402 Jones — Review of Chamberlin's Groundwork 



tion such areal denudation has always been followed by 

 uplift of the denuded area. On the other hand, it is 

 probably true that accumulations of sediments do not 

 cause down-sagging though their weight may be additive. 

 In a large measure sedimentation goes on because of 

 down-sagging instead of down-sagging being the result 

 of sedimentation. The fundamental difference in the 

 effects of denudation and sedimentation is undoubtedly 

 due to the relative areas they affect. Denudation is not 

 constricted to narrow belts while sedimentation is, and 

 in spite of the probability of greater load per unit area 

 in depositional belts than underload per unit area in pene- 

 planed areas, these latter areas respond to the under- 

 loading and rise. It is a question then, not of underload 

 or overload per unit area, but of areal distribution of 

 load. The evidence back of this is conclusive. The 

 smaller, or narrower, the area the greater the load per 

 unit area necessary to cause down-sinking. In a holo- 

 crystalline earth these vertical movements would be 

 entirely controlled, in so far as surficial loading could 

 cause movement, by load per unit area rather than areal 

 distribution of load. If these loads are the result of 

 greater or less density extending to great depths there 

 is no apparent reason why a broad area so loaded should 

 more easily sink or rise than a smaller area similarly 

 loaded. The mere fact of the control by areal distribu- 

 tion rather than by unit area load implies a solid crust 

 of some strength resting on a yield zone beneath. 



Direct proof of ready yielding under not excessive load 

 per unit area but broadly distributed is to be found in 

 the very definite record of crustal sinking beneath the 

 Pleistocene ice caps and the rebound following deglacia- 

 tion. Furthermore, it is becoming increasingly evident 

 that this sinking was accompanied by a corresponding 

 upwarp 21 at some distance in front of the advancing ice. 

 The on-moving load, in this case, was actually "sliding 

 on thin ice." 



Furthermore the universal association of down-sag and 

 upwarp through geologic history is difficult of explana- 



21 Joseph. Barrell : Movements of the Strand-line in Pleistocene and Post- 

 Pleistocene, this Journal (4) vol. 40, p. 13, 1914. 



R. A. Daly: Oscillation of Level in Belts Peripheral to the Pleistocene 

 Ice Caps, Bull. Geol. Soc. Am., vol. 31, pp. 303-318, 1920. 



