Febeuaby 10, 1911] 



SCIENCE 



207 



may cause a region which has been eroded 

 from mountains to a peneplain to sink 

 thereafter. Fourthly, such reasoning as 

 that cited ignores the history of the region 

 before the mountains were there. That 

 early history is essential to a full under- 

 standing of late movements. Fifthly, such 

 reasoning entirely ignores the relations of 

 the region considered to adjacent regions. 

 As the evidence shows that the material 

 concerned in isostatic readjustment is 76 

 miles deep, it is but sensible to estimate 

 that the influences concerned in any one 

 movement of isostatic readjustment extend 

 over horizontal distances of at least 76 

 miles, probably over distances as great as 

 200 miles. Therefore, valid reasoning in 

 regard to the peneplain in question should 

 include a consideration of the conditions 

 surrounding it to a distance of 200 miles, 

 whereas in fact in the reasoning cited the 

 surrounding conditions were entirely ig- 

 nored. 



How is it possible to form an estimate of 

 the relative effectiveness of gravitation 

 tending to produce isostatic readjustment, 

 on the one hand, and of all other forces 

 acting on the outer portion of the earth, 

 on the other hand? Gravitation is the 

 only force which continuously tends to pro- 

 duce isostatic readjustment. The rigidity 

 of the material tends continuously to op- 

 pose the readjustment toward isostasy. 

 Other forces than gravitation are equally 

 likely to help or to oppose gravitation. 

 Therefore, the fact that the isostatic com- 

 pensation is everywhere nearly complete is 

 a proof, first, that the material composing 

 the outer portions of the earth has but 

 small effective rigidity, and, second, that 

 the forces in operation other than gravita- 

 tion are relatively ineffective. If either of 

 these propositions were untrue the present 

 close approach to complete isostatic com- 

 pensation would not exist. 



Before closing let me remind you that 

 the geodetic evidence shows that the outer 

 portion of the earth is not a solid crust a 

 few miles thick floating on a liquid sub- 

 stratum of slightly greater density.'^ 



The existence of isostasy is now thor- 

 oughly established by evidence which is 

 available in print. The time has come to 

 speculate upon the manner in which the 

 isostatic readjustments are produced, to 

 look for the relations between the known 

 condition, isostasy, and other known facts. 

 This address is a map showing the results 

 of a reconnaissance in this field. The 

 reconnaissance has involved much more 

 thought than I have been able to put into 

 words here. Some of the statements have 

 been made in rather a dogmatic form. 

 That is simply because I have tried to 

 draw a clear reconnaissance map with few 

 strokes, not that I have forgotten that the 

 field work has been merely reconnaissance. 

 I feel confident, however, that in due 

 course of time when careful surveys shall 

 have been substituted for this reconnais- 

 sance, the main features of this reconnais- 

 sance map will be found to be correct. 



In closing let me give you the recon- 

 naissance map on a small scale with details 

 omitted. 



Readjustment toward isostatic conditions 

 has been in progress throughout geologic 

 time. 



The differences in density involved in 

 complete isostatic compensation are very 

 small, usually less than one per cent., sel- 

 dom more than three per cent. 



With reference to such small changes of 

 density the earth is not incompressible 

 under the influence of stresses which are 

 applied continuously for geologic ages. 



Erosion and deposition cause changes of 

 pressure, which in turn bring about 



' " The Figure of the Earth and Isostasy," pp. 

 163-164. 



