I9I5.] THE GEOPHYSICAL STANDPOINT. , 307 



otherwise occur. Possibly this elastic change is much larger than 

 the estimate here given. Of course if the erosion or deposition takes 

 place in a small area only, such elastic response will be largely in- 

 hibited by surrounding material on which the load has not been di- 

 rectly changed. But under large areas of erosion or deposition such 

 action must take place and extend to depths possibly as great as 122 

 kilometers. 



Study next the modifying effects, on the phenomena already pic- 

 tured, of chemical changes which are probably produced in the earth 

 by changes of pressure. The expression " chemical changes " is 

 here used in the broadest possible sense. A relief of pressure at 

 any given point in the earth necessarily favors such chemical 

 changes as are accompanied by increase in volume and reduction 

 of density. Increase of pressure tends to have the reverse effect. 

 Such changes tend to reinforce and extend in time the effects just 

 referred to which may be computed from the bulk modulus of elas- 

 ticity. It is important to estimate such changes as well as possible 

 from all available evidence, such for example as that furnished by 

 chemists, by geologists, and by such investigations of rock forma- 

 tion as have been conducted at the geophysical laboratory in Wash- 

 ington. I believe the possible effects of this kind will be found to 

 be so large as to be of primary importance. 



Evidence has accumulated during the past few yeai-s which 

 makes it reasonably certain that with increased pressure, as at the 

 great depths in the earth, the rigidity and the viscosity of the 

 material also necessarily increase. This tends to cause the viscous 

 flows to take place at higher levels than they otherwise would. 

 This should be taken into account. 



Next a reexamination of the conceptions so far formed should 

 be made to ascertain to what extent and how they would be modified 

 if one started with some other reasonable assumption as to the limit- 

 ing depth of present isostatic compensation or some other reason- 

 able assumption as to the law of distribution of the compensation 

 with regard to depth. 



Next full and extensive comparisons should be made between 

 the hypothetical phenomena on the one hand pictured as made 

 up primarily of viscous flows, modified by some elastic effects, ini- 



