Uplift and Subsidence. 15 



all of the sedimentary material deposited close to sea- 

 level. 



I have given the question of sedimentary areas consid- 

 erable space in this paper because of its fundamental 

 importance to the theory of isostasy. If we can prove 

 that the areas of sedimentation which have gravity ano- 

 malies of a negative sign, often of large size, are in isos- 

 tatic equilibrium, then we would be justified in concluding 

 that we have local rather than regional isostatic adjust- 

 ment and the earth's isostatic shell would be shown to be 

 far weaker than many are now willing to admit. The 

 writer believes that we have proved that the large Ceno- 

 zoic gravity anomalies are due to the presence of light 

 material close to the station and that we are justified in 

 assuming that the columns under the Cenozoic material 

 are in approximate isostatic equilibrium. 



Compensation exists tinder small areas having high relief. 



I have not given space in this paper to a discussion of 

 the gravity anomalies at stations on the Pre-Cambrian 

 formation, but it is believed that the tendency of the 

 anomalies at stations on that formation to have positive 

 signs is an indication that the columns under the Pre- 

 Cambrian may be in isostatic equilibrium. If we can 

 eliminate or account for the anomalies on the Pre-Cam- 

 brian and Cenozoic formations then we have taken a long 

 step forward in proving that local isostasy exists. It is, 

 of course, a question of how local the area may be that is in 

 isostatic equilibrium. This is a question that cannot be 

 mathematically solved but as isostatic investigations are 

 extended the areas which may escape a high state of 

 equilibrium (at least in elevated regions) become smaller 

 and smaller. They appear to be well within 100 miles 

 square in mountain areas. This question of the size of 

 an area which may be in equilibrium has an important 

 bearing on the question of mountain formations. 



Uplifted masses dive to vertically rather than horizontally acting 



forces. 



From a consideration of the discussion above we may 

 assert that columns under mountain masses and under 

 areas of sedimentation are in equilibrium. Therefore, 

 if a mountain mass is formed over a column which was 

 subjected previously to heavy sedimentation, the moun- 



