i 5 8 



University of California. 



[Vol. 2. 



topography.* His statements refer, without question, to recent, 

 undissected peneplains, standing now at or near the -level at which 

 they were formed; and in this sense (which is quite different from 

 the interpretation given to them by Professor Davis f) they form 

 a valid argument against the unconditional acceptance of the 

 peneplain explanation. 



The absence of peneplains recently formed or in process of for- 

 mation involves (as Professor Tarr points out and as Professor 

 Davis admits) the assumption of different conditions in the past, 

 when peneplains were formed, from those of the present, when 

 peneplains do not form. If we assume that there have been in the 

 past periods of relative quiescence, while the present is a time of 

 comparatively greater movement, a minor difficulty still remains. 

 Not only are there no present peneplains, unelevated and undis- 

 sected, but those of the past, so far as known, are all elevated to a 

 considerable height above sea-level. As it is beyond question that 

 earth movements, at a given time, are not uniform over the earth's 

 surface, nor even over continental areas, but, on the contrary, are 

 very diverse in different regions, we should expect peneplains, which 

 have been subjected to these movements in different parts of the 

 world, to occupy, in consequence, more diverse positions than 

 those in which they are found. That is, the many different oscil- 

 lations would be likely to leave some peneplains at considerable 

 elevations, to bury others, and to return others again to some- 

 where near their original position — near sea-level. If peneplains are 

 so numerous as they are described, it seems strange that not a 

 single example should appear anywhere near sea-level. This objec- 

 tion, however, may be disregarded if it can be shown (as the writer 

 believes) that the occurrence of peneplains has been much less 

 frequent than is commonly assumed. 



Difficulty of Fulfilling Necessary Conditions . — Professor Tarr's 

 next important argument is that the earth's crust would not remain 

 stable long enough for the slow process of denudation to produce 

 a peneplain.;!; It must be noted that while this statement includes 



* Loc. cit., p. 353. . 



t Loc. cit., p. 227. 



% Loc. cit., pp. 354 and 362. 



