THE INTERIOR OF THE EARTH. 247 



any given point in the earth necessarily favors such chemical changes 

 as are accompanied by increase in volmne and reduction of density. 

 Increase of pressure tends to have the reverse effect. Such changes 

 tend to reenforce and extend in time the effects just referred to 

 which may be computed from the bulk modulus of elasticity. 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 formation as have 

 been conducted at the geophysical laboratory in Washington. 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 years which 

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

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

 terial also necessarily increases. 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, initiated 

 in part by surface transfers of load, modified by changes of tempera- 

 ture, modified by chemical changes and in the other ways, and on 

 the other hand the facts of the past as to the behavior of the earth 

 recorded in the rocks and read by geologists and others. This com- 

 parison should be used to the fullest possible extent to evaluate the 

 relative importance of the various elements in the actions. 



In making this comparison of various hypothetical phenomena 

 with the great accumulated mass of geological facts it should be 

 recognized at once that it is false logic to reason that if a given 

 hypothesis does not account for all the observed facts the hypothesis 

 is necessarily erroneous. On the contrary it is true logic in dealing 

 Avith such a problem as the earth seen from a physical standpoint 

 to reason that the more facts are accounted for by a given hypothesis 

 the more certain it is that said hypothesis is a statement of a con- 

 trolling element in the complex phenomena and then to study the 

 facts which appear neutral, or conflicting, with reference to the 

 hypothesis, considering them as indicators of other elements of the 



