THE GEODETIC EVIDENCE OF ISOSTASY 37 



changes as are accompanied by decrease of volume, that is, in- 

 crease of density. 



So, too, it is a law well established by laboratory investiga- 

 tions that the mass of a given gas that will remain in solution 

 in a given liquid is proportional to the pressure. According to 

 this law, known as Henry's Law, wherever beneath the surface 

 of the earth gases and liquids are in contact an increase of pres- 

 sure will drive more gas into solution and so increase the den- 

 sity of the mixture. A decrease of pressure will cause apart of 

 the gas to come out of solution and decrease the density of the 

 mixture. 



Considering solution as a chemical process this law is but a 

 specific example of the general law stated a moment ago. 



Many other specific examples might be given of changes in 

 pressure producing changes in chemical state and thereby 

 changes in density. 



Very important among these, because it is a process appar- 

 ently in progress very extensively, is the solution of rock con- 

 stituents in water and redeposition with a net increase of 

 density of the rock so modified. 



A quantitative study shows that changes of these kinds in a 

 small part only of the materials in the heterogeneous mixture 

 which makes up each cubic mile are sufficient to account for a 

 change of 3 per cent, in the average density, and that isostatic 

 readjustment brought about in part in this manner is not at all 

 improbable. 



The consensus of geologic evidence also indicates the exist- 

 ence of this relation of pressure, chemical state and density. 

 For example, rocks which have been under great pressure be- 

 cause they have been deep within the crust are, in general, 

 more dense than those composed of the same proportions of the 

 elements but which have not been subjected to great pressure. 

 So, too, it is a general law of metamorphism that changes going 

 on in rocks which are now near the surface but which formerly 

 were deep-seated are changes which are accompanied by de- 

 crease in density. 



The indications are, therefore, that when an elevated area 

 under which there is complete isostatic compensation is un- 



