more than keep pace with it ; or a slight depres- 

 sion, which, by decreasing the general elevation of the region, 

 might shorten more or less the time required for peneplanation. The 

 absence of given deposits from a region would mean, either that 

 they were wholly removed after their deposition, or else that there 

 never had been such deposits in that region. In the latter case 

 there must have been either relative quiescence during the time in 

 question, or else oscillations, in general above sea-level. 



With variations in movement there have been also variations in 

 other factors which affect the formation of peneplains, as rainfall, 

 climate etc. Dutton * states that the climate of the Colorado 

 region was much moister during the Miocene than at present, and 

 that the excessive rainfall of that time aided in the immense erosion 

 which took place in that region. Some regions may be more 

 readily eroded, and thus more likely to be worn down to the condi- 

 tion of a peneplain than others, on account of differences in general 

 characters of the rocks, in amount of elevation etc., as well as in 

 climate and rainfall. 



From the above it will be seen that the writer's own belief 

 includes the theoretical possibility of peneplains and their probable 

 actual occurrence, but only in rare instances, owing to the nature of 

 the conditions under which they must be formed. If, then, actual 

 peneplains are few, a large proportion of the features thus far 

 described as such are probably due to some other cause. It 



* Loc. cit. 



