12 W. Bowie — Relation of Isostasy to 



Burrard (The Geographic Journal, Royal Geographic 

 Society, London, July, 1920). Burrard shows that the 

 isostatic method can be used to predict what the intensity 

 of gravity or the deflection of the vertical will be before 

 observations have been made. This prediction cannot 

 be made with any degree of reliability if the condition 

 of isostasy is ignored. The methods adopted by the 

 geodesist can stand the test of prediction. Some other 

 method, not yet formulated, may work equally well but 

 it cannot depart materially from the one now in use. 



The density of material in the isostatic shell. 



It has been held by some that the geodesist assumes a 

 density of 2.67 for the density of the material in the isos- 

 tatic shell. Geodesists have never postulated the density 

 of the material below sea-level, except in so far as 

 attempts have been made to show that the presence of 

 Cenozoic and Pre-Cambrian formations affect gravity 

 anomalies. What the geodesists have done is to compute 

 the deviations from normal densities in the column do^vn 

 to the depth of compensation. We may assume that the 

 normal densities for the various zones are A-^, A2, A3, etc. 

 The geodesist computes the deviations from these 

 unknown densities. 



The theory of isostasy is based on the idea that the 

 mean density in a column times the volume of the column 

 is a constant. Each column is assumed to have the 

 same cross section and to extend from the depth of com- 

 pensation to the surface of the earth. 

 ■ The volume of the column and the density of the 

 material are supposed to be normal under the coastal 

 plains. The column under a mountain mass will be 

 longer than normal, its volume will be greater and, there- 

 fore, its density of material must be less. Under the 

 oceans the column of material is shorter than normal, 

 the volume will be less and the density of material will 

 be greater. 



Biver deltas as a test of the strength of the earth's crust. 



In his series of papers entitled ^'The Strength of the 

 Earth's Crust," Barrell attempted to prove that the 

 crust or isostatic shell is able to hold large masses as 

 extra loads, because it is able to hold up the sedimentary 

 material deposited at the mouths of rivers. He used 



