38 HAYFORD 



loaded by erosion the underlying material to a depth of 71 

 miles increases in volume mainly because of chemical changes 

 induced by the decrease in pressure, and partly also because of 

 changes in the gases from solution to the free state. This in- 

 crease in volume raises the surface. It also increases the pres- 

 sure at each level above the 71-mile depth, and tends to bring 

 it back toward the value which it had at that level before the 

 unloading. 



This expansion process alone is not sufficient, however, to 

 maintain an isostatic adjustment indefinitely. 



As the process progresses — a continuous expansion in the 

 underlying material keeping pace approximately with continu- 

 ous unloading by erosion at the surface — the pressure near the 

 bottom of the expanding column will become considerably less 

 than it is at the same level in other areas at which no unloading 

 by erosion is taking place. So, too, near the top of the expand- 

 ing column the pressures will tend to be somewhat greater than 

 at the same level in other areas. The result of these differences 

 in pressure at any given horizontal surfaces will be to set up, 

 sooner or later, a great slow undertow from the ocean areas 

 toward the continents, and a tendency to outward creeping at 

 the surface from the continents toward the oceans. 



Let me now emphasize the idea that the theory briefly sketched 

 in the last few minutes is one which correlates many groups of 

 observed facts. 



It obviously accounts for the marked general tendency for 

 areas unloading by erosion to rise and those loading by deposi- 

 tion to subside. 



The theory indicates how the changes in density which ac- 

 company matamorphism are a part of the process of continent 

 building. 



The theory also accounts for the tangential stresses along 

 the earth's surface of which the crumpled strata, especially of 

 mountainous areas, are the evidence. For the great undertow 

 toward the continents is attached to the surface strata by con- 

 tinuous material and tends to carry them inward. A great con- 

 test is waged between the shearing stresses developed between 

 the undertow and the surface strata on the one side, and the 



