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SCIENCE 



[N. S. Vol. XXXIII. No. 841 



have been started by erosion and deposi- 

 tion and continued by the action of grav- 

 itation tending to bring about a readjust- 

 ment to isostatic conditions. 



Attention is invited to the thought that 

 during the actions indicated the pressure, 

 temperature and relative movements in 

 any small portion of the material are a 

 function not simply of the facts at that 

 place but also largely of the facts at many 

 other places, some at considerable dis- 

 tances. 



For example, in the neutral region be- 

 tween a region of erosion and one of depo- 

 sition there may be movements beneath 

 the surface (the undertow), changes of 

 temperature in the flowing material, a 

 crumpling of the surface material, changes 

 of volume and changes of elevation of the 

 surface, all of which are dependent pri- 

 marily upon the facts in adjacent regions 

 of erosion and deposition. 



Let us limit our thoughts still to the 

 cycle of changes which have been sketched 

 roughly. Keep it in mind that the actions 

 at any given point in the material depend 

 on the facts at many other points. Keep 

 it in mind also that a region of erosion in 

 one age may, and often does, become a re- 

 gion of deposition in another, and that, 

 therefore, the actions taking place at any 

 instant in a given portion of the material 

 are necessarily dependent upon the past 

 as well as the present conditions. Is it not 

 evident that even if the cycles of action 

 which have been indicated were the only 

 actions taking place in the outer portions 

 of the earth, the resulting series of move- 

 ments observable at the surface would be 

 very complicated 1 Is it at all certain that 

 under the influence of such actions the geo- 

 logical record at the earth's surface at the 

 end of 50 to 100 million years would be 

 appreciably less complicated than the 

 geologic record which is actually before 



us ? I think that it would be fully as com- 

 plicated as the actual record. 



Let me illustrate, by a single example, 

 the kind of reasoning which the considera- 

 tions just stated should lead one to avoid. 



It has been stated to me that mountains 

 are sometimes eroded to a peneplain, and 

 that thereafter the peneplain sometimes 

 sinks. It has been suggested to me that 

 such a case can not be reconciled with the 

 theory of isostasy. It is said that as the 

 material is eroded from the surface the 

 underlying material must increase in vol- 

 ume to keep the isostatic compensation 

 complete, hence that according to the 

 theory of isostasy a peneplain may rise but 

 never sink. 



This reasoning contains several errors. 

 In the first place, in Fig. 1, after a portion 

 of the surface of column A has been car- 

 ried away by erosion and the pressure at 

 the bottom of the column thereby reduced 

 to less than that under column B, the mere 

 vertical expansion of column A will not 

 reestablish equality of pressure. The 

 equality may be reestablished only by re- 

 storing mass to column A by forcing ma- 

 terial into it from some other column. 

 This gravitation tends to do. Secondly, 

 when gravitation, by producing an under- 

 tow, forces material into column A the new 

 material may enter by processes which in- 

 crease the density of column A. Column 

 A may thus become heavier without any 

 raising of its upper end. An eroded sur- 

 face does not necessarily rise. Thirdly, a 

 time may come when by virtue of the low- 

 ering of the temperature by erosion the 

 material in column A may increase in den- 

 sity by thermal contraction and the sur- 

 face may thus be lowered without any 

 masses passing to other columns. If so, 

 the isostatic condition remains unchanged, 

 the relative pressures at the bases of A and 

 B remaining unchanged. Such a process 



