YIELDING OF THE EARTHS CRUST BOWIE. 



237 



dealt especially with the relation between the theory of isostasy and 

 the value of the intensity of gravity. 



COASTAL PLAIN 



PLATEAU 



Sea fyvel 



harth blocks have .. 

 These k ^, e t fe same 



te se % l/olumes and, densit 



%%H is the dens,ty - 



the 



same cross-sectin 

 mass or Weiqht nc 



es. The longer t n , 



j indt^^J 1 -*^ concise 



stress^ 5 - ?**# 



Fig. 2. — Isostatic equilibrium in the earth's crust. 



The isostatic investigations show that blocks of the earth's crust of different lengths 

 have the same mass ; therefore the pressure of the blocks at a certain distance below 

 sea level where the material changes from a resisting to a yielding solid will be the 

 same throughout. The longer blocks are composed of less dense material than the 

 shorter ones. 



While it is impossible to determine definitely the depth within 

 which the isostatic compensation occurs, the investigations have 









as e 



V S COAST AND CEOtrCTiC SURVEY 



clesteo jovti 



DIRECTOR- 

 DISTRIBUTION Of GRAVITY STATIONS 



1321 



Fig. 3. — Gravity stations in the United States. 

 There are now 286 gravity stations in the United States, located as shown on the 

 diagram. The values of gravity are used in the isostatic investigation. 



shown that the depth is of the order of magnitude of 60 miles. 

 At or just below that depth the material of the earth is assumed 

 to change from a solid which resists stresses acting in a hori- 

 zontal direction to one which will yield plastically to those stresses. 



