36 JOSEPH BARRELL 



the load eroded. Third, he assumes a crustal undertow from 

 heavy toward high areas which would not only fold the surface 

 rocks and heat them in the region of undertow but restore the 

 equilibrium of mass in the regions of erosion and deposition.^ It 

 may be said of all of these factors that when they are subjected to 

 quantitative statement they appear so trifling as to fail wholly to 

 explain the magnitude and breadth and periodicity of crust move- 

 ments. The inadequacy of the temperature effects has been 

 pointed out clearly by Harmon Lewis.^ The assumption of the 

 high coefhcient of compressibility involves more instead of less 

 difficulty for the high isostasist. The inadequacy of isostatic 

 undertow to account for folding has been discussed briefly by the 

 present writer elsewhere.-' On the other hand, the control of the 

 level of the earth's surface during epochs of quiet by the forces of 

 planation and not by forces making for close isostatic adjustment 

 has been discussed convincingly by Chamberlin in his present series 

 of articles. It seems clear, then, that in the study of cycles of 

 erosion and deposition much may be determined in regard to the 

 limits of terrestrial rigidity. The subject could be developed 

 further, but it is preferred to place the emphasis of this paper upon 

 the more readily estimated loads produced by the building of deltas. 



THE EVIDENCE FROM DEPOSITION 



Preliminary statement. — -The waters deposit sediment upon the 

 depressed areas of the crust. To what extent may such areas be 

 loaded before yielding of the base and resultant subsidence take 

 place? The geologic record makes it clear that subsidence and 

 deposition are necessarily related. It has been stated often that 

 deposition was the cause and subsidence the effect, the two being 

 regarded as in deHcate isostatic adjustment. But this is in reahty 

 an assumption, for such a supposed relationship overlooks the extent 

 to which subsidence might have gone forward without deposition 

 and ignores the external load which may have been necessary to 



' "The Relations of Isostasy to Geodesy, Geophysics, and Geology," Science, 

 N.S., XXXIII (1911}, 199-208. 



^ "The Theory of Isostasy," Jour. GeoL, XIX (191 1), 622, 623. 

 3 Joseph Barren, Science, N.S., XXIX (1909), 259, 260. 



