192 SOUTHERN CALIFORNIA GEOLOGY 



THE THEORY OF ISOSTASY 



We find on consulting the leading present-day 

 authorities — mostly men unknown to the masses 

 whose conclusions have not as yet appeared in text 

 books — that the contractional theory has been dis- 

 carded, and in its place another has been substituted 

 which might be called the equilibrium theory, but 

 which is actually called "isostasy." Isostasy, then, 

 deals with flotational movements of adjacent segments 

 of the outer layers of the earth's crust — the zone of 

 fracture — above a lower, heavier, more plastic zone of 

 flowage. The up and down movements are due to the 

 subtraction and addition of load from certain segments 

 of the crust, causing them to rise or sink in the plastic 

 substratum. Movements or change of load from one 

 part of the earth's surface to another produce isos- 

 tatic equilibrium. Such movements are the ultimate 

 accomplishments of the work of volcanism, which 

 transfers load from the sub-surface to the surface; 

 of drifting clouds which transfer moisture from the 

 ocean to the land where it makes rainfall and glacial 

 snow deposits; and of the transportative work of the 

 streams and wind which transfer load from the high- 

 lands to the ocean border. 



The doctrine of isostasy conceives that the great 

 interior of the earth is so dense and rigid that further 

 shrinkage there is impossible and that the movement 

 must be within the outer, or rock girdle. This rock 

 girdle, or so-called crust of the earth, is subdividable 

 into a cooler, lighter, upper zone of fracture and a 

 lower, hotter, heavier, plastic zone, or zone of flowage. 

 The rocks of the fracture zone are less dense than 



